Freshness and Age of the Milk
Apart from specific regulations concerning a few types of fresh pasteurized milk (e.g., high-quality fresh pasteurized milk in Italy) defining the maximum time between the first milking in a supply and the moment of the only heat treatment to be given to the milk, and some provisions prescribed in the individual standards of a few PDO cheeses (e.g., Mozzarella di Bufala Campana cheese), currently there is no legal restriction to the age of the milk at the moment of its conversion into dairy products.
However, dairy technologists know quite well that milk freshness may play a fundamental role in achieving the desired quality level of finished products.
On the one hand, milk refrigeration at the farm undoubtedly improves its hygienic quality, but on the other hand, even when this process is well controlled, it might be the origin of important end-product problems due to psychrotrophic bacterial growth and changes of the milk physico-chemistry.
From a microbiological point of view, from the moment of milking down to its arrival to the dairy, the milk is subjected to an increasing microbial load, variable in nature, origin, and composition, as a consequence of the hygienic status of the animals, udders and milking parlor, GHP application to any tool and equipment in contact with milk, as well as of refrigeration patterns (cooling speed and keeping temperature and time at the farm and during delivery to the dairy). To reduce costs, the frequency of milk collection at the farms has been progressively reduced, passing from twice a day to daily, every other day, or even—in some regions—every 72 hours—that is, from every two to every four or even six milkings. This reduction of the collection frequency has an impact on the microbiology of raw milk and should be done only when reducing the milk temperature from 4°-6°C to 2°C.
At 4°-6°C, psychrotrophic bacteria can proliferate and reach counts exceeding 1 million per milliliter in 48 hours. Chilled milk psychrotrophs are mainly represented by bacteria of the genus Pseudomonas (most frequently, Ps. fluorescens or Ps. putida). At the end of their exponential growth, psychrotrophic bacteria can produce extracellular enzymes, exposing milk to protein and lipid degradation. Proteolytic and lipolytic enzymes from psychrotrophic bacteria are extremely heat resistant, and as a consequence, they can remain active in milk even if their parent bacteria are killed by heat (in UHT milk, their reactivation takes time at room temperature and leads to the defect known as sweet coagulum).
Chilled milk, both raw and pasteurized, if kept at the temperature of 4°-6°C, should be transformed into dairy products within 48 hours from the first milking or at maximum within 72 hours. Longer times should not be considered in absence of strong actions on milking and collection hygiene, milk cooling rate, and keeping temperature.
In any case, GMPs should not consider manufacturing high-quality dairy products from milk aged more than 5 days. CE regulation 853/2004 establishes a maximum limit of 300,000 cfu/mL for TVC at the end of storage immediately before its processing for raw milk and max 100,000 cfu/mL for heat-treated milk.
From the physico-chemical point of view, refrigeration at the farm has represented a big step forward in reducing raw milk collection costs while improving milk microbiological quality and work conditions both at the farm and the dairy. Nonetheless, its impact on caseins and salt equilibria in milk cannot be omitted as well as their consequences on the composition and the dimensions of casein micelles.
Cooling in fact increases the solubility of caseins (especially of fi-casein although K- and as are interested to a lesser degree), which consequently tend to be extracted from the micelles to form soluble casein. The proportion of fi-, as-, and K-casein solubilized after storage for 48 hours at 4°C may reach, respectively, 18%, 5%, and 5% of the total casein fraction in raw milk (Downy et al., 1970; O'Connor and Fox, 1973; Ali et al., 1980a,b).
In addition to this, another phenomenon takes place—the release of plasmin from the micellar to the serum phase and its consequent activation. fi-casein being its preferred substrate, an increase of the levels of gamma-casein and fractions 5 and 8 of the peptone proteoses may be observed (Reimerdes & Herlitz, 1979). This proteolysis may bear effects of technological importance when the milk is stored cold for more than 48 hours.
Cooling produces a significant increase in the levels of soluble calcium and inorganic phosphate as well, at the expense of their micellar forms.
In synthesis, cold storage alters the equilibrium between colloidal and soluble forms of calcium and phosphate, leading to disassembling casein micelles. Using cold storage for 48 hours at 4°C increases the levels of dissolved calcium and ionic calcium of nearly 10% and 20%, while the level of soluble inorganic phosphorus rises by about 8% to 10% (Leone et al., 1981; Ali et al., 1980a).
The result of this phenomenon is that the dimensions of the micelles are reduced, their hydratation is increased, and the cooled milk colloidal phase is more finely dispersed.
These effects are reversible, but the reversibility is slow. About 24 hours at 20°C are required to bring the micelles' size and soluble calcium content back to their original values. It is not certain, however, that the micelles will fully recover their original form and composition.
The effects of cooling are known especially to cheesemakers, since the milk coagulability by rennet is definitely influenced by the level of colloidal calcium phosphate and the size of the micelles.
Cooling has repercussions on the renneting behavior of milk: the coagulation time is lengthened, the firmness of the gel is reduced, and whey drainage becomes more difficult.
In conclusion, cooling milk and keeping it at a low temperature for a long time reduces its fitness for transformation into high-quality products, unless special precautions are taken, both in GHP/GMP all along the supply chain and in dairy technology. Milk freshness in terms of milk age at the time of processing is a real added value for the level of product quality perceived by the consumer and for the ease of manufacturing with 0 defects by the dairy industry.