MARKET MILK

Module 6. Common dairy operations

Lesson 33
UHT MILK - HOMOGENIZATION, PACKAGING AND DEFECTS

33.1 Introduction

For products containing high fat, homogenization must be done to prevent fat separation. In UHT processing, there are two positions for the homogenizers i.e. upstream or downstream. Homogenization may be carried out before or after UHT heating. If the latter arrangement is adopted, it will cost more, since the process has to be carried out aseptically. In order to maintain high quality in UHT processed products – flavour-wise, microbiologically and nutritionally – aseptic packaging becomes a necessity. ‘Aseptic’ in the context of the milk packaging industry means the elimination of microbial recontamination of UHT–processed milk to prolong shelf life at ambient temperature. In spite of all efforts, some defects may occur in the physical properties of UHT milk.

33.2 Upstream and Downstream Homogenization

Many UHT plants operate with the homogenizer in the upstream position. In such cases, homogenizers are not in the sterile part of the plant and do not have to operate under aseptic conditions, thereby removing the requirement of incorporating a sterile block. The milk flow rate is controlled by the upstream homogenizer in indirect systems. A variable speed homogenizer may be used to vary the throughput if so desired from the view point of filling requirements. When a de-aerator is provided, equal rates of milk supply and removal from the vessel are maintained by various means depending on the supplier of the system. The drawback of this process is that the higher temperatures used in the processing zone or the high shear rates found in the plate heat exchangers might destabilize the emulsion and affect its stability during storage. It is, however, the best position where stability and sedimentation are not major problems.

In case of downstream homogenization, it is necessary to provide a sterile block, wherein the pistons move through an atmosphere of steam. However, this position increases the risk of post-contamination. In all sterilizers of the direct type, homogenization of the product should be done on the down–stream and/or sterile side. The downstream location of the homogenizer in the direct heating systems is needed to minimize certain texture defects. Also fat tends to agglomerate upon direct heating of previously homogenized milk. There is, however, an advantage in homogenization after UHT heating since it prevents or reverses protein–protein and fat globule–protein aggregation. It also retards the formation of sediment comprising of precipitated whey proteins. The homogenizer is made aseptic by providing steam seals.

33.3 Aseptic Packaging

Aseptic packaging involves

• sterilization of packaging material

• creation of a sterile environment while forming the container and/or filling the product and

• sealing of containers thus preventing re-infection.

33.3.1 Packaging material

The packaging material is built up in the following way:

Starting from the outside, the material is covered with an external coating which might be either plastic or wax below of which is a layer of printing ink on the paper. The latter may be a twin layer (duplex) or single layer paper. Next we have the laminating layer of plastic material by which the aluminium foil is attached to the paper base.

The aluminium foil has three functions:

• It provides protection against light.

• It provides protection against entry of air.

• It is necessary to achieve satisfactory transversal seams.

On the inside either one or two polyethylene coats are applied (extruded) on top of the aluminium foil depending on the kind of product to be packaged and the extrusion equipments available.

The bacterial contents of the packaging materials food contact surface as found after extrusion in the paper conversion factory have been checked. An average total count of 0.02 to 0.05 microorganisms per cm2 was found. This comes to about 40 bacteria per 1 L brick carton. The groups of microorganisms present are 20% fungi, 10% yeast and 70% bacteria, mainly micrococci.

33.4 Tetra Brick Aseptic Packaging

The material fed into the machine from a reel of packaging material. Depending on carton size, each reel will give between 2300 (1 L size) and 5,000 (1/2 L size) units. The web travels up the machine. A strip is applied to one edge of the paper. This strip has two functions: to reinforce the longitudinal seam, and to prevent the product from coming in contact with the paper edge. After application of this strip, the material passes through a hydrogen peroxide bath. A pair of pressure rollers removes surplus hydrogen peroxide.

While travelling down, the material is formed into a longitudinally sealed tube. Remaining hydrogen peroxide is evaporated by heat, and the product is admitted. The level of the product remains above the zone where the transversal sealing is done. The product outlet is below the product level in the tube and the tube is sealed transversally. Finally the cartons are given their final shape.

Since the packaging web is a plastic coated material, the material is electrically charged. There is also a certain amount of bacteria in the surrounding air. Again, these bacteria carry electric changes. There will be attraction between the bacteria and the packaging material.

While the paper is travelling up through the air in the packaging room, a certain amount of infection is picked up. The degree and kind of infection vary largely depending on the hygienic conditions in the room. Having gone up the machine, the paper passes through a hydrogen peroxide bath. The peroxide bath contains a solution of hydrogen peroxide, with a concentration of 15 to 20% in the tetrahedron system and about 35% in the aseptic brick system. Packaging material with bacteria on it enters the bath. Some of the bacteria are washed off and some are killed. But there will still be survivors, mostly spores. When the material travels down in the machine, it is formed into a tube.

To prevent re-infection, the tube heat element produces a hot air current which protects the paper from the surrounding air. Simultaneously, hydrogen peroxide vapour escaping from the tube heater zone is channelled to a ventilation system. The tube heater is an electrically powered radiation element working at different temperatures (400- 600°C) depending on the diameter of the tube.

First of all, water is evaporated from the hydrogen peroxide with an increase in both, temperature and concentration of H2O2. Eventually, the hydrogen peroxide evaporates. The temperature is measured in the inside of the paper tube in the plastic layer. The highest temperature at this point is about 120°C.

The sterilization efficiency of the system is the combined effect of the hydrogen peroxide bath and the time of exposure of the web. During passage from the bath down to the axis, the web passes radially across the face of the disk where excess H2O2 is discharged at the periphery. Product is preheated to about 80°C by tubular regenerators before it enters the sterilizer. It is heated to 130-150°C in about 0.02 sec by friction, the final temperature being determined and controlled by a valve which regulates the product flow rate. Cooling is done by regeneration. Some homogenization occurs due to the mechanical forces applied to the product. Because of the rapid rate of heating, the friction sterilizer is likely to be similar in performance to the conventional direct heating sterilizers.

33.5 Defects in UHT Milk

33.5.1 Colour

Milk heated by the UHT process does not undergo browning because the time of heating is too short. On the contrary, such milk often appears whiter than the original milk because soluble proteins and casein micelles are partly disintegrated. In UHT milk, significant browning occurs during storage, particularly at higher temperatures.

33.5.2 Flavour

UHT milk after treatment has a hydrogen sulphide odour (cabbage) and a cooked flavour which disappears within 24 hrs. Sulphahydryl groups disappear within a few days as a result of oxidation processes. Potassium iodate (10-20 ppm) has been used to reduce the amount of cooked flavour in milk by causing the oxidation of any exposed -SH groups. Potassium iodate also reduces the denaturation of ?-lactalbumin. Oxidized or stale flavours appear after the cooked flavour has disappeared owing to the presence of methyl ketones and aldehydes.

33.5.3 Texture and structure

33.5.3.1 Fat separation

It is mainly due to inefficient homogenization.

33.5.3.2 Sediment

Formation of sediment is higher in direct heating process. This is due to fat-protein complex formation which increases with increasing homogenization pressure, viscosity and fat, solids and calcium contents as well as decreasing heat stability. Agglomeration of fat/protein in UHT milk can be prevented if the homogenizer is arranged downstream.

33.5.3.3 Age thickening and gelation

UHT milk shows a greater tendency to thicken and coagulate during storage than conventionally sterilized milk and these tendencies are related to the severity of the heat treatment. The gel structure is believed to be caused by casein micelles which will also trap fat globules and whey proteins in a three dimensional net work.

• Casein proteolysis due to enzyme reaction.

• Maillard reaction.

33.6 Nutritive Value of UHT Milk

UHT processing causes fairly extensive denaturation of whey proteins. The direct process generally causes less denaturation (60-70%) than the indirect (75-80%) and with both processes the ?-lactoglobulin is affected to a much greater extent than the ?-lactalbumin. Some slight modification of the milk caseins- size and composition occurs. There are no changes in nutritional status of lipids, minerals and carbohydrates on UHT processing though there may be some loss of unsaturated fatty acids in triglycerides. Vitamins A, D, E and K are affected very little by UHT processing. Of the B complex vitamins, thiamine, riboflavin, pantothenic acid, biotin, nicotinic acid, vitamin B6 and vitamin B12 suffer no significant loss but up to 20% of folic acid may be destroyed. Processing destroys any dehydroascorbic acid present in milk, but does not affect the content of ascorbic acid.