CONDENSED AND DRIED MILKS

Module 4. Manufacturing techniques

Lesson 7
MANUFACTURE OF SWEETENED CONDENSED MILK

7.1 Introduction

The basic principle underlying the production of condensed milk is that milk of high quality is filtered or clarified, standardized, forewarmed, and condensed to the desired level as shown in Fig. 7.1. The preservation of sweetened condensed milk is by the addition of sugar whereas the evaporated milk is preserved by heat sterilization.

7.2 Manufacture of Sweetened Condensed Milk

Sweetened condensed milk is milk that is concentrated by evaporation, to which sucrose is added to form an almost saturated sugar solution, after which it is packed. The high sugar concentration is primarily responsible for the keeping quality of the product and for its fairly long shelf life, even after the can has been opened, although it then will eventually become moldy.

7.3 Selection of Milk

The milk for the concentration purpose should have the quality as discussed in Lesson 3. As far as possible, milk should be fresh, sweet and should be free from off-flavors. Milk having off – flavours should be rejected because it is not possible in this case to remove all off- flavours. It should not have sediments. Milk after weighing should be clarified and filtered to remove visible foreign matter, which is not only unsightly but also cause consumer complaints. It is a common practice to preheat the milk before clarification in the warm clarifiers in order to increase the efficiency of the operation. It should be cooled to 4.4°C if it is not be used immediately.

7.4 Standardization of Milk

This is done to conform to the legal standards in the end product. The standardization of milk meant for condensed milk preparation is done with a view:

1. To bring the Fat: SNF ratio to the desired level.

2. To establish the desired ratio between the added sugar and total milk solids.

3. To adjusts the concentration of the finished product to the desired percentage of total solids.

7.5 Correcting the Fat Shortage of the Batch

The fat shortage of the batch is corrected by the addition of cream.

Example

We want 8.55 % Fat and 28.00% TS in the finished product. We have got 100,000 kgs of milk having 3.6% Fat and 12.11 % TS. How much cream of 40% Fat must be added to provide the desired ratio of fat to SNF?

Solution

The finished product must contain 28.0% - TS, 8.55% fat and 19.45% SNF.

Now ratio of Fat to SNF desired is: 8.55/ 19.45 = 0.4395:1

So for each kg of SNF there must be 0.4395 kgs of fat.

We know that milk on hand posses 12.11% TS and 3.6% fat, so

Therefore, milk on hand contains 12.11% TS, 3.60% fat and 8.51% SNF.

Hence, the fat content should be: 8.51 x 0.4395 = 3.74 %

If 1 kg of SNF contains 0.4395 kg of fat, therefore 8.51 kgs of SNF contains 3.74 kgs fat.

Therefore, fat deficiency is equal to: 3.74 - 3.60 = 0.014% fat

Now, 100 kgs of milk = 0.14 kgs of fat deficit.

Therefore, 100000 kgs of milk = (100000 x 0.014) / 100 = 140 kgs of fat shortage.

This fat shortage can be made up by addition of the cream of 40% Fat. However, cream also contains SNF. The Skim milk portion of 3.6% fat milk contains 8.82% SNF.

Therefore, 40% fat cream contains: {(100 – 40) / 100} x 8.82 = 5.3 % SNF

Hence, in order to balance or obtain the desired ratio of Fat to SNF, it is necessary to reserve

5.3 x 0.4395 = 2.33 % Fat to balance the SNF added with the cream.

This then leaves the fat available in the cream to (40 - 2.33) = 37.67% Fat to recover the fat shortage of 140 kgs and hence requires the addition of (140 x 100) / 37.67 = 372 kgs of cream.

So to have desired ratio of Fat to SNF, it is necessary to add 372 kgs of cream of 40% Fat in the 100000 kgs of 3.6% Fat milk.

Where ,

C = Kgs of cream to add

M = Kgs of milk on hand

F = % Fat in milk on hand.

F1 = % Fat in Cream.

SNF = % SNF in milk on hand.

SNF1 = % SNF in cream from this milk

SNF2 = % SNF in skim milk from this milk

R1 = Fat / SNF desired

7.6 Correcting the Fat Surplus Batch

The fat surplus is corrected by the addition of skim milk.

Example

We want 8.55% Fat, 28.00% TS in the finished product. Raw materials available are:

(1) 100,000 kgs of milk having 4.2% Fat and 12.95% TS.

(2) Skim milk testing 0.05% Fat.

How much skim milk must be added to provide the desired ratio of Fat to SNF?

Solution

The ratio of Fat to SNF desired in the finished product is

8.55 / (28.00 – 8.55) = 8.55 / 19.45 = 0.4395 : 1

The SNF content of the fresh milk = 12.95 TS - 4.2% Fat = 8.75% SNF with a fat content of 4.2% the SNF in the milk must be (4.2) / (0.5395) = 9.556%

Hence the deficiency of SNF = 9.556 – 8.75 = 0.806 %

The deficiency of SNF can be recovered by the addition of skim milk.

Skim milk is separated from 4.2% milk, hence it contains (100 x 8.75) / 95.8 = 9.13 % SNF

This skim milk contains 0.05% Fat. So we are adding both fat as well as SNF.

We must therefore reserve (0.05 / 0.4395) = 0.1138% SNF to balance the 0.05% fat contained in the added Skim milk.

This then leaves (9.13 – 0.1138) = 9.016 SNF available to cover the SNF shortage of 0.806 kgs per 100 kg of milk.

Therefore, for 100,000 kgs the requirement will be 806 kgs SNF.

The amount of skim milk needed therefore is (806 x 100) / 9.016 = 8940 kgs of skim milk.

So to have decided ratio of Fat to SNF in the finished product, 8940 kgs of skim milk must be added to 100,000 kgs of 4.2% Fat milk.

Where,

S = Kgs of skim milk

M = Kgs of milk on hand

F = % of fat in milk

F2 = % of fat in skim milk

R1 = Ratio of Fat to SNF desired

SNF = % SNF of milk on hand

SNF2 = % SNF of skim milk from above milk

7.6.1 Forewarming

The standardized milk is Forewarmed / Pasteurized prior to condensing for purposes as discussed in working of the evaporator’s part. Pathogens and potential spoilage organisms must be killed. Among the enzymes, milk lipase should primarily be inactivated; bacterial lipases are not inactivated and, if present, can cause severe rancidity. Deterioration caused by proteinases has not been reported. The heating intensity considerably affects viscosity and also age thickening and gelation of the product, so the actual heat treatment must be adjusted to these properties. UHT heating at about 130 to 140°C is commonly applied.

7.6.2 Homogenization

Creaming is often not a major problem, and therefore homogenization is not always done. Currently, however, sweetened condensed milk is made less viscous (and exhibits less thickening) than previously. The density difference between fat globules and continuous phase is large, so homogenization is often done at low pressure, i.e., 2 to 6 MPa.

7.6.3 Addition of sugar

Sugar is added to the milk for the purpose of preserving the condensed milk without resorting to sterilization by heat. This can simply be added to the original milk. The amount added can be adjusted readily, and the sugar is pasteurized along with the milk. However, this procedure causes fairly extensive Maillard reactions during heating and evaporation, and above all, a faster age thickening. Alternatively, a concentrated sugar solution, which should be sufficiently heat-treated to kill any osmophilic yeasts, is added at the end of the evaporation step. The sugar should be refined and be devoid of invert sugar to prevent excessive Maillard reactions.

7.6.3.1 The Quality of Sugar desired

(1) It must have known preservative property.

(2) It must not be readily fermentable

(3) It must have the power of inhibiting the activity of the germ life.

(4) Its solubility at required concentration should be such that it has sufficient concentration to avoid super-saturation and crystallization within the range of temperature to which it is supposed to be exposed. Most commonly used sugar is sucrose.

(5) The sugar used must convey to the milk flavour that is pleasing to the palate of the consuming public. Sucrose is used, which is properly refined, ferments with difficulty in concentrated solution. It inhibits the growth of bacteria and other ferments present in the milk. Its flavour has universal approval of the human palate.

(6) Dextrose has disadvantages of

a. Age thickening and

b. Discolorations. So, for exporting or long period storage of sweetened condensed milk, it is not recommended.

(7) The chief advantage of dextrose lies in its cheapness.

7.6.3.2 Quality and Care of the Sugar

1. Low grade sucrose is an unsafe product for use because it contains sufficient quantities of acid of invert sugar which damage the quality.

2. Sugar stock should be kept sealed in barrels and should be in a place that is kept clean and dry as sucrose has hygroscopic property. In the absence of above precaution there is danger of gassy fermentation.

7.6.3.3 Amount of Sugar / Sugar Ratio

(1) Amount of sugar in sweetened condensed milk should be such that concentration will avoid activity of micro organisms.

(2) It will be referred as “Sugar Ratio “.

(3) The sugar ratio is expressed in percent.

(4) A 62.5% sugar ratio means that the water present in the sweetened condensed milk contains 62.5% of added sugar. This ratio of 62.5% is adequate to protect the product from age deterioration due to bacterial causes provided

i. The milk is of good quality and

ii. Is efficiently forewarmed in manufacture and there is

iii. No post forewarming contamination.

(5) This ratio will not prevent development of defects in condensed milk heavily contaminated with sucrose fermenting yeasts and molds.

(6) Age thickening or thinning etc. will also not be avoided by this.

7.6.3.4 Method of Adding Sugar

The temperature and time of adding the sugar to the milk in the batch have a definite effect on keeping quality and age thickening of the finished product.

(1) The presence of added sugar in the fresh milk at the time of forewarming increases the heat resistance of the microorganisms and enzymes. So higher temperature for longer time has to be taken.

(2) Furthermore, the presence of added sugar in milk at forewarming temperature decreases the physical stability and increases the tendency to age thickening of the finished product.

(3) It is therefore recommended to dissolve sugar in water at 88°C or above making about 65% sugar solution and drawing this solution in to the pan separately during the condensing period. This eliminates stability weakening effect of sugar in the milk at high temperature.

(4) In order to ensure freedom from foreign material, the syrup must be passed through pressure filter or a centrifugal clarifier.

7.6.3.5 Precaution when using Dextrose

(1) The tendency of progressive thickening & discolouration of sweetened condensed milk containing dextrose can be largely avoided by dissolving dextrose.

(2) When dextrose is used in place of part of the sucrose, there is definite tendency to

a. Early discolouration and

b. Age thickening of the finished product, unless special care is taken in adding dextrose.

(3) These defects are due to a peculiar action of dextrose on the casein, forming a protein dextrose complex in hot water making 60 – 65% dextrose in water solution.

(4) Forewarming the milk and the dextrose syrup separately and drawing the latter into the vacuum pan separately near the end of the condensing period helps in avoiding this defect.

(5) The preparation and forewarming of the dextrose syrup should be done in the equipment that has been washed and rinsed completely free of milk remnants and of alkaline washing solution. It is also important that the water used to make the syrup, be on the acid side. In the case of hard water, it is advisable to add a small quantity of acid, such as acetic or hydrochloric acid, reducing the pH to ~ 6.8 in the Bromo - Thymol Blue test.

7.6.3.6 Amount of sugar to be added

Suppose we want 8.5 % Fat and 30 % of milk solids in finished product with 60 % sugar ratio, then the above combination of milk solids and sugar ratio yields a sugar content in the finished product of:

{(100 – 30) / 100} x 60 = 42 %

Example 1

Condensed milk contains 30% TS and 43.75% added sugar. Find out the sugar ratio?

Answer

In (100 – 30) = 70, i.e. 70 parts of water, sugar is 43.75 parts.

Therefore, Sugar Ratio = (100 x 43.75) / 70 = 62.5.

Example 2

Condensed milk contains 30% TS and the sugar ratio desired is 62.5%. What must be the % of sugar in condensed milk?

Answer

In 100 parts of water 62.5 parts of sugar is desired.

Therefore, in (100 – 30) = 70 parts water sugar required will be (70 x 62.5) / 100 = 43.75 %.

Example 3

Condensed milk containing 8.5% Fat, 21.5% SNF and 43.75% sugar is required. The fresh milk standardized to the above ratio of Fat to SNF contains 3.47% Fat and 8.78% SNF. How much sugar must be added to the fresh milk to give the condensed milk a sugar content of 43.75%?