CONDENSED AND DRIED MILKS

Module 4. Manufacturing techniques

Lesson 10
PILOT STERILIZATION AND HEAT STABILIZATION FOR EVAPORATED MILK

10.1 Introduction

To ensure that the evaporated, homogenized milk does not coagulate during sterilization and at the same time does acquire a desirable viscosity, a series of sterilization tests is often done on small quantities of the evaporated milk to which varying amounts of a stabilizing salt (for the most part, Na₂HPO₄) are added. The tests are needed because variation occurs among batches of milk. Essentially, the addition of the salt means adjusting the pH. Because further processing must be postponed until the test results are available, this necessitates cooling the evaporated milk after its homogenization and storing it for a while. However, long-term storage should be avoided to prevent bacterial growth; moreover, cold storage of the milk increases the tendency of age thickening. The stabilizing salt is added as an aqueous solution, which dilutes the evaporated milk slightly. Therefore, the milk is often concentrated somewhat too far and is re-standardized to the correct dry-matter content during stabilization.

10.2 Pilot Sterilization

The purpose of this method of technical control of evaporated milk is to have control on sterilization process by controlling heat stability and viscosity of finished product . It makes possible

(1) The adoption of standardized process of sterilization designed and adopted for evaporated milk of superior quality for process. This process provides a very narrow range of variation of temperature and of time exposure, in order to limit the personal factor with its inevitable uncertainties to the minimum.

(2) A standard method of determining by means of a pilot sterilizer, a viscosimeter and a colour test, the proper viscosity and colour that the evaporated milk should have when it comes from sterilizer.

(3) A standard method of determining the amount of stabilizer that must be added to any given batch of EM in case its properties are such that it is unsafe to subject it, without such treatment to the temperature conditions that fall within the range of the standardized process of sterilization.

10.3 Use of Stabilizer

The amount of stabilizer to add to any given batch of Evaporated Milk is determined by trial. A number of sample cans are used and the test is made in the pilot sterilizer. The samples are prepared by transferring various accurately measured amount of the stabilizer solution to individual sample tins which are subsequently filled with exact weight of the evaporated milk to be tested.

From the research of Sommer and Hart, it is found that most cases of unbalanced salts in Evaporated Milk, a range of 6 – 62.5 gm of dry citrate or phosphate / 100 kgs of evaporated milk will cover the amount required to restore the balance and to accomplish a satisfactory sterilizing process. But some times isolated samples also require as high as 125 gm /100 kgs of evaporated milk.

10.4 Preparation of Stabilizer Solution

The citrate or phosphate is added in the form of a solution. The strength of the solution should be such that one tenth of a cubic centimeter of a solution is equivalent to the addition of 6.25 gm of the dry salt / 100 kgs of EM. This requires a solution containing 10.63 gm of dry salt dissolved in enough water to make up 100 ml of solution that had been added to the 170 ml can, the inspection and test of which show the viscosity desired.

10.5 Preparation of Sample Tins

Use 1 ml pipette. In order to prevent loss of solution due to some of it being blown out with the air that escapes past the tin of pipette, it is best to add the solution to the Cans before they are filled with the milk.

10.6 Compensating For Effect of Dilution of Sample

The addition of a salt solution dilutes the EM. This factor alone has a slight influence on the heat coagulation point of the milk. In order to eliminate this factor it is advisable to equalize the dilution in all samples of a given series.

10.6.1 Process

Milk giving alcohol negative test, after standardization to Fat/SNF = 8/18 , forwarmed to >90 C for a period of 10 minutes and concentrated to 26-28% milk TS, homogenized to 100 Kg/Sq.cm at 55-60°C temperature is used for Pilot Sterilization using following procedure:

1. Prepare a number of sample test bottles and proceed as shown in Table 10.1.

2. Prepare a 10 % solution of stabilizer (by dissolving 10 gms of dry stabilizer in enough water to make 100 cc of solution).

3. Add stabilizer solution and distilled water to evaporated milk bottles as per table given.

4. Fill the sample bottles containing the measured portion of stabilizer solution with evaporated milk from the batch and seal them.

5. Place the sealed bottles in the Pilot Sterilizer and subject them to a heat treatment for 15 minutes at 116-117 °C with a 15-20 minutes coming up time.

6. Cool the bottles rapidly to 24°C with help of air flow.

7. Examine the bottle contents for smoothness and then test for viscosity.

8. Select the bottle whose contents show a desirable body and smooth texture.

9. Determine whether stability correction is necessary.

10. Based on the correction corresponding to the Pilot Test (to give the best viscosity), calculate the total quantity of stabilizer required for the bulk of the evaporated milk to be prepared.

11. Just add enough water for complete solution of the stabilizer in a vessel. Add the mixture slowly to the evaporated milk in the storage tank, ensuring thorough mixing during addition and for several hours after its solution.

12. For subsequent batches of evaporated milk add a portion of the required amount of stabilizer (determined by previous day's experience) to the standardized fluid milk at the forewarmer and then complete the correction by adding the remainder to the evaporated milk in the storage tank.

10.7 Method or preparing solution

Table 10.1 Concentrations of stabilizer solution used for pilot sterilization test

In table, the first sample represents the evaporated milk without any addition. The second sample contains no stabilizer but represents the milk in dilute form, the rate of dilution being the same as that of all the other samples. The remaining samples represent addition of stabilizer in increasing amounts but with the dilution in all the cases equal to the second sample. By this procedure, it is possible to determine to what content the improvement in heat stability is attributed to the dilution and to the stabilizer respectively.

10.7.1 Sterilizing the sample

The sample content determining the measured portions of stabilizer solution are now filled with evaporated milk from batch. They are sealed and kept in pilot sterilizer where they are subjected to heat treatments. A heat treatment is given by bringing the temperature from 110 to 117°C which is brought up in 2 minutes. The milk is held at the temperature for 15 minutes. The bottles then are cooled rapidly by opening exhaust and cold water intake valve. As temperature influence the viscosity, it is advisable to cool it to standard 24°C. As soon as cooled, the bottles are opened, examined for smoothness and colour and tested for viscosity.

10.7.2 Adding the correct amount of stabilizer to the batch

The stabilizer should be added to the evaporated milk in the form of a solution using just enough water for complete solution of the crystals. The mixture should be added slowly, keeping the evaporated milk thoroughly agitated both, during addition and for several hours after its solution

The casein – stabilizing reaction of the Na-salt is most pronounced when the added stabilizer is in the milk during forewarming and condensing. It has been found advantageous, therefore, to add at-least a portion of the required amount to the milk at the forewarmer and then complete the correction by adding the remainder to the evaporated milk in the storage tank.

10.8 Factors Influencing the Viscosity

10.8.1 Factors that increase viscosity and curdling tendency

1. Presence of developed acidity.

2. High Protein Content.

3. Excess or Deficiency of Ca⁺⁺ in Ca-casein combination

4. Presence of Bacteria producing rennet like substance.

5. Low forewarming temperature

6. High Concentration.

7. High homogenizing pressure especially in case of milk of inferior quality.

10.8.2 Factors that decrease the viscosity

1. Absence of developed acidity.

2. High temperature quick heat, short holding heat treatment in forewarming

3. Low concentration.

4. High temperature quick heat, short held heat treatment of concentrated milk

5. Optimum Ca ⁺⁺ casein balance.

10.9 Canning

Packaging in cans is common. Evaporated milk intended for use in coffee is usually packaged in bottles that are closed with a crown cork or a screw cap. Can should meet all the sanitary standards and its metal should be non-toxic to the product or should not impart any flavour in it. The evaporated milk container must withstand the pressure changes in the process of sterilization. Its seal must be absolutely air- tight and it must be strong.

a) Temperature of Milk

The temperature of evaporated milk at the time of filling needs attention because it is a factor affecting the tendency of milk to foam. Foaming can seriously interfere with proper filling of the Cans. In addition, excessively low temperature increases the tendency of flapping. The most effective temperature for filling is considered to be 4.4°C.

b) Leak Detection

Immediately after sealing and before they leave the last turn table of the sealing machines, the Cans are inspected for visible defective seals. In order to minimize loss of Cans and contents due to leaks in any part of Can, in the seams as well in the seals it is important to test all Cans by means of leak detector before they reach the sterilizer. Most commonly can pass submerged through hot water bath and expansion of the air in the Can cause slight pressure. In case of leaky can, the pressure expands some of excess air. The escaping air percolates upward in water in the form of air bubbles that are readily seen.

10.10 Sterilization of Evaporated Milk Cans

In-bottle or in-can sterilization can be applied batch wise (in an autoclave) or continuously. Machines that have rotary air locks (to maintain the pressure) may be applied for cans and hydrostatic sterilizers for bottles.

The filled and sealed cans that have successfully passed inspection for freedom from defective seals and seams are now ready for sterilizer. If they cannot be sterilized within an hour or two, they should be held under refrigeration at 4.4 – 7.2°C. This precaution is especially important during hot weather season.

a) Purpose of Sterilization

(1) The primary purpose of sterilization is to destroy all germ life and enzymes present therefore preserving the product permanently.

(2) In addition, sterilization process gives the milk

a. Increased viscosity

b. Improves the body of the finished product

c. Gives the evaporated milk a more creamy consistency

d. Tends it a pleasing richness

e. Assists in keeping fat in homogeneous emulsion and

f. Lessening its tendency to separation after manufacture.

b) Sterilizers

There are fundamentally two types of sterilizers in use

10.10.1 Batch sterilizer

The batch sterilizer consists of large boiler like horizontal steam drum opening at the top or at one or both ends. Its hollow interior consists of revolving frame of spider cage into which the cans are loaded. A perforated steam distributing pipe near the bottom extends over the entire length of sterilizer drum. This pipe has a separate steam inlet to each end. Near the top, there is a water distributing pipe with connections in the water main. In the bottom of the shell, there is a drain. On the sterilizer drum are mounted also a pressure safety vent, water steam and vacuum gauges, high temperature thermometer and preferably temperature control recording and safety devices. Process consists of filling the frame with water and heating it by steam and cans are kept rotating in it; temperature maintenance and time of heat treatment are controlled.

Temperature of heating & importance of uniform heating

A uniform temperature keeping is necessary in all the parts of sterilizer to keep away defects due to not uniform treatment to all cans and all parts of each can. It will keep uniform colour, consistency etc. Rapidity and uniformity of heat transfer are enhanced by the use of water in sterilizer. Sterilizing process must provide a ratio of temperature - time exposure that is lethal to even most resistant type of spore forming bacteria.

It is recommended that the temperature should be raised from room temperature to 115.5°C and held at that temperature for not less than 15 min and not more than 20 min. It is further recommended that the rise in temperature during last 10 min of the coming up period be at about 3°C for every minute and the temperature of holding should be between 115.5 – 118.3°C and time not less than 15 minutes.

Shaking

The purpose of shaking the evaporated milk is to mechanically breakdown the curd that may have formed during sterilization to a smooth homogeneous consistency. Violent shaking causes a sharp decrease in viscosity. Excessive shaking is therefore avoided. The period of shaking also should be minimized. Usual shaking period kept is 15 sec to 2 minutes and is sufficient to break normal soft curd.

Cooling after sterilization

Immediately after the holding time, the evaporated milk is cooled. The steam is turned off, exhaust and drain are opened and cold water is turned into the sterilizer with the real revolving and cooling is continued until the temperature of milk is reached to 21 to 27°C. It should not take more than 15 minutes time. Rapid and uniform cooling is important.

10.10.2 Continuous sterilizer

It is best adopted in large plants. Its advantage lies in a large reduction of packing labour, elimination of incubation and of hand inspection and automatic rejection of leakers. On the other hand the continuous sterilizer is built for a standard size cans only. It will not handle odd size and larger size cans.

The evaporated milk continuous sterilizer consists of three principal units.

(1) Pre – heater

(2) The Sterilizer or Cooker

(3) Cooler

Principle behind the process is to avoid excessive adversities of pressure in sterilizer to Cans. Milk is preheated just to boiling point and then in sterilizer, temperature is raised and then milk is cooled.

10.10.3 UHT sterilization

UHT sterilization kills bacterial spores more effectively than in-bottle sterilization. The combination of preheating and UHT treatment of the concentrate suffices to inactivate plasmin. Preheating is also required to prevent excessive heat coagulation in and fouling of the UHT sterilizer. Some heat coagulation nearly always occurs, and the subsequent homogenization also serves to reduce the size of the protein aggregates formed. Aseptic homogenization must be applied. Indirect UHT sterilization in a tubular heat exchanger allows the pump of the homogenizer to be fitted before the heater and the homogenizing valve behind it. Thereby, the risk of recontamination with bacteria is diminished. The addition of stabilizing salt can often be omitted if UHT sterilization is applied, or the amount to be added is not so critical that sterilization tests must be carried out. It implies that the whole process from preheating up to and including aseptic packaging can proceed without interruption.