I. Composition of milk

Average values:
Solids: 12.75%
of which:
Milk fat 3.95 %
Protein 3.30 %
Milk sugar 4.85 %
Ash content 0.65 %
Water: 87.25 %

Milk fat is mostly dispersed in milk serum in the form of an emulsion of fat globules. Around 99 % is made up of triglycerides, which are esters of glycerol and fatty acids. When homogenized the milk globules are reduced in size and do not rise to the surface.

Milk protein comprises two main groups: casein (approx. 80 %) and whey protein (approx. 20 %). The casein content has a major effect on production yield, particularly of cheeses.

Milk sugar is made up of glucose and galactose. It is characterized by its low sweetness level. It is broken down by microorganisms into lactic acid and other products which are important for souring processes. Milk contains minerals such as calcium, potassium, chlorine and phosphorus, amongst others.

Milk pasteurization is a heat treatment which prevents the action of pathogenic microorganisms which can be found in milk. During pasteurization the temperature and heating time are combined. We thus distinguish between long, economical and high pasteurization. High pasteurization is used most frequently (85 degrees Celsius for several seconds), for example with consumer goods, and economical pasteurization (72 degrees Celsius for 15 seconds) is used particularly in the production of cheeses. Products made of pasteurized milk must be kept in a cold chain.

II. Pasteurization and dairy cultures

The pasteurization of raw milk usually involves skimming it and modifying the fat content.

When spun in a centrifuge at high drum revolution speeds, where there is a series of plates, the milk is separated into cream and skimmed milk. The skimming process is based on the separation of substances in accordance with their differing specific weights – the lighter, fat phase (cream) is expelled down the middle while the skimmed milk is drawn off from the perimeter of the drum.

Pasteurization removes almost all microorganisms from milk. The effectiveness of pasteurization – although dependent on many factors – isusually shown to be 99.9 %. The number of germs destroyed in the pasteurization process does not depend on the initial number of microorganisms, but on the typeof these microorganisms. This means that judging pasteurized milk on the total number of microorganisms it contains immediately after pasteurization has nogreat significance. Unfortunately pasteurization, as well as getting rid of undesirable organisms, also destroys the useful ones which are needed for biochemical processes which most dairy produce is based on.

Pure dairy cultures are essential, particularlyin the manufacture of sour products and cheeses. These can be divided up intobacteria, yeasts and mould.

In addition to the tradition forms of milk bacteria cultures, which are used by way of parent and operative starters, new forms for direct injection into a tank, machine, or fermentation unit are also used in production. These are highly concentrated and standardized cultures which do not require preactivation before use. These cultures are either deep-frozen or freeze-dried.

There are many manufacturers involved in the production of pure cultures; in the Czech Republic there is MILCOM a. s., Laktoflora Praha plant.

Pure dairy cultures are microorganisms whose enzymes act to give each product its own flavour, consistency and aroma. For the biochemical process to work properly, the optimum growth conditions for the microorganisms in question must be main tained.

III. Pure dairy cultures

Pure dairy cultures are responsible for the biochemical processes involved in the manufacture of dairy products. They are called pure because they are purely cultivated individual strains containing no “wild” microorganisms, which are to be found, for example,in self-acidified milk. The biochemical processes which take place in milk or ina product include particularly souring, the breakdown of proteins and the break down of fat, amongst others.

The most well-known culture is butter starter, which is used to make butter from sour cream, culture buttermilk, starters, as well as the basic culture for the production of cheeses. It is a mesophilic hybrid culture and, depending on thechoice of strain, can cause heterofermentative souring and leads to the creationof aromatic substances - diacetyl, but also to the creation of carbon dioxide. This can cause lids or cartons to swell, which many consumers see as a defect. Or it is possible to choose a culture with homofermentative souring – i.e. without CO2, but also with those aromatic substances, which detracts from thecharacter of the product. Hybrid cultures are mostly used. Recently there hasbeen a rise in popularity of what are known as bifidobacteria, which have amilder souring effect and therefore a milder flavour, both with sour products aswell as with yoghurts.

The right choice of culturesis important in the production of cheeses. In addition to the basic cultures(starters), special cheesemaking cultures are used, depending on the type of cheese being produced; for example, with mould cheeses with mould inside (Niva) it is Penicillium roqueforti, with mould on the surface (Camembert, Kamadet) it is Penicillium candidum and camemberti. With cheeses with a waxy paste on the surface (Romadur), the yeast is applied first, to break down the acidity to an almost neutral reaction, and then a mass culture is applied, generally Brevibacterium linens.

If we are growing a culture ourselves, we must know its growth curve to allow us to achieve optimum productivity. If we intend to use the culture for direct dosage, we need to acquire information about how it works and its properties – souring curve, creation of maturing products, maturing conditions, phagoresistence, etc.

Even when ever more refined and effective cultures are used there remains one variable which is difficult to influence, this being milk, the composition of which changes (effect of the time of year, fodder, breed, etc.). It goes without saying that it must not contain any inhibitory substances and that supplies of raw milk should not be kept for any length of time.