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Dairy industry - Milk storage

Dairy industry - Milk storage

Milk storage

Before being sent to its destination (direct consumption and often also the one used for the preparation of the cheese), the milk must undergo a remediation (or remediation) which is normally carried out by heat. Law no. 306 of 8.7.75, which transposed an EEC Regulation, provides for the obligation to pay milk on the basis of quality according to the analytical standard. This involves the following analyzes: content of coliform bacteria, total bacterial content, spores of butyric bacteria, titratable acidity, aptitude for coagulation, percentage of fat and casein, white blood cell count, possible presence of antibiotics.

Skimming and titration

Arrived at the factory, after checking the chemical characteristics (milk samples are often taken and some analyzes carried out directly on the livestock farms), it is discharged into a stainless steel warehouse while undergoing ordinary filtration to remove the coarse material. Then it is subjected to a skimming: if you want to obtain low-fat milk, skimmers are used; while for the production of drinking milk with a certain fat content, it is necessary to have skimmers-titrators to standardize the fat content.

Pasteurization

Its purpose is to eliminate all pathogenic germs and to reduce the number of trivial ones by more than 99% so that milk can be kept for a time that allows it to be distributed as food or transformed into various derivative products. Pasteurization consists in heating the milk to a temperature lower than its boiling point, it can be of two types: low and slow or high and fast. In the first process, now out of use, the milk was brought to the temperature of 63 ° C for about thirty minutes, in the second the temperatures reached 72-78 ° C for 10-20 s. In the latter case, plate heat exchangers are mainly used, consisting of a series of contiguous, thin-layer plates, where milk and hot water flow respectively in the opposite direction. To increase the contact surface between milk and hot water, the plates are corrugated. The whole process is continuous and therefore allows high work capacity. Pasteurized milk, called short-term storage, can be kept for 3-4 days at a temperature of 4-6 ° C.

Sterilization

It is a process which, by destroying any vital or vegetative microbial form (spores), aims to conserve milk longer. In fact, absolute sterility is never achieved, but commercial sterility which ensures the preservation of milk for at least 3 months or 6 months (at room temperature) depending on the type of treatment, after which the enzymes are reactivated.
Currently, discontinued discontinuous sterilization by means of autoclave heating, it takes place continuously according to two main systems: indirect sterilization of the milk in the containers and direct sterilization (UHT system: ultra high temperature). The first was used when there were no systems capable of aseptically packing the milk and heating took place on the product already packed in glass and polyethylene containers. The UHT system is now prevalent and can be created using heat exchangers similar to those used for pasteurization or systems in which the sterilization of milk takes place by direct contact with the steam (uperization). Sterilization with heat exchangers, which is carried out using plate or tubular exchangers, lends itself to greater heat recovery, with consequent energy savings, and favors a more prolonged shelf life of the product, but causes greater alterations of the milk components. In the process of uperization, the milk is previously pasteurized with a normal plate heat exchanger and then sent to a chamber where it is sprayed and invested from below by a high temperature steam current that heats it to 140-150 ° C for 2 -3 s. In order to be able to store sterilized milk for a long time (long-life), the subsequent filling and packaging operations must also be carried out aseptically (the tetrapak containers, today the most used, are sterilized before packaging and closed under sterile conditions).

Classification of milk intended for human consumption

Milk intended for human consumption is classified according to law 3/5/89 n. 169 and subsequent ministerial implementing decrees. They are distinguished:

- pasteurized milk, produced with low and slow pasteurization (poorly marketed);
- fresh pasteurized milk, produced with high and rapid pasteurization;
- high quality fresh pasteurized milk, coming from controlled national stables, characterized by high fat (3.5%) and protein (3.2%) contents;
- long-life sterilized milk, rehabilitated with indirect sterilization (in container) and expiring in 180 days;
- long-life UHT milk, rehabilitated with direct sterilization and 90-day expiration.
High quality pasteurized fresh milk must be marketed only whole, while the other types of milk can be marketed as whole, partially skimmed and low-fat milk.
The high milk sterilization temperatures can cause the alteration of some of its components. For example, the reaction between lactose and proteins leads to the formation of melanins which lead to a yellowing of the product; whey proteins release groups -SH which give the taste of cooked; some amino acids (lysine, methionine, alanine, valine, cysteine) of the whey proteins are denatured; the destruction of the most temperature-sensitive vitamins (A, C and some of the group B).

Homogenization

It is not a remediation technique, but it is a stabilization treatment of the milk inserted in the sterilization technology. With this process, the fat globules are crushed so that the reduced size prevents their spontaneous surfacing during storage and makes the milk more digestible. Homogenization is always carried out before sterilization as it allows for a more regular and uniform recovery of milk. The operation consists in projecting the milk, which comes out of a nozzle under very high pressure, against skeins of stainless steel.

Concentrated milk and milk powder

The concentration of milk is a preservation medium that is obtained by partial evaporation of the water under vacuum at a temperature of 40-50 ° C (concentrated milk - about 70% of H2O). For the production of condensed milk (or sweetened concentrated milk - about 25% of H2O), the raw material is concentrated and added with sucrose to ensure its shelf life without resorting to sterilization. With the most frequent total drying, milk powder is obtained, which has a percentage of residual water ranging from 2 to 7%. This product is generally prepared according to two methods: the Roller system and the Spray system. In the first case, the milk falls on two cylinders rotating in the opposite direction and heated internally with steam at a high temperature (135-140 ° C). A thin film of dried milk is formed on the two cylinders which is scraped by two sheets and collected on the bottom. In the Spray drying systems (by spraying) the milk is dropped into towers several meters high and, upon encountering an upward current of air at a temperature of 130-140 ° C, it dries instantly. Both systems determine alterations in fat and lactose and are mainly used to obtain milk powder for animal feed.
Powdered milk for food use is obtained through other systems (e.g. foam drying) that do not cause alterations in the fat and give a product that regenerates perfectly with the addition of water.

Fermented milks

They are products obtained by coagulation of milk by microorganisms of acidic or acid-alcoholic fermentation. The milk is homogenized, pasteurized, concentrated up to 14% of dry residue (to give the product a certain consistency) and inoculated with lactic acid bacteria left to incubate for a few hours, until pH 4.0 is reached. The product thus obtained can be added with fresh or frozen fruit (with the addition of sucrose).
Fermented milks are grouped into two major categories:
- acid milks, also called yogurt
- acid-alcohol milks, also called kefir.
To obtain yogurt, bacteria such as Streptococcus thermophilus and Lactobacillus bulgaricus are grafted, which transform lactose into lactic acid.
Kefir is obtained from the fermenting action of lactic acid bacteria (Streptococcus caucasicus) and alcoholic yeasts (various species of Torulopsis). This product is fluid and frothy, with an acidulous and slightly alcoholic taste (1-1.5 alcoholic degrees).

Per capita consumption of milk, butter and cheeses, in Italy (1970-1993)

Milk65.3 (1970) 82.9 (1993)
Butter2.0 (1970) 2.1 (1993)
Cheeses10.2 (1970) 17.3 (1993)


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