Physical and Enzymatic Treatments
To prepare a milk mix ready to be fermented, it is of utmost importance to start from a high quality, inhibitors-free milk.
Milk fat and protein standardization is key if the cultured milk product is expected to have the right texture in absence of syneresis. Evaporation and reverse osmosis remove some water from milk, increasing its dry matter content, whereas ultrafiltration enriches milk in protein, removing with the permeate mainly lactose and minerals. Adding milk components, it is possible to reach the same goal, taking advantage of the functional properties specific to each of them. The target protein content is a function of the type of fermented milk (skimmed, whole/double fat, stirred/set, spoonable/drinkable) and that of added ingredients. As a general rule, as fat decreases, protein content must increase: from 3% or even less for whole fat fermented milks to 4.5% to 5.0% for fruit- added skimmed products. Concentrated cultured milks like Greek traditional strained yogurt are richer in proteins (7% to 10%), mostly in skimmed products with 0.1% fat.
Milk, once standardized in fat and protein, is submitted to relatively high-temperature heat treatment and high-pressure homogenization (180-300 bar) in order to obtain the stabilization of the fat-in-water emulsion as well as a certain level of modification of proteins structure that improves fat microglobules' incorporation into the protein matrix and water retention, preventing whey release. For stirred yogurt as homogenization pressure and milk fat content increases product viscosity improves. In the case of set yogurt on the contrary homogenization pressure appears to be inversely proportional to milk dry matter (Puhan, 1988). Homogenization is more effective for increasing viscosity and stabilizing the product when it takes place after heat treatment. Ingredients and additives, if not included in special preparations, must necessarily be added to the milk mix before heat treatment and homogenization.
Heat treatment, often following the evaporation step, depurates the milk blend from microorganisms and their nonthermoresistant enzymes and leads to whey protein denaturation and aggregation to casein micelles, improving the water-holding capacity of the gel resulting from the following lactic fermentation. It also improves the milk blend attitude to be fermented, thanks to formiate and peptides formation stimulating the starter bacteria to develop. The parameters adopted are those of a high-temperature pasteurization, but for longer times (e.g., 85°C per 15-20 minutes or 90°C per 5-10 minutes), because an elevated level of protein, denaturation is sought (e.g., 80% to 85%) (Puhan, 1988).
Milk blend degassing, when not already achieved through evaporative concentration, must immediately follow heat treatment. Undegassed milk may significatively delay lactic fermentation (around 3 hours for fermentations at 31°C, 0.75 hours at 45°C; see Driessen, 1984).
The milk blend, after cooling at fermentation temperature, is ready to be inoculated with specific lactic cultures.
The above-described milk preparation is applied for manufacturing fermented milks made with a single starter culture, simple or complex. When the target product is a blend of two different fermented milks (e.g., yogurt + probiotic cultured milk), then it must be taken into account the specific needs of each culture choosing the best milk preparation process. This could mean, for some probiotics uncapable to grow well in milk in standard industrial conditions, the need for fermenting aseptically in the absence or at reduced pressure of oxygen UHT milk, eventually fortified with growth factors, for relatively long time at optimum temperature, to achieve maximum growth. This result is of utmost importance because several countries consider that probiotics should be present, live and alive until the end of the expiry date, at the minimum quantity of 1 x 107 cfu/g, target sometimes difficult to reach, especially “cofermenting” lactic starter cultures and probiotics.
Last but not least, an enzymatic treatment of the milk blend must be taken into account. Enzymes like lactase or transglutaminase can act either before or during the fermentation. Once again, in those countries that require the absence of active added enzymes in the finished product, it is necessary to add the enzymes to the milk blend before the heat treatment and keep it for longer time at low temperature. In industrial practice, where technically feasible, it is always advantageous to add the enzymes, eventually dissolved in a small quantity of sterile milk, into the fermentation tanks, together with starter cultures.