Factors affecting gross milk composition
Species (see also Table 1)
Cheese making principles are similar for milk of all species with some modifications required to account for high solids of some species such as buffalo and sheep. Cows' milk and goat's milk have similar cheese making properties except that:
- Goats' milk cheese tends to ripen by lipolysis (fat breakdown) more than cows' milk cheese.
- Goats' milk has smaller fat globules which allows higher fat recovery and possibly a smoother texture..
- Cheese making quality of goats' milk is also improved due to higher levels of alpha-S1 casein in goats' milk which permit better coagulation.
Through out the modern history of dairying, farmers have selectively bred dairy cattle to increase production or fat content or both. Recently, genetic selection has focussed on other milk properties such as increasing the proportion of milk protein to fat. Three genetic effects are most relevant to cheese making.
(1) Relative proportions of fat and protein (P/F ratio)
Fat content and protein content generally increase or decrease in parallel, but fat varies more with feed and season then protein. The same is true for breed (genetic) effects, such that genetic selection has produced the the following practical effects in modern dairying.
- Higher fat breeds have lower protein/fat ratio. For example, a typical protein/fat ratio in Jersey milk is 0.7 relative to 0.84 in Holstein milk (Table 4.4).
- Genetic selection over the last 100 years has produced considerable within breed improvement with respect to both milk production and increased protein and fat content. The result is greatly increased per cow production of milk protein and fat. However, once again, because fat responds more to genetic selection than protein, the result in many areas has been a gradual decrease in the average P/F ratio. For example in Ontario, the P/F ratio decreased from about 0.88 in 1970 to 0.85 in 1995 (estimated from data provided by Laboratory Services Division, University of Guelph.
(2) Relative proportions of fat and protein to other solids
With respect to other solids, mineral content (mainly Ca, Mg, and P) generally varies in proportion to protein content and lactose content is relatively stable. Because lactose is largely a wasted component, increasing protein and fat by feed or genetic selection has economic advantages in terms of feed conversion, milk transportation costs, and waste handling.
(3) Stage of lactation
Fat content tends to increase during lactation as milk production decreases. The result is that the relative proportion of protein to fat (protein/fat ratio or P/F) is highest at the peak of lactation (about 60 days of lactation) and lowest at the end of lactation. Protein distribution also changes during lactation with resulting effects on cheese ripening and flavour. In particular, the proportion of alpha-caseins decrease during lactation while the proportion of beta-casein increases.
Depending on the relative demand for butter fat versus milk non-fat solids, there may be incentive to change the relative proportions of milk protein and fat. The only short term means to do this is by changing the diet. Generally less roughage and more high energy feeds will encourage lower fat content with little decrease in protein content to provide a higher P/F ratio.
Seasonal variation in milk composition is most important to cheese yield efficiency and composition control. Some important seasonal effects are listed below and illustrated in Figure 4.1. These observations are based on Ontario data.
- Fat content reaches a minimum in August and a maximum in October.
- Protein content changes roughly in parallel with fat content, but the seasonal variations are smaller, causing high protein fat ratios (P/F) during the summer and low P/F ratios in the winter.
- Casein content also varies with season which is most important because cheese making is dependent on casein NOT on total protein (more on this in Treatment of milk for cheese making, Standardization of milk for cheese making and Yield efficiency).