11.0 Heat Treatment of Milk

11.1 Cooking: Of the various methods for preserving food, the use of heat finds very wide application. The simple acts of cooking, frying, broiling, or otherwise heating our foods prior to consumption are forms of food preservation. In addition to making food more tender and palatable, cooked foods will have a large proportion of their microflora and natural enzymes destroyed increasing the storage life. However, cooking will generally not sterilize a product, and so even if protected from recontamination, food will eventually spoil in a comparatively short time. The time before onset of spoilage is prolonged if the cooked food is refrigerated. These are common household p ractices. Another feature of cooking is that it is usually the last treatment food receives prior to consumption. The toxin formed by Clostridium botulinum is destroyed by a 10-minute exposure to moist heat at 100°C. Thus, proper cooking provides a safeguard against this toxin. However, cooking is a drastic form of heat treatment. Boiling milk at 100°C for undetermined time will cause very significant changes to both the structure of the milk components and the physical stability of the product and will certainly cause marked changes in flavor. Yet, boiling of milk does not render it sterile and it will unavoidably spoil after some period of time.

11.2 Time-Temperature Combinations: If the purpose of the heat treatment is exclusively to destroy pathogenic bacteria, then careful selection of time-temperature combinations will achieve the purpose without excessive destruction of the milk components. It is important to know that for any temperature above 63°C, different holding times may be specified to cause the needed destruction of pathogenic bacteria. Thus, at 63°C a holding time of 30 minutes is required for the certain destruction of the most resistant milk pathogen, Mycobacterium tuberculosis. However, as higher temperatures are chosen, the required holding time becomes less, and at 72°C, only 15 seconds of holding is required to destroy the organism.

11.2.1

Pasteurization of milk as defined by the US Code of Federal Regulations

11.3 Pasteurization: This is a comparatively low order of heat treatment, which must be sufficient to destroy all pathogenic bacteria. Pasteurization may be defined as: "Any heat treatment of milk which secures the certain destruction of tuberculosis bacteria without markedly affecting the physical and chemical properties." The term pasteurization was coined to honor the French physician, Louis Pasteur, who in the middle of the 19th century made his fundamental studies of the lethal effect of heat on microorganisms and the use of heat treatment as a preservation technique. Pasteurized products remain perishable and should be treated as such.

11.4 LTLT Pasteurization: This method is called the holder method or the low-temperature-long-time method. This is a typical batch method where a quantity of milk is placed in an open vat and heated to 63°C and held at that temperature for 30 min. Sometimes filled and sealed bottles of milk are heat-treated in shallow vats by that method and subsequently cooled by running water.

11.5 HTST Pasteurization: The term is an abbreviation of high-temperature-short-time. The HTST process for milk involves heating it to 72-75°C with a holding time of 15-20 seconds before it is cooled. Depending upon the quality of the raw milk and the degree of refrigeration, the shelf life may be from 2 days to 16 days.

11.6 Ultra Pasteurization. Not to be confused with UHT (see next). Ultra pasteurization is a process to increase the shelf life beyond what is traditionally expected. Heating milk to 125-138°C for 2 - 4 seconds and cooling it to below 7°C is the basis for the longer shelf life. However, the product is not sterile and will eventually spoil.

11.8 Conventional Sterilization: This is the original form of sterilization which involves in-container sterilization usually at temperatures from 115-120°C for 20 - 30 minutes. Sterilization is a process which causes complete destruction of microorganisms and their spores. Commercial sterilization does not always meet this definition, because some harmless, heat resistant bacteria may still be present. The criterion for food sterility remains to be a process, which will ensure no surviving botulism bacteria or their spores. The common guideline is to use a multiple of 12 for the D (121°C)-value of C. botulinum, or its equivalent.

11.9 Forewarming. This is a heat treatment given in some instances to milk where the primary purpose is to alter the physical-chemical state of the protein/mineral system so as to increase the stability of the milk system to subsequent sterilizing temperatures. This type of treatment is important for manufacture of canned evaporated milk.

11.10 Regenerative Heating and Cooling. The term refers to an engineering principle in equipment design and operation which has had enormous importance for the economy of the modern dairy industry. In many cases a product must first be heated for a certain treatment and then cooled. Pasteurization is such an example. Chilled milk is heated from, perhaps, 4°C to a pasteurizing temperature of 72°C, held at that temperature for maybe 15 seconds and then chilled again to 4°C. In regenerative cooling, the heat of the just pasteurized milk is used to warm the incoming cold milk. Thus, the outgoing hot milk is the heating medium for the cold, raw milk. At the same time, the cold milk is the cooling medium for the hot milk. The process takes place in a heat-exchanger and is called regenerative heat recovery. As much as 94-95% of the heat content of pasteurized milk may be recycled in this manner.

11.11 Counter-Current, Plate Pasteurizer with Regenerative Effect . In the following diagram, the homogenizer and the pasteurizer are in a sense "a unit". The milk enters the pasteurizer at the heat regeneration section, where it gets heated by the hot milk emerging from the holding section. The warmed milk is now piped to the homogenizer. After homogenization, it is piped back into the pasteurizer where it enters the high heat section, that also serves as the holding section (80°C/30sec). From the holding section, the milk enters the heat regeneration side, where it is used to warm the incoming milk. Finally, it is sent through the cooling side to emerge as cold, homogenized, pasteurized milk.

11.11.1 Diagram of Plate Pasteurizer (-and Homogenizer)

11.11.2 Counter-Current Flow. The diagram shows the opposite flow directions of cold milk to be warmed and hot milk to be cooled. In this arrangement, the temperature difference between the two liquids at any point can be established within a few degrees of each other. Counter-current flow is our most effective way of heat exchange.

11.11.3 Concurrent Flow. You should note that in concurrent flow (liquids flowing in the same direction) it is impossible to heat the incoming cold product to a higher temperature than that which would be obtained by mixing the heating medium with the cooling medium. This limittion does not exist with counter-current flow.

11.11. 4 Partition Plates in the Pasteurizer. Counter-current flow in the plate pasteurizer is achieved by assembling a great many partion plates in a continous stack. These stainless steel plates have been machined to fit snugly together, preventing leaks, and have been engineered so that milk will pass on one side of the plate while the heating or cooling medium flows in counter-current direction on the other side. The thickness of the plates has been maintained at a narrow tolerance to permit efficient heat transfer. The heat transfer area is determined by the number of plates in the pasteurizer. The factors that must be taken into account in selecting and dimensioning a pasteurizer are: 1. Product flow rate 2. Physical properties of the liquids (e.g.viscosity) 3. Temperature requirements 4. Permitted pressure drops 5. Heat exchanger design 6. Cleanability requirements 7. Required running times The permitted pressure drop is an important consideration because the pressure on the product side must be higher than on the side for the incoming milk, so that there will be no leakage of raw milk into the pasteurized milk.

11.11.6 Calculation of Holding Cell Dimensions.

11.12 Study Question:

1. Calulate dimensions of a holding tube for a holding time of 15 sec. with a capacity of 10,000 L/hour. The inner diameter of the tube is 48.5 mm. Efficiency factor is estimated to be 0.90.

2. What is understood by regenerative heating?

3. Describe the concept of counter-current flow.