Dairy Engineering

Module 6. Sterilizing & packing equipment

Lesson 21
DIFFERENT TYPES OF STERILIZER, IN BOTTLE STERILIZERS, AUTOCLAVES

21.1 Introduction

Sterilization is the term used for process to destroy all the microorganisms present, both vegetative forms and spores, or at least make them incapable of growth in the product, so that a long keeping quality is obtained without refrigerated storage.

The term”sterilized” here does not imply that the product is sterile in the strict microbiological sense. It has been clear for many years that absolute sterility cannot be obtained by any heat treatment processes. If the treated product is incubated at appropriate temperature, spoilage will be noticed at some parts. This spoilage can be reduced by increasing the time-temperature of the heat treatment process. However, this will cause undesirable chemical changes, color, flavour and or nutritional changes.

Typical time – temperature combinations for in-container sterilization processes are 105-120°C with effective processing times of 10-30 minutes, the higher temperatures being associated with the shorter times. This sterilization effect can be obtained by using much higher processing temperatures like 135-150°C, with shorter holding times of about few seconds. These processes cause much less chemical changes as a result of heat treatment than that of conventional sterilization processes. The higher temperature processes are frequently considered as different processes, which have been given the description “Ultra High Temperature “(UHT). With in-container processes, with limited heat transfer rates, do not allow sufficiently rapid change of product temperature. Therefore UHT processing involves the treatment of product in continuous flow in a heat exchanger.

Packaging of UHT milk has to be done in aseptic condition. In comparison to In-can sterilized milk, UHT milk cannot completely inactivate the more heat-resistant, proteolytic enzymes, particularly when initially present at high concentrations.

21.2 Batch Sterilization Equipment

Fig. 21.1 Batch sterilization equipment

The Batch Sterilization equipment, usually called Retorts, consists of a cylindrical tank, under pressure, into which the cans containing the food products, placed in cages are lowered. Steam flows into the retort from the top. After heating, cooled water is sprayed into the retort. To prevent the pressure in the cans from becoming higher than that in the retort itself, compressed air is blown into the retort to compensate for the pressure drop, and when the temperature is reduced to below 100°C the retort may be opened.

The advantage of this type of sterilization equipment is the flexibility with regard to package size and the relative ease to maintain constant temperature. However, the operation of loading is laborious, and the heating is slow because of not agitation of the inside liquid.

21.3 Continuous-CAN Sterilization Equipment

These are more suitable to larger capacity of handling. It has basically three sections. In the first section, the cans are heated, in the second section the cans are cooled under high pressure and in the third section they are cooled at atmospheric pressure. The cans are transported in a helix positioned on the periphery of a barrel, and when they have been transported one revolution of the barrel they have at the same time been moved one step along the retort. Simultaneously, the can rotates around its own axis. The effect of this is a flow pattern in the package enhancing heat transport in liquids. As shown in figure the cans are transported from one zone to another.

There is high pressure in the sterilizer to prevent differences in pressure in the can and sterilizer. The disadvantage with these sterilizers is the limitation on the size of the cans, as they are built for one can size and only small difference in size can be accepted in the equipment.

Fig. 21.2 Continuous sterilization equipment

21.4 Hydrostatic Sterilizer

A Hydrostatic sterilizer consists of a chamber equipped with steam injection. The chamber is connected to two water columns (barometric lock) which are used to adjust the pressure in the chamber. If the height of the water columns is changed the stream pressure is changed and thus the maximum temperature obtainable also changes. The normal height of water column to get 121°C temperature is 13.7m. These sterilizers are often very tall. A conveyor, suitable to size of can, travels through the steam chamber carrying the cans. The sterilization time may be changed by varying the speed of the conveyor. The flexibility and the capacity are the major advantages of this type of sterilizer. The height and cost are the main disadvantages.

Fig. 21.3 Hydrostatic retort

21.5 Hydrolock Sterilizer

Fig. 21.4 Hydrolock retort

This equipment consists of a continuous sterilizer that has a rotating pressure lock, called hydrolock, which is partly submerged under water. The cans pass the hydrolock into a preheating chamber filled with water. They are then transported into the steam chamber. The bottom part of this chamber contains hot water thus further heating the cans. The cans are then transported to the upper part of the chamber containing steam and here sterilization takes place. The product then passes through a cooling section. Hydrolock sterilizers are flexible but they are rather large and costly. Leakage in the rotating valve of the hydrolock is common.