Manufacture of Evaporated Milk

Unsweetened concentrated milk is known as evaporated milk and manufactured practically in the same manner as the sweetened condensed milk. In this product no sugar is added as preservative. To achieve preservation the product is sterilized by heat after concentration and sold in the hermetically sealed container in which it has been sterilize.

Flow Diagram of Manufacture of Evaporated Milk

i) Receiving milk/filtration/clarification/standardization: The raw milk used in the preparation of evaporated milk is treated in a similar manner to that which applies in case of sweetened condensed milk as discussed earlier. It is important that the raw milk should be of very good quality in order to ensure that it is free from any heat resisting microorganisms. Only fresh milk can be used since, if the acidity of evaporated milk is above normal it will clot or curdle during sterilization and will thus be rendered un-saleable.

ii) Prheating/Forewarming of milk: To improve the heat stability of concentrated product and for imparting optimum viscosity to the finished product the fluid milk is preheated before it is condensed. Milk may be preheated either at 95-1000 C for few minutes (5-10) or at 140-1450 C with not holding time.

iii) Concentration: Preheated milk is concentrated in an evaporator. Multiple effect evaporators are used for handling large amount of milk to have continuous operation.

iv) Homogenization: After concentration the milk is homogenized. Fat separation in evaporated milk during storage is a major defect. This defect can be reduced or eliminated by homogenization of condensed milk. Homogenization refers to a process of forcing milk under pressure through equipment called homogenizer.This is an essential part of production of evaporated milk. The homogenizer reduces the mean size of the fat globules in the milk so that they are uniformly distributed in milk and do not rise to the top to form the creamy layer during storage. In raw milk, the diameters of the fat particles (globules) vary from 1 to 20 micron while a diameter of about 2 micron or less is required to keep the fat distributed uniformly. In this process milk is forced through a small orifice at a high pressure by means of a positive displacement pump. The mechanical forces, which are set up as the fat particles pass quickly through the orifice,cause the particles to split. Condensed milk removed from evaporator is passed through two stage homogenizer at about 50°C with a pressure of 175 kg/sq. cm at first stage and 35 kg/sq cm at second stage.

v) Cooling: After homogenization concentrated milk is cooled to 5°C and held in storage tank. Stabilizing salts may be added to concentrated milk in tank as indicated by pilot sterilization test.

vi) Pilot Sterilization: The purpose is to determine the amount and type of chemical stabilizers to be added to any given batch of condensed milk for most satisfactory heat stability. Heat coagulation of milk is influenced by many factors such as initial quality of raw milk, fore warming, concentration and sterilization. In raw milk it has been suggested that salt balance of milk i.e. ratio of calcium +Magnesium to citrate + Phosphate is important for the stability of milk towards heat. If the ratio of Ca + Mg/Citrate + Phosphate is disturbed the heat stability may be affected. The more common cause of heat coagulation is the disturbed salt balance. Depending on the location of pH of maximum stability with respect to natural pH of concentrated milk either sodium salt of orthophosphate (disodium phosphate) or calcium chloride/monosodium phosphate are used as stabilizers. This is the basis of a process of pilot-sterilization in which various quantities of selected stabilizers are added to the concentrated milk in 170 ml (60 oz) tins which are then sterilized and the condition of the product carefully examined in order to determine whether any stabilizer is necessary and if so in what amount it should be used. Depending upon the type of milk and the processing condition used 100 to 300 gm of stabilizing salts per 100 kg of evaporated milk may be needed to improve the heat stability. Calculated amount of salt is added to the evaporated milk in the form of a solution using just enough water to dissolve it.

vii) Packaging: The evaporated milk is filled in suitable cans or containers either manually or mechanically. It is essential that the packaging is done as quickly as possible. After cans have been filled they should be sealed immediately. The sealing process requires care, as the seal has to withstand the heat and pressure of sterilization.

viii) Sterilization: The object is to destroy all bacteria their spores and enzymes thereby preserve the product for a longer time. In addition sterilization process is also used to increase the viscosity and improve body and texture to give a creamy consistency to the product.

Two important factors in sterilization process are:

•  The temperature attained
•  The period during which this temperature is maintained.

The temperature time of heating during sterilization should be such as to ensure sterility consistent with sufficient body and texture without causing objectionable discolouration or excessive cooked flavour.

Two systems are in common use for sterilization:

Batch Sterilization: This method is suitable for small scale operation and is also useful for cans of various sizes. In this system the cans leaving the filling machine are placed in racks. The racks are then loaded into a steam-tight boiler. The racks are moved by a revolving mechanism, which keeps them in motion throughout the process,the speed of rotation varies between 6- 12 revolutions per minute. Steam is admitted in to the sterilizer until the temperature reaches 118 °C and the sterilizer is maintained at this temperature for not less than 15 min. After sterilization is completed, the steam is shut off and cans are cooled to 25- 30 °C either by means of water spray or by immersing in cold water. After cooing the cans are passed through an oscillating machine, which breaks up any curd formed during the process of sterilization and retains the smooth homogeneous texture of evaporated milk.

Continuous Sterilization: This is used for large scale continuous operation. This system consists of many compartments, which are maintained at different temperatures/ pressures. The filled cans entering the apparatus roll through the chambers by means of spiral track, which ensures that the heat distribution to each can is uniform. This rolling motion of cans takes the place of the methods of agitation provided in the batch method. The filled cans are preheated before they pass to the main sterilizing section. The movement of cans is controlled so that they remain in the sterilizing chamber for 15 min. After sterilization process is complete the cans pass to the cooling section, which under similar pressure conditions to those of the sterilizer.Both systems have their merits and demerits. During sterilization the temperature of evaporated milk is raised rapidly to 116-118°C and held at this temperature for 15 minutes and their cooled.

Storage: After sterilization cans of evaporated milk are unloaded from the sterilizer, and cooled by water spray. The cans are then shaken mechanically to break any curd or lump which might have formed during sterilization to have homogenous consistency. Excessive shaking is avoided as it may decrease viscosity. As evaporated milk is a sterilized product it may be stored at or below room temperature. The present trend is to store at below room temperature to check deterioration in quality and thereby prolong keeping quality.