MARKET MILK

Module 6. Common dairy operations

Lesson 26
PRINCIPLES OF HEAT TREATMENT

26.1 Introduction

In the dairy industry, thermal processing is accepted terminology to describe heat treatment required to eliminate/minimize chances of spoilage of milk and occurrence of food borne illness there-from. Pasteurization is one type of thermal processing designed for a specific pathogenic microorganism, but it does result in a shelf stable product without refrigeration. Thermal treatment is the transfer of heat energy into milk. Such heat treatment of milk consumes a lot of energy. The objective of the heat treatment is to completely inactivate all microorganisms and most of the enzymes contained in milk. Primarily, heat treatment is a hygiene-oriented activity within the entire framework of processing. On the other hand, it is a dominant factor for improving the shelf life of all fresh milk products and is a legal obligation. The most common thermal process is pasteurization.

26.2 Pasteurization

It is the thermal inactivation of microorganisms at temperatures around 72-78°C for specific time period, which improves the hygienic quality of milk and achieves a certain level of preservation.

The main objective is to kill pathogens to avoid health hazard. At the same time, a significant reduction in the total bacterial count is accomplished increasing the shelf life of milk and milk products made thereof. Apart from the hygiene aspect, gentle heat treatment must be achieved which results in time-temperature profiles meeting the following requirements:

• The percent destruction of microorganisms has to be > 99%; for pathogenic germs, it must be 100%.

• Milk must be treated gently in order to retain the nutrients and vitamins to the maximum possible extent as well as preserving its organoleptic quality.

• Process economics must be profitable, and the installation cost must be low.

Other processes that can decrease the microbial population in milk include bactofugation.

Other methods include:

• Radiation with UV, X-rays or gamma-rays.

• Ohmic heating. This is the advanced thermal processing method where in the milk, which serves as an electrical resistor, is heated by passing electricity through it. Electrical energy is dissipated into heat, which results in rapid and uniform heating. Ohmic heating is also called electrical resistance heating, joule heating, or electro heating.

• High-pressure process, using pressures of 2000-6000 bar at temperatures of 40-¬60°C.

• Ultrasound process.

• Microwave treatment.

The last process is a very interesting one for the thermal treatment of foods. During heating with microwaves, the product is exposed to an electromagnetic field having a frequency of 800-3000 Hz. The heat is caused by the rotation of the dipoles and oscillation of ions in the food. The microwave energy is created by transforming a current in a generator magnetron or klystron, from which the microwaves are fed through a rectangular channel (to conduct the waves) into an applicator and heat treatment unit. The product enters the sealed and protected chamber, and the microwaves are fed into it (with an antenna, slit and/or other sharp inserts and other facilities for distributing the microwaves), and penetrate the food evenly.

These processes are not yet widely applied. Their main disadvantage is their low inactivation effect, legal barriers, and high costs.

26.3 Influence of Heat Treatment on Milk

The inactivation effect and the chemical-physical, nutritional and organoleptic changes in milk during heat treatment are characterized by the following parameters:

• Temperature and time profile

• Type of microorganisms and the initial level of contamination

• Acidity of milk

• Flow conditions and heat transfer in the installations

26.4 Time and Temperature

Regarding microorganism inactivation, there is a nearly logarithmic relationship between temperature and time for a given range. If temperature is increased by 10°C, the bacterial count is reduced to 1/10th of the initial level. If we assume that for a total inactivation of the microorganisms in milk at 100°C the time required is 30 minutes, then we can obtain the following residence time if temperature is raised by 10°C:

100°C = 30 min 130°C = 1.8s
110°C= 3min 140°C= 0.18s
120°C=0.3min or 18 s

The quotient, which expresses the decrease in inactivation time when the temperature is increased by 10°C is called the Q₁₀ factor. This value indicates how much time would be required for complete germ inactivation when the temperature is raised by 10°C. The Q₁₀ factor for non-spore forming microorganisms is 9-13, whereas for spore formers it is about 30. If a type of bacteria is inactivated at 11.5 times more rapidly at 90°C than at 80°C, then the Q₁₀-factor is 11.5. The velocity of chemical reactions in milk increases by a factor of 2-4, when temperature is increased by 10°C; the physical changes increase by a factor of about 2.

Another indicator for complete inactivation (sterility) is the F-value.

26.5 Changes in Milk Components

The relatively low Q₁₀ values for chemical and physical modifications appear to permit very high heat treatment for a short time; however, we have to consider modification in the milk components even with temperatures slightly above 70°C with residence time of a few seconds.

With increasing temperatures and residence times, whey proteins are modified; e.g., the solubility decreases to such an extent that they coagulate with casein at a pH of 4.6. Another consequence of the heat treatment of proteins is the liberation of sulfhydryl groups (-SH groups), thus increasing the anti-oxidative properties of milk, leading to the development of ‘cooked taste’. At the same time, we observe a percent decrease in whey proteins and a quantitative increase in casein. In particular, we observe complex formation between ?-casein and ?-lactoglobulin, resulting in an additional protective colloidal effect.

Fig. 26.1 Influence of the heat treatment process on milk quality

This results in delayed release of the peptides when rennet is added to cheese milk. Hence, cheese milk should be pasteurized at minimum temperature-time combination suggested for milk. Casein remains nearly unchanged at normal pasteurization temperature.

A firm gel is formed due to the concomitantly increased water-binding capacity of the heat denatured proteins culminating in reduced syneresis, which is favorable in the manufacture of yogurt/dahi.

The heat treatment also leads to disturbance in the salt equilibrium in milk. Soluble calcium and phosphorus are bound in the form of calcium phosphate, which precipitates on the casein micelle. This activity is reversible up to a temperature of 80°C; at higher temperatures, calcium precipitates and forms stone-like deposits on the heat exchanger surface.

Longer residence times at temperatures >100°C can lead to complex compounds be¬tween casein and lactose. The observed browning is due to Maillard reaction. At this temperature, nearly all enzymes are inactivated, and vitamin losses of 20-30% are observed. But even at lower temperatures, significant vitamin degradation occurs, especially of B complex vitamins.

Heat treatment has only a minor effect on fat. Only the membranes of the fat globules with their heat-sensitive protein compounds experience some modifications, influencing the agglomeration of fat globules and their creaming.

26.6 Germ Inactivation

The germ inactivation effect depends largely on the type of microorganisms present in milk. Most of the vegetative forms, especially the lactic acid bacteria, and pathogens are destroyed at temperatures of 70-90°C and residence time of a few seconds to few minutes. Excluded are the thermophilic lactic acid bacteria which can survive slightly higher temperatures and the spores of surviving bacilli/Clostridia; temperatures > 100°C are necessary for their destruction. The time required for germ inactivation is a function of the total count, and the initial count determines the total count of the milk which is to be treated

Fig. 26.2 Germ Inactivation effect during pasteurization

26.7 Heat Treatment Processes

The holding time at a specific temperature is a function of the residence time in the heating where milk reaches the preset temperature due to heat transfer as well as residence time in a post heating holding section, where no heat transfer takes place. The time-temperature combinations of the heating process for milk are shown graphically in Fig. 26.2. The short time treatment guarantees relatively gentle heat treatment and is the most widely applied system. Due to the large-scale handling requirements of milk, the low-temperature long-time (LTLT) treatment is without any industrial significance. The high-temperature short-time (HTST) treatment is a relatively gentle process and is the most widely applied system.

Table 26.1 Heat treatment processes

Thermization is used mainly for heat treatment of raw milk in order to stabilize its quality during long storage by inactivating psychrotrophic microorganisms.

The high heat process is used mainly for heating of cream. It is also used eventually for a very low quality of raw milk. The high heat process with direct steam injection is used in order to increase the shelf life of fresh milk under conditions similar to those for the short-time process. This process is also used for cheese milk and milk concentrates for long-shelf-life products.

26.8 Time-Temperature Profiles for Heating

Ultra high temperature (UHT) process leads to organoleptic changes in milk without changing the nutritional value significantly. Direct or indirect heating is used for preparing UHT milk. Sterilization is used only, when filled products must be preserved for a period longer than 5 months at ambient temperature. Heat treatment can also be used in combination. A thermization or a short-time heat treatment can be used before applying a high-heat pasteurization, ultra-high heat treatment or sterilization (Fig. 26.3). Sterilization leads to a completely sterile product. The high-heat pasteurization and ultra-high heat treatment achieves ‘commercial sterility’; i.e., the product is free of live microorganisms, except for few spore formers.

Fig. 26.3 Time-temperature profiles for heating process

26.9 Control of Heat Treatment

Ensuring adequate heat treatment must be established in two different ways. One way is by recording the temperature; the chart shows the date of processing which should be kept for two years. For short shelf stable products, the storage period for the records is shortened to two months after expiry or minimum shelf life date.

The second test is alkaline phosphatase test. Raw milk contains alkaline phosphatase. It is destroyed at the temperature necessary for efficient pasteurization. When milk containing phosphatase is incubated with pnitro-phenyl di-sodium orthophosphate, the liberated para-nitro-phenyl gives a yellow color under the alkaline conditions of the test. The color is a measure of the phosphatase content of the milk sample. Therefore, if phosphatase is present, it indicates that the milk has not been heated to right temperature or got contaminated after the heating process by raw milk. If the high heat pasteurization is done properly, the enzyme peroxidase (inactivated at temperatures ? 85°C) would not show any activity in test. If HTST pasteurization is applied, the enzyme alkaline phosphatase would be completely inactivated (inactivated at temperatures ? 71°C).