Effect of subclinical intrammamay infection on milk quality in dairy sheep: I. Fresh-soft cheese produced from milk of uninfected and infected glands and from their blends

Abstract

Subclinical intramammary infection (IMI) is associated with a decrease in milk yield and changes in milk composition. The effects of IMI on fresh-soft cheese yield and quality were evaluated by: (i) the effect on the composition and coagulation properties; (ii) the effect of three levels of coagulating enzyme and three temperatures in the presence and absence of added CaCl2 on milk coagulation properties (milk from uninfected and infected halves and a blend of 50:50); (iii) the effect the milk source: uninfected, infected halves and a blend of 50:50 and 75:25 (uninfected:infected) on the production of small cheese blocks. Somatic cell count was significantly higher in infected glands of both Manchega and Lacaune dairy sheep breeds. In milk from infected glands, 25–30% of the milk did not coagulate. Rennet clotting time (RCT) of milk from infected glands was doubled and curd firmness (CF) was much lower in comparison to samples taken from the contra-lateral uninfected glands. Addition of Ca and temperature levels did not influence RCT in any of the milk combinations, while CF was significantly higher in milk from uninfected glands and 50:50 blends and also was higher as temperature increased. Enzyme concentration significantly influenced RCT and CF in milk from both uninfected and in the 50:50 blends but not in the milk from infected glands. IMI affected significantly milk syneresis, which was slower in the infected and in the 50:50 blends. The changes in syneresis were reflected in higher moisture in the curd of milk sampled from infected glands. The contents of fat, protein and dry matter in the cheese were significantly lower in the 50:50 blends compared to uninfected milk, whereas the contents of these organic components in the 75:25 blends were in-between. The distance required to penetrate the cheese before crushing was significantly deeper in cheese made from a 50:50 blend than that of cheese made from uninfected milk and 75:25 blend, which indicated that IMI modified cheese structure into softer and more elastic texture. The study also indicated that during storage of the milk before processing, milk quality (lower pH, increased RCT and decreased CF) from 50:50 blends deteriorated faster in comparison to uninfected milk.