Physical and Enzymatic Treatments
In the case of pasteurized milk, due to the fact that milk can generally be heat treated only once, milk preparation is generally made inline before pasteurization although some unit operation can be made separately.
The fat content is generally normalized at 40°C by an automatic centrifuge separator, then homogenized to increase the stability of fat-in-water emulsion and finally pasteurized at the target time/temperature combination (min 72°C for 15 seconds: Codex Alimentarius, 2004; USDHHS, 2009) before being chilled and stored at 4°C in a buffer tank that feeds the filler machines.
Eventual additional nutrients such as proteins, vitamins, and minerals are generally dissolved in a small aliquot of the raw milk and then mixed to the rest before pasteurization.
In the case of fresh pasteurized lactose-free milk (residual lactose content lower than 0.01%), due to its short shelf life, the enzymatic hydrolysis is necessarily made on raw milk at 4°C. The complete hydrolysis may take around 8 hours.
If lactose-free milk is intended to be submitted to a stronger heat-treatment, like in the case of ESL or UHT milk, it is not necessary to hydrolyse lactose in raw milk. Sometimes, as in the case of goat's milk, a heat pretreatment associated with the use of additives like citrates and/or phosphates may be beneficial. To reach a good level of protein stability, essentially due to a new saline equilibrium that favors the solubilization of casein micelles, these stabilizers should be added to cold thermized/pasteurized goat's milk and the latter left calm for a few hours.
As a general rule, when needed, all the ingredients and/or additives are added to preheated milk (the whole quantity or an aliquota to be integrated in the rest) to constitute a separate operation, generally in batches, before the final treatment.
Likewise, when membrane techniques are employed to enrich the milk protein content or to fractionate and riassemble milk components in new proportions in the finished product, one or several operation units, eventually associated to intermediate storages, are necessary to reach the final recipe.
The availability of new systems capable of adding sterile solutions aseptically downstream in the UHT treatment (e.g., Flexdos™ TetraPak) has offered new perspectives for the addition of enzymes or other heat-sensible microingredients into ultrapasteur- ized or UHT sterilized milk. Thanks to this innovation, it is possible to delactosate UHT during its quarantine period at room temperature. Lactase stays active in the finished product, but this constitutes neither a problem nor an advantage for the consumer. However, it must be noted that the food regulation of some countries requires enzymes used as manufacturing process aids to be inactive in the finished product.
Among the physical treatments used in the preparation of market milk, a special mention must be reserved for bactofugation and microfiltration. Both of them, when used for fresh pasteurized milk production, are integrated in the primary process before the final heat treatment. In the case of ESL or UHT milk, on the contrary, they really constitute preparative operations of the milk mix to be furtherly processed.
Bactofugation and microfiltration share the same objective: purifying milk from live or dead microorganisms and their endoenzymes, somatic cells, and other impurities. Such “purified” milk may be submitted to either milder heat treatments (time/temperature) for better nutritional quality or, keeping standard process parameters, reach a product longer shelf life (provided shelf life is not imposed by law, as in the case of fresh pasteurized milk in Italy).
Bactofugation is a process in which a specially designed centrifuge is used to separate microorganisms from milk. It was originally developed to extend the shelf life of market milk but now is widely used to improve the bacteriological quality of milk (and whey) intended for other dairy products, too (e.g., cheese, ricotta, milk, and whey powders for infant formula). As far as liquid drinking milk is concerned, bactofugation may be usefully applied both for pasteurized/ESL milk and for UHT milk, provided high-quality short- aged raw milk is used. It must be taken into account that exocellular enzymes like proteases and lipases from Pseudomonas spp are not eliminated in the bactofugate and will be able to reactivate in UHT milk at medium term during the product shelf life if the same is prepared from a milk where these bacteria grew at more than 105 to 106 cfu/mL counts.