Composition and properties of bovine milk: A study from dairy farms in northern Sweden; Part Ⅱ. Effect of monthly variation

摘要

This study investigated the influence of monthlyvariation on the composition and properties of rawfarm milk collected as part of a full-scale cheese-makingtrial in a region in northern Sweden. In our companionpaper, the contribution of on-farm factors to the variationin milk quality attributes is described. In total,42 dairy farms were recruited for the study, and farmmilk samples were collected monthly over 1 yr andcharacterized for quality attributes of importance forcheese making. Principal component analysis suggestedthat milk samples collected during the outdoor period(June–September) were different from milk samples collectedduring the indoor period. Despite the interactionwith the milking system, the results showed that fatand protein concentrations were lower in milk collectedduring May through August, and lactose concentrationwas higher in milk collected during April throughJuly than for the other months. Concentrations of freefatty acids were generally low, with the highest value(0.86 mmol/100 g of fat) observed in February andthe lowest (0.70 mmol/100 g of fat) observed in June.Plasmin and plasminogen-derived activities varied withsampling month without a clear seasonal pattern. ThepH of farm tank milk ranged from 6.60 to 6.82, withthe lowest and highest values in September and February,respectively. The highest somatic cell count wasobserved in August (201 × 10~3 cells/mL) and the lowestin April (143 × 10~3 cells/mL). The highest value of gelstrength, was recorded in December (88 Pa) and thelowest in July (64 Pa). Rennet coagulation time andgel strength were inversely correlated, with the lowestrennet coagulation time value observed in December.Orthogonal projections to latent structures (OPLS)and discriminant analysis adaptation of OPLS identifiedcasein micelle size and total proteolysis as the milkquality attributes with major responses to samplingmonth, with smaller casein micelle size and higher totalproteolysis associated with the outdoor months. Usingdiscriminant analysis adaptation of OPLS to furtherinvestigate causes behind the variation in milk traitsrevealed that there were factors in addition to feedingon pasture that differed between outdoor and indoormonths. Because fresh grass was seldom the primaryfeed in the region during the outdoor period, grazingwas not considered the sole reason for the observeddifference between outdoor and indoor periods in rawmilk quality attributes.