CHEESE MILK PASTEURIZATION (heat treatment):

1 BACTERIOLOGICAL CONDITION:

Results from comparing cheese manufactured from raw versus pasteurized milk will often show that cheese manufactured from raw milk has a better taste and more aroma then doe’s cheese made from pasteurized milk. Sometimes this statement though, depends in part on variety, handling, etc. 

Cheese made from pasteurized milk will be more "flat" in taste and often is more acid and more bitter.  This will occur providing the milk is of good bacteriological quality, whereas use of a bacteriolically poor cheese milk as a rule will have the result of a cheese manufactured from pasteurized milk, though its deficiency, will show to give the best results.

With the exception of spore formers (for example butyric acid bacteria) most micro organisms which are harmful to cheese such as coliform bacteria, yeast and mould can be destroyed by pasteurization. Pasteurization of cheese milk will therefore prevent the appearance of a number of fermentation defects in cheese. 

It is also important to destroy pathogenic bacteria in the milk. 

U.S., German and Swiss research has proven that we can find live tuberculosis bacteria in nine month old cheese manufactured from raw milk. For that reason alone, pasteurization of cheese milk is desirable. 

While the undesirable bacteria are being destroyed, the lactic acid bacteria unfortunately are also being destroyed, as these bacteria belong to the most heat sensitive micro organisms of milk.  One of the conditions for making cheese is that a favorable fermentation takes place.  When working with pasteurized milk, it is necessary to replace the destroyed lactic acid bacteria by addition of a starter culture. 

Lactic acid bacteria are important in cheese making first and foremost in their ability to form lactic acid, and secondly in assisting rennet in reducing the cheese moisture content, and third in aiding the preservation of cheese.  The indigenous lactic acid bacteria will normally be more versatile and better acclimatized in the milk than the added lactic acid bacteria.  This is one of the reasons for the difference in quality, of cheese produced from a good raw versus pasteurized milk.  It is very important to make it clear that this is an all important difference in working with raw and pasteurized milk as often it seems unexplainable how we can get so many fermentation defects in cheese of pasteurized milk when it is possible to manufacture cheese of raw milk containing a certain number of harmful bacteria. 

It is therefore very important that work must be done with a higher degree of cleanliness than that which is necessary for cheese made from raw milk.  A great part of the fermentation defects which appear in our cheese today, will disappear if these conditions are looked after.  From the moment the cheese milk leaves the holding section of the HTST (the place where it has the highest temperature) careful consideration should be given, that no contamination of milk takes place.  This means that the regeneration section in the HTST, all pipes and possible side connections and all valves must be sterile before cheese milk passes through.  It is important that all valves, especially two-way valves, don't leak, so that no pasteurized milk flows reverse. If necessary the side connections should be disconnected.  All fluctuations in pasteurization temperature must be avoided.  If the temperature is below the minimum, the flow of milk into the cheese vat will contain some of the harmful bacteria, while lactic acid bacteria will possibly be destroyed.  This will, of course, have extremely unfavorable results. 

The cheese vat and everything which comes in contact with the milk and whey under cheese making should be sanitized carefully. The cream or skim milk, which are added to standardize fat content, must be sufficiently pasteurized, as well, as all other additives (except coagulant and starter culture of course).

Despite adequate pasteurization and sanitation measures, harmful bacteria in the milk have been found.  During heat treatment in the HTST a precipitation may form insulations that may allow survival of producing bacteria.  This rarely happens.  In most cases, after preheating in the regeneration section milk passes through a separator, where soil particles as well as precipitated milk parts are removed. 

2 PASTEURIZING EFFECT ON CHEESE CURD:

Heating of milk may change its chemical and physical properties.  All matter in milk including salts and enzymes etc. will be affected and this plays an important role in the cheese process. 

Pasteurizing will generally prolong coagulation time, giving a softer curd and a curd which will shrink and drain the whey at a different speed.  The degree to these changes is determined by pasteurizing temperature.  Normal pasteurization (71°C in 16 seconds) produces relatively small change, but measurable (time for coagulation and amount of whey drained). 

The actual reason for these changes is determined by the colloidal solution.  The heating causes albumin and globulin to become denaturized. 

Pasteurization means that we get a change in the salt balance in the milk. 

The heating causes calcium to precipitate as calcium phosphate (CaADVANCE \U 0.03ADVANCE
\U 0.0(P0ADVANCE
\U 0.04ADVANCE
\U 0.0)ADVANCE
\U 0.02ADVANCE
\U 0.0).  Citrate is also very sensitive to heating, but not significant.  This means that particle size decreases.  This change of particle size in rennet curd has been determined by electron microscopy. 

It is known that an addition of CaClADVANCE
\U 0.02ADVANCE
\U 0.0 to milk after pasteurization can help curd properties.  The amount should be limited, to 5 - 15 g.  CaClADVANCE
\U 0.02ADVANCE
\U 0.0/100 liter milk, as larger amounts may cause bitterness.  It goes without saying that the higher the temperature we use, the more difficult it is, to get a produce normal coagulation. 

Prolonged coagulation time, which is caused by precipitation of CaADVANCE \U 0.0++ADVANCE
\U 0.0, can to some extent be regained with suitable storage of the milk before cheese making.  Trials have shown that storing milk at 16 - 20°C gives gradual improvements in 5 hours.  During this time the precipitated CaADVANCE
\U 0.0++ADVANCE
\U 0.0 dissolves unites with casein molecules to form larger casein particles. 

Trials done at the Danish Government Dairy Research Plant (128. Report) show that fastest whey drainage occurs from the curd at pasteurization temperature around 70 C.  Whey drainage at higher or lower temperature is restricted.  Similarly pasteurizing influences cheese pH.  As pH gets lower when pasteurizing temperature is moved away from 70 C both up and down.  By pasteurizing at that temperature the least amount of calcium goes into solution, so that the dissolved amount of calcium both with more or less pasteurization is a little bit more. 

The reason why the curd moisture binding ability is poorest at 70 C in 16 seconds and improves by stronger or weaker pasteurization is still obscure. That the cheese pH falls faster and that the cheese becomes more acidic then that which is obtained by 70 C, is on the other hand a natural course of the slower whey drainage, as the whey is more acidic, when it leaves the cheese curds, so that it removes more phosphate, thereby reducing the cheese buffer ability.

It is generally known that to get a sufficient bacteriological effect (coli free) it is necessary to pasteurize to 70 - 72°C in 16 seconds, therefore an increase in the temperature will mean that the curds water binding ability will increase and the cheese gets more acidic, as at the same time a greater bacteriological effect is obtained.  With very strong pasteurizing there will be a considerable albumin precipitation as a result, the cheese yield will increase, but in most cases will the regards for cheese quality beforehand hinder an initialization of that condition. 

The need for pasteurizing changes with the season and with the cleanliness, by which the milk is produced and it is therefore not possible to give exact temperature(s) for pasteurizing, which will be suitable in most cases.  Normally temperatures between 70 - 74 C and a holding time of 15 - 20 seconds will be sufficient (16 seconds minimum in Canada), but the final determination must in each case happen with consideration to milk, equipment cheese type.  The effect on whey drainage etc. is traced clearly by manufacturing of semi hard and especially with soft cheese, where it nearly exclusively is the cheese milks own whey drainage ability, which determines whey drainage and thereby cheese course.  In the soft cheese only a gentle and in some cases no pasteurization (for example, Mycella).  For Emmentaler raw milk is desirable, as with the use of pasteurized milk it is extremely difficult to get this cheese characteristic taste (U.S. sources claim the opposite to be true).