Raw milk quality
4.1 The Principal Milk Components
See also Dairy Chemistry and Physics in the Dairy Science and Technology Education website.
Cheese can be made from the milk of many mammals including goats, sheep, buffalo, reindeer, camel, llama, zebra and yak. The milk of ruminants is the best milk for cheese making because it contains high levels of the milk protein casein which is required to provide an adequate coagulum. Our consideration of milk composition will include only a summary of the proximate analyses of the most common dairy species and a few, relevant with respect to cheese making, comments about each component.
Gross composition of food (also referred to as proximate analysis) means distribution of the total amounts of fats, proteins, carbohydrates, ash (mainly minerals such as calcium) and moisture or total solids. Typical proximate analysis profiles for cows', sheeps' and goats' milk are listed in Table 4.1. Further discussion refers only to cows' milk unless otherwise stated.
Fat content ranges from 2.0 to 7.0 kg/hl. An approximate average for regions where Holstein Fresian cattle predominate is about 3.9 kg/hl. With respect to cheese manufacture and quality the following properties are important.
- Most diverse of all natural fats. We are routinely quantifying over 100 fatty acids ranging from four carbons to 22 carbons in length. Many more have been identified.
- Traditional, but not entirely justified, nutritional concerns are: (1) About 70% of milk fatty acids are saturated; and (2) It contains cholesterol.
- Positive nutritional factors are butyric acid (anticarcinogenic), saturated but mid to short length fatty acids (antihypertensive), and rumenic acid (anticarcinogenic).
- Unique flavour of dairy fat is due to short chain fatty acids, especially butyric acid
- The two major spoilage reactions in milk fat are (1) break down of the triglyceride fat structure releasing fatty acids such as butyric to create a rancid flavor; and (2) Oxidation of unsaturated fats creating an oxidized flavour (flat, cardboard flavour)
- The melting properties of butter fat are significant to cheese texture.
Total milk protein ranges from about 2.5 to 5.5 kg/hl. The average for regions in which Holstein Fresians predominate is about 3.3 kg/hl. There are two major groups of proteins, the caseins (about 2.6 kg/hl) which I refer to as the 'cheese proteins' and the whey proteins (about 0.7 kg/hl) which as the name suggests are usually lost in the whey during cheese making. Caseins are not water soluble and so the cow packages them in water dispersable particles called micelles which along with caseins include most of the milk calcium, magnesium, phosphate and citrate (more about casein micelles in Coagulation. Unlike whey proteins which are very sensitive to heat, caseins are little affected by heating except that they react with heat denatured whey proteins. Table 4.2 lists the principal caseins and some properties which are most relevant to cheese making. Similarly, Table 4.3 lists some properties of the principal whey proteins.