The only step in the dairy processing system in which inactivation of pathogenic microorganisms is guaranteed is pasteurization. For this reason, pasteurization is the most critical segment of the processing line. Pasteurization may be conducted in a vat batch system, also called low-temperature long-time (LTLT) or in a continuous high-temperature short-time (HTST) system. Of course, strict sanitation is critical up to and beyond pasteurization to assure the safety and quality of dairy products.
In a LTLT system, milk, yogurt mix, or sour cream mix is continuously agitated in a single tank, at a set temperature (legally at least 62.8 °C) for a given time (legally at least 30 min if at 62.8 °C) to guarantee inactivation of pathogens (16). In the case of HTST, milk must be held at 72°C for 15 sec to be legally pasteurized (16). For the production of yogurt
and sour cream, higher heat treatments are utilized in order to denature whey proteins, which enhances water retention and viscosity. Specific time-temperature combinations will be discussed in Secs. III and IV, independently.
There are several stages to the HTST pasteurization process:
1. Balance tank. Milk entering the HTST unit is collected in a balance tank, which contains a floater to prevent overfilling.
2. Regenerative preheating. Incoming milk is heated by outgoing milk (which is simultaneously cooled), separated by plates in the regeneration unit. Positive pressure is exerted on the side of pasteurized milk so pasteurized milk will not be contaminated if there is a system error.
3. Fat modification. Milk or mix is typically clarified/separated prior to homoge-nization, which occurs prior to final heating.
4. Heating section. Warmed milk enters the heating chamber, where it is heated by hot water/steam to 72°C (or higher).
5. Holding tube. Milk enters the holding tube, where it is held at a prespecified temperature and time (for instance, 72° C for 15 sec).
6. Flow diversion. If milk temperature drops below the target temperature (72°C), it is directed by a flow diversion valve back to the balance tank because every particle ofmilkmust be maintained at 72°C for 15 sec for legal pasteurization. At startup, all milk is diverted to the balance tank until the target temperature is reached.
7. Regenerative cooling. After holding, milk enters the regeneration unit, where it is cooled by incoming milk (which is simultaneously heated as in #2).
8. Cooling. If fluid milk is the desired product, the milk is cooled to a final temperature of 4°C in a cooling section with ice water or glycol. If a mix is to be used in the production of sour cream, yogurt, or another product, the mix may be cooled to the appropriate inoculation temperature rather than to 4° C.
The HTST system has many checks and balances to assure product safety. Temperature charts are documentation and legal proof that adequate pasteurization temperature and time have been employed. The holding tube and flow diversion valve guarantee that every particle of milk is legally pasteurized. Higher pressure on the pasteurized product side than the raw product side and leak detectors prevent cross-contamination.
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