Milk as a growth medium
Cheese making depends on the growth of bacteria to produce acidity, flavour compounds, and ripening enzymes. It is, therefore, important to understand the characteristics of milk as a growth medium.
Milk is a good source of all principal nutrients, including carbon, nitrogen and macro-minerals. Many micronutrients such as vitamins and micro-minerals are also available. However, milk is unique with respect to its sugar.
Carbohydrates, especially simple sugars such as sucrose (table sugar), can be utilized as sources of energy more quickly than fats and proteins. However, the energy currency of the cell is glucose (also called dextrose) so to use available carbohydrates, microorganisms must be able to convert them to glucose.
The only sugar naturally present is milk is lactose. Most microorganisms lack the enzyme lactase which is required to break lactose into its two component sugars, namely, glucose and galactose. Lactic acid bacteria which do have lactase readily break down lactose and use glucose as an energy source. Lactic acid bacteria, therefore, have a competitive advantage in milk; that is, they are able to out grow other bacteria which are unable to obtain glucose from lactose. Further, some lactic acid bacteria are able to convert galactose to glucose.
Acidity as measured by pH is one of the most critical parameters with respect to both food safety and both process and quality control of fermented foods such as cheese. The concepts of acidity and pH are explained in Sections 3.5. The titratable acidity of milk typically varies from 0.12 to 0.19% lactic acid depending on composition, especially protein content. The pH of milk is near the physiological pH of 6.8 which, considering the following points, means that milk is a good growth medium with respect to acidity (pH).
- Most organisms grow best at pH near physiological pH of 6.8. As explained in Section 3.5, titratable acidity (TA) is not a good predictor of acid effects on microbial growth and chemical properties such as protein functionality.
- The major groups of microorganisms important to food preservation are in order of increasing acid tolerance: bacteria, yeasts and moulds.
- Natural fermentation of warm raw milk by lactic acid bacteria reduces milk pH to less than 4.0 which prevents the growth of pathogenic bacteria and most spoilage bacteria
Milk has a high moisture content (typically 87% for cows' milk) and with respect to available moisture, is an excellent growth medium. But, it must be understood that with respect to microbial growth, the critical parameter is water activity not moisture content. Water activity (aw) is an index of the availability of water for microbial growth. It is the availability of water in the food reported as a fraction of the availability of water from pure water. In other words, the aw of water is 1 and the aw of other substances is reported as decimal fractions of 1. Water activity is reduced by dissolved substances, varying directly with number of dissolved molecules rather than the weight. For this reason, relative to large molecules such as proteins, small molecules such as sugar and salt have a large effect on water activity. For example, jams are preserved by their high sugar content.
Microorganisms vary greatly in their ability to survive and/or grow at reduced water activity. However, acknowledging that exceptions exist, the minimum water activity for the principal groups of microorganisms are as follows:
- Most bacteria: 0.90 - 0.91
- Most yeast: 0.87 - 0.94
- Most moulds: 0.70 - 0.80
Compare these values with typical aw values for milk, cheese and a few other foods.
- Milk, fresh fruits and vegetables, fresh meats 60 - 98% moisture, aw 0.97 - 1.00
- Most baked products, some cheese, some cured meats, 20-60% moisture, aw 0.88 - 0.96.
- Dehydrated foods such as breakfast cereals. Less than 5% moisture, aw 0.20 - 0.30
Typical aw values for some cheese at the marketing stage are given below (Eck and Gillis, 2000). See also typical aw values for cheese families in Table 1.1.
Availability of oxygen
- Cottage 0.988
- Brie 0.980
- Munster 0.977
- Saint-Paulin 0.968
- Edam 0.960
- Cheddar 0.950
- Parmesan 0.917
Availability of oxygen
With respect to oxygen requirements, microorganisms may be:
- Aerobic: must have oxygen to grow
- Anaerobic: can only grow in the absence of oxygen
- Microaerophilic: require small amounts of oxygen
- Able to grow with or with out oxygen.
Moulds require oxygen, so they can be eliminated by vacuum or gas flush packaging. Most yeast are aerobic (require oxygen) but some can grow anaerobically (in the absence of oxygen). Bacteria may fall into any of these categories, but lactic acid bacteria are micoaerophilic or anaerobic.
Milk will acquire some dissolved oxygen during milking, storage and handling, but it is used up quickly during bacterial growth.