THERMIZATION-
When collection of milk on alternate days was introduced, cheese producers who had to use such milk noticed that the quality of the cheese frequently deteriorated. This tendency was particularly noticeable when the milk had to be stored a further day after reception, even when it was chilled to 4 °C in conjunction with transfer from road tanker to storage tank. Even longer storage times may be expected when working weeks are limited to six or even five days.
During cold storage, the milk protein and milk salts change character, which tends to impair cheesemaking properties. It has been shown that about 25 % of the calcium precipitates as phosphate after 24 hours storage at +5 °C. This reduction, however, is temporary. When the milk is pasteurized, the calcium redissolves and the coagulating properties of the milk are almost completely restored. b-casein also leaves the complex casein micelle system during cold storage, which further contributes to reducing the cheesemaking properties. However, this reduction too is almost completely restored by pasteurization.
Another equally important phenomenon is that the microflora introduced into the milk by recontamination – especially Pseudomonas spp – will adapt to the low temperature at which their enzymes, proteinases and lipases, will decompose protein and fat respectively. The result is a “bitter" flavour emanating from decomposition of the β-casein that has left the casein micelle during low-temperature storage.
The proteolytic and lipolytic enzymes formed by Pseudomonas may also co-operate to penetrate the membranes of the fat globules. This symbiotic co-operation leads to liberation of fatty acids, especially the lower ones, by lipase action, giving the milk a rancid flavour.
So, if milk that is already at least 24 – 48 hours old cannot be processed within about 12 hours after arrival at the dairy, it is advisable to chill it to about +4 °C or, preferably, thermize it.
Thermization means moderate heat treatment, 65 °C for 15 seconds, followed by cooling to +4 °C, after which the milk is still phosphatase positive. This technique was basically introduced for the purpose of arresting growth of psychrotrophic flora when milk was stored for a further 12 – 48 hours after arrival at the dairy. As mentioned in Chapter 1, the “critical age” of raw milk kept at +4 °C normally falls between 48 and 72 hours after milking. Figure 14.2 shows the arrangement of a milk reception station.
PASTEURIZATION-
Before the actual cheesemaking begins, the milk usually undergoes pre-treatment designed to create optimum conditions for production.
Milk intended for cheese that requires more than one month of ripening needs not necessarily be pasteurized, but usually is. National legislation often stipulates if the milk has to be pasteurized or not.
From Table 14.1, you can see that milk intended for unripened cheese (fresh cheese) must be pasteurized. This implies that cheese milk for types needing a ripening period of at least one month need not be pasteurized.
On the other hand, whey used for fodder must be pasteurized, to prevent it from spreading bovine diseases. However, if the cheese milk is pasteurized, it is not necessary to pasteurize the whey separately.
Milk intended for original Emmenthal, Parmesan and Grana, some extra hard types of cheese, must not be heated to more than 40 °C, to avoid affecting flavour, aroma and whey expulsion. Milk intended for these types of cheese normally comes from selected dairy farms with frequent veterinary inspection of the herds.
Although cheese made from unpasteurized milk is considered to have a better flavour and aroma, most producers (except makers of the extra hard types) pasteurize the milk, because its quality is seldom so dependable that they are willing to take the risk of not pasteurizing it. Pasteurization equalizes the bacterial composition of the milk from one day to the next, eliminating disturbances in an automatic or time-controlled process.
Pasteurization must be sufficient to kill bacteria capable of affecting the quality of the cheese, e.g. Coliforms, which can cause early “blowing” and a disagreeable taste. It must also kill most of the natural pathogenic bacteria.
Regular HTST pasteurization at 72 – 73 °C for 15 – 20 seconds is therefore most commonly applied. (Phosphatase negative).
However, spore-forming microorganisms in the spore state survive pasteurization and can cause serious problems during the ripening process. One example is Clostridium tyrobutyricum, which forms butyric acid and large volumes of hydrogen gas by fermenting lactic acid. The butyric acid has an unsavoury taste, and the gas destroys the texture of the cheese completely.
More intense heat treatment would reduce this particular risk, but would also seriously impair the general cheesemaking properties of the milk, as it increases the level of denatured whey proteins. This is unacceptable in terms of both quality and legal requirements. Other means of reducing thermo-tolerant bacteria are therefore used.
Traditionally, certain chemicals have been added to cheese milk prior to production to prevent “blowing” and development of the unpleasant flavour caused by heat-resistant, spore-forming bacteria (principally Clostridium tyrobutyricum). The most commonly used chemical is sodium nitrate (NaNO3), but in the production of Emmenthal cheese, hydrogen peroxide (H2O2) is also used. However, as the use of chemicals has been widely criticized, mechanical means of reducing the number of unwanted microorganisms have been adopted, particularly in countries where the use of chemical inhibitors is banned. These inhibitors can also affect some of the added bacteria in the starter culture.
MECHANICAL REDUCTION OF BACTERIA-
SPORE AND BACTERIA REMOVING SEPARATORS BACTOFUGE
As discussed in Chapter 6.2, specially designed hermetic separators for spore and bacteria removal is used to separate bacteria, and especially the spores formed by specific bacteria strains, from milk.
The use of bacteria and spore removing separators has proved to be an efficient way of reducing the number of spores in milk, since their density is higher than that of milk. These separators normally separates the milk into a fraction that is more or less free from bacteria, and a concentrate , which contains both spores and bacteria in general and amounts to up to 3 % of the feed to the separator .
In applications where quality milk for cheese and powder production is the objective, the spore and bacteria removing separator is installed in series with the milk separator, either downstream or upstream of it.
The same temperature is often chosen for spore and bacteria removal as for separation, typically 55 – 60 °C.
There are two types of spore and bacteria removing separators:
- Two-phase type
- One-phase type
The two-phase type has two outlets at the top:
One for continuous discharge of the heavy phase via aspecial top disc, and
One for the spore and bacteria -reduced phase
The one-phase type has one outlet at the top of the bowl, for bacteria-reduced milk. The concentrate is collected in the sludge space of the bowl and discharged at pre-set intervals through ports in the bowl body.
These two types make it possible to choose various combinations of equipment to optimize the bacteriological status of milk used for both cheesemaking and other purposes.
It should be mentioned at this point that whey, if intended for production of whey protein concentrate as an ingredient in infant formulae, should be processed after recovery of fines and fat.
Process alternatives-
There are about ten possible ways to configure a line with spore and bacteria removing separators ; three examples are given here:
Two-phase spore and bacteria removing separator with continuous discharge of concentrate
This concept, shown in Figure 14.3, works under airtight conditions and produces a continuous flow of air-free bacteria concentrate as the heavy phase. This phase, comprising up to 3 % of the feed flow (adjusted by an external pump with variable speed control) is often sterilized and remixed with the main flow.
The sterilizer can be of different types; plate heat exchanger, tubular or infusion heater. Typical heat treatment is 120 °C for one minute, which is sufficient to inactivate spores from Clostridia microorganisms. After cooling, the concentrate can be remixed with the clarified milk before it is pasteurized at 72 °C for 15 seconds followed by regenerative cooling to the renneting temperature.
The spore and bacteria removing separator with continuous discharge of concentrate is used in applications where:
Remixing of sterilized concentrate is possible
There is an alternative use for the concentrate in a product where the heat treatment is strong enough to inactivate the microorganisms
At nominal capacity a spore and bacteria removing separator reduce approximately 98 % of spores from Cl. tyrobutyricum and 95 % of the aerobic forming spores.
