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Dairy Foods/National Mastitis Council Symposium Review Symposium Review| Volume 4, ISSUE 1, P51-54, January 2023

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Farm factors associated with increased free fatty acids in bulk tank milk*

Open AccessPublished:December 01, 2022DOI:https://doi.org/10.3168/jdsc.2022-0301

      Highlights

      • Free fatty acids (FFA) result from milk fat breakdown.
      • The presence of elevated FFA (>1.2 mmol/100 g of fat) is a milk quality concern.
      • FFA concentration is multifactorial and many factors arise at the farm level.
      • These factors stem from milk production, harvest, transport, and storage.

      Abstract

      An elevated amount of free fatty acids (FFA) is a milk quality concern that can contribute to off-flavor, rancidity, reduced foaming ability, and inhibited fermentation that affects cheese coagulation. Free fatty acid concentrations >1.2 mmol/100 g of fat are considered high, although there are various thresholds and units used to quantify levels. The FFA result from milk fat breakdown through spontaneous, bacterial, or induced lipolysis. The amount of bulk tank milk FFA can vary between farms as well as within farms daily, and this mini-review aimed to identify those risk factors at the farm level associated with elevated FFA. A search of the literature identified 5 current sources selected for this review based on relevance. Cows that were milked by automated milking systems (AMS) are suggested to produce milk that is higher in FFA compared with conventional parlors. Factors associated with AMS contributing to spontaneous lipolysis include higher milking frequencies, reduced milking intervals, and low milk yields at each robot visit. Automated milking systems also have characteristics of quarterly milking and high milk lines that can increase vacuum fluctuations and air admission contributing to induced lipolysis. Both AMS and conventional systems with poor tank cooling or without precooling mechanisms can be at risk for higher bulk tank FFA. Bacterial lipolysis can occur when milk temperatures fluctuate and rise, or when there is insufficient milking system cleaning and sanitization. Feed factors such as high saturated fatty acid diets can increase the likelihood of spontaneous lipolysis. We concluded that the major factors associated with increased levels of FFA are non-parlor milking systems, increased air admission, the absence of additional cooling, temperature fluctuations in the bulk tank, and rations high in saturated fatty acids. Future research further investigating these factors can help to minimize FFA and ensure milk quality.

      Graphical Abstract

      Figure thumbnail fx1
      Graphical AbstractSummary: Elevations in free fatty acids (FFA) in milk can result from multiple farm factors. These include cow characteristics, ration composition, milk harvest equipment, milk transport, and milk storage. Although through different mechanisms, each of these factors can result in increased lipase enzyme breakdown of triacylglycerol (TAG) molecules contained inside milk fat globules. The result is a release of FFA that, when in high concentrations, can negatively affect the quality of dairy products.
      Milk quality is arguably the most important aspect of milk production. Consumer satisfaction in dairy products is key to the sustainability of the dairy industry. Milk quality has traditionally been defined in terms of the absence of antibiotics and hormones as well as low somatic cell and bacteria counts. However, several European researchers suggest that free fatty acid (FFA) concentration should also be considered a milk quality attribute (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). This is driven by the fact that elevated FFA concentrations in milk are associated with off-flavor, rancidity, reduced foaming ability, and inhibited milk fermentation that affects cheese coagulation (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      Fat is an important component of milk, with most existing in the form of triglycerides (or triacylglycerol, TAG) (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). Free fatty acids are products of fat breakdown by lipolysis (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ), a catabolic process that results in the cleavage of TAG to form 3 fatty acids and a glycerol molecule through hydrolysis (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ).
      There is inconsistency in the literature with respect to a suggested sensory threshold for FFA because fatty acid composition in milk varies, and various analytical techniques are used to quantify their levels (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ) suggest that any level >1.2 mmol/100 g of fat (as measured by infrared spectroscopy) is concerning and can lead to milk quality issues.
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ) conducted a study on 124 herds and classified “high FFA” as any concentration >0.78 mmol/100 g of fat. The units for reporting FFA can also vary, but most studies report levels in millimoles per 100 g of fat (mmol/100 g of fat), milliequivalents per 100 g of fat (mEq/100 g of fat), or milliequivalents per liter of milk (mEq/L) (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). Although quantifying the sensory threshold of FFA remains a challenge, the consequences on milk quality are well documented.
      There are different forms of lipolysis in milk, but the effects of increased FFA are cumulative. Spontaneous lipolysis is catalyzed by lipoprotein lipases (LPL) and influenced by the physiological state of the cow and her nutrition (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). Bacterial lipolysis is catalyzed by bacterial lipases and is influenced by hygiene during milk harvest and milk cooling (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). Induced lipolysis is a result of physical stress on the milk fat globule membrane and is influenced by milk transport and handling factors (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ).
      Lipoprotein lipase is an endogenous milk enzyme that catalyzes spontaneous lipolysis by cleaving TAG to separate fatty acids from the glycerol molecule (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). It is produced in the mammary gland and its function is important in the uptake of blood lipids to construct milk fat (
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). Although LPL is abundant in normal milk, a protective milk fat globule membrane prevents its access to the TAG (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). If the milk fat globule membrane endures mechanical stress or agitation, it can expose hydrophobic sites where casein micelles and whey proteins can adsorb (
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ). Because LPL is most often associated with the casein micelle, adsorption of casein brings LPL closer to the TAG and risks their cleavage to produce FFA (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). A similar process occurs in bacterial lipolysis; however, bacterial lipases catalyze the milk fat breakdown instead of LPL (
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ). For induced lipolysis, physical or chemical damage to the milk fat globule membrane itself exposes the TAG to LPL or bacterial lipases, making them easier to target (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ).
      The amount of FFA in bulk tank milk leaving the dairy farm varies daily, with this variation resulting from many factors causing spontaneous, bacterial, and induced lipolysis (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). These can be broadly classified as cow factors (genetics, parity, stage of lactation), herd management factors (feed), and milk harvest factors (equipment, milk transfer, storage) (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). Although there are likely postharvest elements (transportation and processing) to consider, this review focuses on the on-farm factors affecting the level of FFA in bulk tank milk.
      Optimal lactating rations are essential to produce a high yield of high-quality milk that is low in FFA (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). Underfeeding cows can increase cow stress and the susceptibility of their milk to lipolysis, which could influence FFA (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ) conducted a study on 262 herds from the Netherlands and reported an association between an increased number of water and feeding areas and lower FFA to support this statement.
      The composition of the lactating ration also affects FFA concentrations. Feeding a ration higher in saturated fat (either natural or through supplementation) increases milk fat yields, but it can also increase FFA (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). Saturated fatty acids produce large and unstable TAG that increase the size of the milk fat globule, making it more susceptible to lipolysis (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ).
      Lactating ration ingredients can also influence FFA, and it is thought that diets higher in UFA can be protective (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). In an experimental feeding crossover design that compared lactating rations of 8 cows, lower FFA concentrations were observed in milk of cows on a high-UFA diet compared with a high-SFA diet (
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ). Higher levels of UFA stimulate de novo milk synthesis and produce milk fat globules that are smaller, have greater surface tension, and are more stable during mechanical processes, such as pumping milk (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). This explains the observation of reduced FFA during the summer months for herds with cows on pasture: fresh pasture feed contains higher levels of UFA (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ), in 3,585 herds (259 organic), observed that organic herds had a lower concentration of FFA (averaging 0.62 mmol/100 g of fat) than conventional herds (averaging 0.66 mmol/100 g of fat); the differences were most evident during the summer months when cattle were grazing. However, in that study, correlations between feeding regimens and FFA explained only 19% of the variation in FFA concentration (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      On most dairy farms, milk is harvested using 1 of 3 systems: tie-stalls, parlor, or milking robots. Tie-stalls and parlors are referred to as conventional milking systems, whereas robots are referred to as automated milking systems (AMS). An association between milking system and higher levels of FFA has been reported in the literature, with tie-stalls and AMS having, on average, higher FFA levels compared with parlors (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ), although the between-farm variability across all 3 systems is substantial.
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ) reported that although most (71–100%, depending on the milking system) of their 3,585 study herds had FFA concentrations between 0.8 and 1.3 mmol/100 g of fat, herringbone parlors had the greatest proportion of herds with FFA <0.8 mmol/100 g of fat (12.3% for organic and 14.5% for conventional).
      Although tie-stalls are associated with increased FFA concentrations compared with parlor milking systems, a more substantial difference in FFA has been reported between parlors and AMS (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ;
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ).
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ) reported that milk from 5,614 conventional herds had an average FFA of 0.75 mEq/L, which was lower than that for AMS models (0.77–0.92 mEq/L, depending on the model).
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ) reported an increase in FFA concentrations from 0.39 to 0.57 mmol/100 g of fat after the 262 conventional parlor farms transitioned to AMS.
      Despite milk quality concerns including increased FFA (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ), AMS are becoming more popular in the dairy industry because they can reduce labor, enhance animal welfare, and increase milk yield by up to 18% (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). An increase in milk yield is most likely due to increased milking frequencies because an AMS allows cows to be milked more than twice a day (
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). However, an increased herd average milking frequency in AMS is also the most notable explanation for higher FFA concentrations (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ) reported a previous study where AMS herds had increased FFA concentration (from 0.42 to 0.71 mmol/100 g of fat) when going from 2 to 3 milkings per day.
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ) reported that conventional Dutch farms (n = 262) that milked 3 times a day had significantly higher concentrations of FFA than those milking twice a day.
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ) reported an increase from 1.14 to 1.49 mmol/100 g of fat in milk from 2 teats (half of the udder) that went from being milked twice to 4 times a day. This FFA increase was seen as early as 5 d after the milking frequency increased (
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ).
      Both mechanical and biological theories have been proposed to explain elevated FFA with increased milking frequency (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). One is that the milk fat globule membrane is not given sufficient time to form due to decreased time between milkings (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ), which makes it more susceptible to damage and the resultant hydrolysis of TAG to yield FFA. Increased milking frequency is correlated with higher rates of milking failures, and
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ) suggested that this can be another indicator of milk at higher risk for elevated FFA concentration. An increased milking frequency is also associated with a greater proportion of large milk fat globules, which are more unstable and prone to spontaneous lipolysis (
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ).
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ) reported that the volume-weighted diameter of the milk fat globules increased from 4.21 to 4.36 μm in 11 cows transitioned from twice to 4 times a day milking.
      Individual cow milk yields at each milking are lower with increased milking frequency, which also leads to higher FFA concentrations (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). This relationship is especially significant for late-lactation, low-yielding cows (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). For the 20 cows in their study,
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ) reported an average decrease of 0.16 mmol/100 g of fat in FFA for every additional liter of milk at a single milking.
      The opposite association between milk yield and elevated FFA was observed when analyzing total milk yield. Herds with higher-yielding cows are associated with increased levels of FFA (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). The reason for this is not clear and could be confounded by milking system.
      Excess air in the milking system can contribute to elevated FFA concentration (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). When there is a greater air-to-milk ratio in the milking equipment, the added air can rupture the milk fat globule membrane, creating “bubbling” (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). Ensuring optimal milk yields at individual milkings prevents a high air-to-milk ratio and protects against elevated FFA concentrations (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ) demonstrated that reducing air inlet in the AMS milking cluster of 20 cows from 7 L/min to 1.7 and 0 L/min decreased FFA concentrations from 1.02 to 0.90 and 0.77 mEq/100 g of fat, respectively. Blocking the air inlet completely contributed to higher vacuum fluctuations, but no direct effects on FFA concentration were reported (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ).
      The international standard for total air admission in a cluster is 4 to 12 L/min, but AMS surpass this rate (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). In AMS, each quarter milker allows an air admission of 4 to 7 L/min and, because there are 4 quarters per milking unit, the accumulation of air can reach 15 to 28 L/min (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). Automatic milking systems likely have an increased air admission requirement due to their individual quarter milkers, long and narrow milk tubes with elevation changes, individual shut-off valves, foremilk separators, and separate milk meters (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ).
      In conventional milking systems, air leakage and excessive air in the cluster are common issues leading to reduced milk quality (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). When these are paired with high-line pipelines that require additional air to lift milk, there is an increased risk of air-to-milk contact that may cause mechanical stress on the milk fat globule membranes (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ;
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ) reported that of 31 conventionally milked herds in their study that received a milk inspector visit, 71% had faults in air leakage and 61% had faults related to excess air in the cluster. The conventional farms with air leaks had, on average, FFA concentrations 0.16 mEq/L higher than those of herds with faults other than air leakages (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). After inspection, these affected farms' FFA concentrations decreased from 1.82 to 1.32 mEq/L, demonstrating that targeting air admission concerns could reduce FFA concentrations (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ).
      Pumping milk through pipelines to reach the bulk tank induces physical stress on milk fat globules that can result in membrane breakage and release of FFA (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). The effect on FFA can be amplified when excessive air is present, as in the case of AMS (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). Although there have been no effects of specific pump types and increased FFA reported (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ), a longer post-run milk pump has been associated with elevated FFA concentrations (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ). This can be a concern in high-line milking systems because of the larger force required to lift and transport milk to the tank (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ) reported that of 56 study herds visited by a milk inspector, 67% of AMS and 29% of conventional systems were identified as having faults in the pumping of milk. After the milk inspection visit, FFA concentrations were reduced by an average of 0.50 mEq/L of milk, demonstrating the protective effect that proper milk pump function has on reducing FFA concentrations.
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ) conducted a study that found an increase in FFA of 1.03 mmol/100 g of fat when milk from 8 mid-lactation Holstein cows was immediately pumped at 31°C compared with 4°C. This can be explained by the coalescence of milk fat globules, which occurs at higher milk temperatures. Lower temperatures induce partial crystallization of the milk fat globule, which stabilizes the milk fat globule membrane and helps it withstand the physical stress during pumping and agitation (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ) concluded that pumping milk immediately (within 15 min) after cooling to 4°C is important to limit FFA concentrations. Compared with milk that was held for 60 min at 4°C, milk that was pumped within 15 min of cooling to 4°C had lower FFA concentrations, by 0.24 mmol/100 g of fat. This effect can be explained by the adsorption of plasma proteins and thus LPL to the milk fat globules (because the majority of LPL is bound to caseins) during cold milk storage (
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      Effective precooling mechanisms (if any) implemented on a farm include a plate cooling exchanger to efficiently lower the milk temperature (
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ).
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ), studying 277 AMS farms, showed a decrease in FFA concentration by 0.25 mmol/100 g of fat for those with a plate cooler compared with those that did not use a plate cooler. Currently in the dairy industry, farms either do not have additional cooling or have a precooling mechanism situated after the milk pump (not before). Therefore, recommendations by
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ) to install additional cooling could be taken into consideration to potentially decrease FFA concentrations.
      Maintaining bulk tank function within regulatory requirements and routine system checks are important to prevent elevated FFA concentrations (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ;
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). In AMS, the risk of milk freezing in the bulk tank is more common due to lower milk volumes entering an empty tank and the cooler being turned on too quickly; freezing of milk has been associated with elevated FFA concentrations (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ;
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ) reported that 58% of the 24 AMS herds that received a milk quality inspection visit due to FFA concerns had issues with overcooling their milk. After correcting the cooling concern(s), FFA concentration decreased by, on average, 0.52 mEq/L. Similarly, 79% of those AMS herds had issues with excessive bulk tank agitation and, after correction, FFA decreased (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). For the 31 conventional systems, resolving agitation concerns resulted in a 0.55 mEq/L decrease in FFA concentrations (
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). The protective effect in all milking systems emphasizes the importance of correct tank agitation onset and settings to prevent elevated FFA concentrations.
      Temperature fluctuations in the bulk tank can add stress to the milk fat globule membrane and risks higher FFA concentrations (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). This can occur in AMS with a buffer tank, where milk may sit for hours during milk pick-up and tank washing (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). When the milk is then transferred (normally by gravity flow) to a hot bulk tank that was recently sanitized, the large temperature difference can increase FFA concentrations (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). In conventional systems, warm milk entering and mixing with cold tank milk can also cause temperature fluctuations that increase FFA concentrations (
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ). This effect could be minimized with the implementation of a precooling mechanism (
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ).
      Minimizing and responding to any bulk tank alarms has also been associated with reduced FFA concentration (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ). Farms with regular checks to the attention list and cleaning systems were associated with lower FFA concentrations, and those with more cooling alarms were associated with higher concentrations of FFA (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ).
      The range and number of farm factors contributing to elevated FFA concentrations could explain why the increase in FFA is greatest within the first 24 h of milking (
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ). It is evident that elevations in FFA concentrations arise from a host of farm factors, including milk production (feeding), milking system, milk transport, and milk storage. Based on this review, we conclude that the major factors associated with increased levels of FFA are non-parlor milking systems, increased air admission, the absence of additional cooling, temperature fluctuations in the bulk tank, and rations high in SFA (
      • de Koning K.
      • Slaghuis B.
      • van der Vorst Y.
      Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
      ;
      • Wiking L.
      • Bertram H.C.
      • Björck L.
      • Nielsen J.H.
      Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
      ,
      • Wiking L.
      • Nielsen J.H.
      • Båvius A.K.
      • Edvardsson A.
      • Svennersten-Sjaunja K.
      Impact of milking frequencies on the level of free fatty acids in milk, fat globule size, and fatty acid composition.
      ,
      • Wiking L.
      • Bjerring M.
      • Lokke M.M.
      • Lovendahl P.
      • Kristensen T.
      Herd factors influencing free fatty acid concentrations in bulk tank milk.
      ;
      • Rasmussen M.D.
      • Wiking L.
      • Bjerring M.
      • Larsen H.C.
      Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
      ). These results can serve as a foundation for future research aimed at maintaining low levels of FFA to ensure milk quality and consumer satisfaction with dairy products.

      Notes

      Funding was provided by the Dairy Farmers of Ontario (DFO), Canada.
      No animals were used in this review, and ethical approval for the use of animals was thus deemed unnecessary.
      The authors have not stated any conflicts of interest.

      References

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        Robotic milking and milk quality: effects on bacterial counts, somatic cell counts, freezing point and free fatty acids.
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        Influence of air intake on the concentration of free fatty acids and vacuum fluctuations during automatic milking.
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        Evaluation of cooling strategies for pumping of milk - Impact of fatty acid composition on free fatty acid levels.
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