Impact of storage at freezing temperatures on casein micelle size, coagulation properties and curd yield in bovine milk

Sammanfattning: Frozen storage is an efficient and convenient way to preserve and transport milk, but the effect from frozen storage needs to be further researched. The objective of this study was to investigate how storage at freezing temperatures (-80°C and -20°C) of frozen and freeze-dried bovine milk samples, respectively, affects pH, casein micelle size, ethanol stability, rheological properties and curd yield. The short-term changes after cold storage at 4°C were also examined. The results were evaluated using one-way ANOVA and Tukey’s pairwise comparison test. A significant decrease in pH was observed during the short-term storage at 4°C. A significant difference was shown in ethanol stability after storage of the freeze-dried milk samples in week 4 while comparing the freezing temperatures. The pH measurements showed a significant difference between frozen and freeze-dried samples stored at -20°C and in week 1 and 2. The result from the coagulation time measurements showed significant difference in all treatments, except during the first week. The gel firmness was significantly different when comparing frozen milk samples stored at -80°C and -20°C, respectively for 2, 3, 5 and 6 weeks. The study concluded that milk is affected by frozen storage, resulting in softer cheese curd, prolonged coagulation time and decreased gel firmness. The study indicated that the optimal freezing temperature and time for frozen bovine milk samples are one week at -80°C. Further research with higher number of replicates and longer storage time could result in stronger conclusions about the effects of frozen storage of milk. Inclusion of caprine and ovine milk samples would make the results more useful for small-scale cheese manufacturers.