Variation in raw milk quality : impact on milk coagulation and cheese ripening

Abstract: This thesis examined some of the causes behind recent increasing variation in ripening time of a traditional, Swedish long-ripening hard cheese. Variations in the composition and properties of farm and dairy silo milk intended for production of the long-ripening cheese were recorded during one year, and the resulting cheese was characterised. Two major types of dairy farming systems associated with differences in milk quality attributes of importance in cheese making were observed. Farms characterised by loose-housing, milking parlour or automatic milking system (AMS), and Swedish Holstein as the dominant breed, were found to be larger than tiestall farms with Swedish Red and other dairy breeds. Levels of free fatty acids, plasminogen-derived activity and gel strength were higher in tank milk from tiestall compared with AMS farms. Some milk quality attributes were influenced by sampling month, e.g. casein micelle size was smaller and total proteolysis higher in milk during outdoor compared with indoor months. The effect of variation in casein micelle size and in calcium and citrate content on milk coagulation was investigated in an experimentally designed study. The results showed that elevated levels of calcium and citrate in the milk altered casein micelle size, while modifying the coagulation properties of the milk. Larger micelles with moderate citrate level led to a firmer gel than smaller micelles with higher citrate level. The process of cheese ripening was visualised using rapid and non-destructive NIR-hyperspectral imaging, through which cheese maturity could be predicted with 76% accuracy. The predictive model can function as a supplementary quality assurance tool for e.g. efficient planning and optimisation of the logistics in cheese ripening facilities. Overall, variation in the quality attributes of dairy silo milk had little impact on cheese ripening time, contradicting expectations. However, sensory and texture scores of the resulting cheese were influenced by plasmin and plasminogen activity. These findings suggest that associations between raw milk characteristics and cheese ripening are weak if the milk is of high quality.

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