Sterilization

i. Definition

Sterilized milk refers to a product obtained by heating milk in a container in a commercial cooker/ retort to temperatures of 110-130OC for 10-30 min. The process is also referred as in-container sterilization. Sterilized milk is generally intended for prolonged storage at room temperature (up to 6 months). The major objective of heat sterilization is to destroy microbial and enzymatic activity. The length of time and magnitude of temperature employed during processing depend on the type of the product, number and heat resistance of microorganisms and enzymes present in milk. The heat resistance of microorganisms or enzymes is generally evaluated in terms of D-value or Z-value. Sterilization load or heat load for sterilization is generally expressed in terms of Fo value.

 

ii. Theoretical Basis

Clostridium botulinum is considered as the index organism for assessing thermal sterility in foods. Under anaerobic conditions, inside a sealed container, it can produce botulin, a toxin, which can be 65% fatal to humans. Therefore, destruction of this organism is a minimum requirement of heat sterilization. As milk is a low acid (pH>4.5) food, it is recommended to achieve 12 decimal reductions for C.botulinum. This can be achieved by heating the product at 121OC for 3 min(Fo = 3). However, this minimum treatment may produce milk that is safe but not necessarily commercially sterile. This is so because there are more heat-resistant spores present in milk. There is B. stearothermophilus or B. sporothermodurans.These spores are not pathogenic. Their presence may require heat treatment equivalent to two (2) or more decimal reductions. This may correspond to an F0 value of 8.Target spoilage rates should be less than one survivor in every 10,000 containers.

 

iii. Types of Sterilization Plants

Sterilizing retorts are either batch type or continuous in operation. Batch type sterilizers may be either vertical or horizontal. Horizontal retorts are easier to load or unload. They have facilities for agitating containers/cages. However, they require more floor space. Typically such horizontal retorts contain concentric cages. Cans are loaded horizontally into the annular space between the cages. When cages are full, the retort is sealed. The cages are supported by guide rails, which slowly rotate them. This stirring of the contents in cans facilitate proper heating. Continuous retorts are generally equipped with better controls. They cause very gradual change in pressure inside the cans. Thus products are heated more uniformly. Can seams are also subjected to less strain in comparison to batch process.

Continuous sterilizers: They are mainly of three types: (a) cooker-coolers; (b)hydrostatic sterilizers; and (c) rotary sterilizers. Cooker-coolers carry cans on a conveyor which pass through three sections of a tunnel. These sections are maintained at different pressures for preheating, sterilization and cooling. The hydrostatic sterilizer consists of a chamber equipped with provision for steam injection. The chamber that is partially full of water is connected to two water columns (12 to 18 meter tall, barometric leg) which are used to adjust pressure in the chamber. If the height of the water columns is changed, the steam pressure is changed and therefore the maximum attainable temperature changes. For example,to get a temperature of 116oC, a difference in height between the two water columns should be 10.7 m while for attaining 121oC temperature in the chamber,the water column difference should be 13.7 m. A conveyor with provision to accommodate cans of different sizes moves through the steam chamber carrying the food cans. The heating time could be regulated by varying the speed of the conveyor. Hydrostatic sterilizers are very flexible and suitable for large capacity plants. However, size of the structure and high capital costs are the major disadvantages of this system.

Continuous rotary sterilizer consists of several horizontal inter linked cylinders which allow for preheating, heating, precooling and cooling in upto four continuous stages.The vessel has a spiral track on the inner wall. A spoke or reel within the centre of the cooker causes the cans to roll along the spiral track. Rotary valves used to interconnect the shells, maintain pressure in the heating and cooling sections. Sealed cans are introduced directly from the sealing machines. The contents inside the cans are mixed as cans travel along the helix and therefore enhance heat transfer and ensure less heat damage to the product. Cans coming out of the cooker are directly taken to labelling and palletizing machine. Rotary sterilizers are particularly suitable for processing of milk and milk based products, which are extremely heat sensitive and susceptible to browning.

 

iv. Description of the Canning Process

Basic operations in conventional retorting/canning process include: preparation of the raw material, filling of the container, exhausting, sealing of container, sterilization,cooling of the cans, labelling and storage.The preparation of raw materials refers to washing, peeling, cutting, blanching, pre-cooking, etc. in case of fruits, vegetables, meat, etc; and preheating, mixing, homogenization, etc; in case of milk. Filling of containers can be carried out either manually or mechanically. Correct and accurate filling is important from economic standpoint as well as for prevention of entrapment of large volume of air/ gas inside the can, which might decrease the intensity of heat treatment. Exhausting is an essential operation in the canning process and involves removal of air/ oxygen from the container before it is closed. Removal of air ensures minimum of strain on the can seams or pouch seals through expansion of air during heat processing. Removal of oxygen is essential to prevent internal corrosion of the container through oxidation and creation of vacuum inside the container while cooling. Absence of oxygen inside the container also delays oxidative deterioration of the product besides destruction of ascorbic acid.

After exhaustion, containers are sealed. Depending on the type of containers (metal cans, glass bottles, flexible pouches); sealing machines are chosen. Glass jars are normally vacuum-sealed while tins are closed with a double sealing on the seal side and may also be vacuum-sealed. Flexible retortable pouches are sealed by fusion of two thermoplastic materials through application of heat by heated plates or jaws.

Product in the closed containers is heated in the sterilizer in an atmosphere of saturated steam or hot water or air-steam mixture. The sterilizing action of steam depends on its latent heat of vaporization as it condenses on the surface of the can.Saturated steam condenses readily and is therefore an efficient sterilizing medium.Displacement of all air present in the retort by steam before the sterilizer is brought to operating temperature is a very essential step. This is also known as venting.The purpose of this processing step is to maintain uniform steam-air mixture in the sterilizer and prevent under processing. Sterilization temperature – time combination in retorts may vary from 110 – 130OC for 10-30 min. Sterilized containers are then cooled and brought to room temperature for labelling and storage. Turbidity test developed by Aschaffenburg is conducted to ensure sterility of the product. This is an indirect test and it measures denatured whey proteins. Complete denaturation indicates that the milk is adequately sterilized.

 

v. Quality of Sterilized Milk

Sterilized milk has a rich creamy appearance and a distinct cooked flavour (rich,nutty, caramelized). It is considerably browner in colour than raw milk. The brown colour develops due to formation of coloured pigments resulting from interactions between free amino groups of proteins and aldehyde group of lactose through Maillard reactions. The intensity of cooked flavour and brown colour depends upon the severity of heat treatment. In-container sterilization causes loss of nearly half of the ascorbic acid (Vitamin C) and sizeable loss of thiamine (30-40%). Vitamin B12 is almost completely destroyed. Fat soluble vitamin A, carotene, riboflavin and nicotinic acid are not affected. Biological value of proteins is only marginally affected.Sterilized milk cannot be coagulated with rennet unless calcium chloride is added externally.