Module 4. Manufacturing techniques

Lesson 8

8.1 Introduction

Concentration for manufacture of sweetened condensed milk is usually done by evaporation. A falling-film evaporator is generally used to remove the bulk of the water and a circulation evaporator to remove the remainder. Relatively high temperatures (up to 80°C) are often applied, which implies a lower viscosity in the evaporator but a higher initial viscosity of the final cooled product. The low water content of the sweetened condensed milk implies a high viscosity and boiling point. Evaporation in continuously operating equipment with many effects is, therefore, not easy. Fouling, thus, readily occurs. It is difficult to accurately adjust the desired water content, which is mostly monitored by means of refractive index.

8.2 Striking the Batch

The term “Striking” refers to the drawing of a sample from the pan and testing it for density. When the boiling milk approaches the desired concentration, there are various indications that suggest to the experienced pan operator that the milk is nearly ‘done’:

  • The general behaviour and appearance of the milk is characteristically different.
  • The milk has “settled down” to a quite boil.
  • Its surface assumes a glossy glistening luster and
  • There is heavy roll from periphery towards centre of the pan, forming a small ‘puddle’ of foam in the centre.

8.3 Method of Sampling

Numerous sampling devices have been developed that make possible the rapid drawing of samples representative of the boiling milk in the pan without interrupting or disturbing pan operations.

8.4 Temperature of Sample for Density Tests

The result of the density test is influenced by the temperature of the sample at the time of the test. For accurate and comparable results, therefore, the adoption of a standard temperature is indispensable. Towards the end of the condensing period, the pan temperature usually drops to near 49°C. Therefore, a standard testing temperature adopted is 49°C and the sample used is to be taken accurately. For accurate density determinations, the temperature of the sample should therefore be taken at the time of the density test. If it varies several °C from 49°C, it should be adjusted to the standard temperature or if this is not practicable, the density readings should be corrected by applying the proper temperature correction factor.

8.5 Density Tests

The density tests that have been found most practical for use at the pan are

(1) Judging Density by appearance to eye

(2) The Picnometer test and

(3) The Hydrometer test.

(4) Refractometer test

(5) Viscometer test.

8.5.1 Temperature correction factor

If the temp of the sample at the time of the density test varies from the adopted standard temperature, the Baume's reading may be corrected to what it would be at the standard temperature by multiplying the temperature deviation. If the temperature of the sample is above the standard temperature, the product is added to the observed Baume's reading. If the temp of the sample is below the standard temperature (120°F or 48.9°C), the product is deducted from the observed Baume's reading.

8.5.2 Formula for calculating the correct Baume's Reading

The Sp. Gr. of sweetened condensed milk of any composition may be calculated by dividing the figure 100 by the sum of the quotients that result from dividing the % of each group of ingredients by its respective Sp. Gr. of sweetened condensed milk at 60° F(15.6°C). It is thus represented by following formula:


8.5.3 Conversion of Specific Gravity to Baume's Degrees

The composite Sp. Gr. values of the milk thus obtained by the use of the above formula are then converted to Baume's Degrees (°Be) by means of the following formula:


This is then corrected to the standard temperature used for the hydrometer test of the pan sample by the “Temperature correction factor”.

8.5.4 Specific gravity values for Sp. Gr. formula

The constituents or groups of constituents, the Sp. Gr. of which determine the composite Sp. Gr. value of sweetened condensed milk consist of the Fat, SNF, added sugar or sucrose and the water. Their respective sp. Gr. at 60° F (15.6°C) are:


8.6 Finishing the Batch

When the desired density has been reached, the condensing process is stopped in the following order:

  • All steam to the pan is shut off,
  • The valve in the water line to the condenser is closed,
  • The vacuum pump is stopped and
  • The vacuum relief valve is opened.

The above operations should be carried out in the order named to prevent milk from burning on to the heating surface and condenser water from flooding the pan. When the vacuum has been dissipated, the condensed milk is then drawn from the pan. This should be done promptly so as to avoid super heating due to the static status of the hot milk in the pan, which has a tendency to accelerate age thickening especially in the case of milk of naturally unstable viscosity.

8.7 Final Standardization

The fat, SNF, and the added sugar are already present in their desired ratio to each other. The third and final standardization, therefore, has to do only with the adjustment of the % of total solids, if such adjustment is necessary or desired.

8.8 Cooling of Sweetened Condensed Milk

It is one of the important steps in manufacture of sweetened condensed milk. It is absolutely essential to cool the product promptly to desired temperature because i n these steps, formation of large lactose crystals must be avoided. Consequently, seed lactose is added. Before that, the condensed milk must be cooled to a temperature at which lactose is supersaturated so that the seed lactose does not dissolve. However, the temperature must not be so low that spontaneous nucleation can occur before the seed crystals are mixed in. After seeding, cooling should be continued to crystallize the lactose.

If we do not cool down quickly

(1) There is tendency of discoloring and

(2) Tendency of age thickening during storage.

Cooling will also help in

(1) Formation of smoother texture in finished product and

(2) Prevent objectionable sugar deposit.

8.8.1 Crystallization

The relative smoothness of product will largely depend upon number and size of crystals present. Our main aim is not to prevent the formation of crystals but our main aim is to form more crystals having smaller size. In this process major constituent responsible is lactose because it has low solubility than sucrose. Also crystals formed are hard and thick. In case of sweetened condensed milk, the rate of lactose crystallization is further impeded (retarded) by

(1) The milk colloids and

(2) The higher viscosity which reduces the rate of diffusion.

Lactose is a disaccharide and reducing sugar. It is not very soluble and not so much sweet as sucrose. It is fermented by lactic and acidophilus organisms etc. The concentration of this sugar varies from 48-50 gm/liter. In solution, lactose exists in two forms i.e. α & β forms. In crystalline form, three forms are available:

  1. α - lactose hydrate
  2. α - lactose anhydrate
  3. β - lactose anhydrate

Solubility of lactose is low at room temperature in pure water. Its solubility is 18 parts/100 parts. α & β lactose in sweetened condensed milk are in definite proportion and when these proportions are changed, they change their forms and again form lactose equilibrium. This change is very slow. So lactose present in sweetened condenses milk takes more time for crystallization. The solubility of these two forms is also varying and α -lactose has lower solubility as compared to β –lactose. So α -lactose will crystallize first. So equilibrium will be changed and β -lactose will turn to α -lactose which will take time and crystallization process will be slow.

8.8.2 Forced-crystallization – importance of mass crystallization in cooling

The problem of ensuring permanently smooth texture in the finished product is not a problem of preventing the formation of lactose crystals during the cooling process. It is a problem of preventing the crystals that are present at the conclusion of cooling process from subsequently growing larger. This is accomplished by providing condition at the conclusion of the condensing period, i.e. in the cooling process that produce mass crystallization. A multitude of crystal nuclei i.e. small crystals, great enough to reduce the state of supersaturating to a state of saturation must be formed while the crystals are still exceedingly small. This eliminates sugar in solution in excess of maturation and thereby the possibility of objectionable further crystal growth after completion of the cooling and agitation period.

8.8.3 Forced crystallization period

The purpose of this period is to provide for mass crystallization. It is a period in cooling process when condensed milk has reached to the temperature which will provide super saturation of lactose with minimum viscosity and such temperature at which the condensed milk is held at the optimum temperature of seeding. The optimum temperature for forced crystallization at which the super saturation takes place varies from 40 - 85.6 °C. In our country, it is varying between 32.2 - 85.6°C, depending upon original lactose content. The milk should be held at this temperature for about 60 minutes, under vigorous agitation after which it is cooled rapidly to the final temperature. For best results, at the end of the condensing process, the hot condensed milk should be cooled from pan temperature to seeding temperature as rapidly as possible, to prevent slow initial crystallization such as would yield a relatively small number of crystals, designed to grow to relatively larger size, giving the finished product a coarse sandy texture. For milks of fairly normal concentration, the optimum temperature for forced crystallization ranges from 30 – 40°C.

8.8.4 Purpose of seeding

The purpose of seeding is to give the lactose present in super saturation form an additional incentive for crystallization. When condensed milk at optimum temperature is seeded with properly prepared lactose in presence of vigorous agitation, numerous crystals of lactose of uniform size will be formed. This operation should be continued for about one hour. The number of crystals centers present at any time, that are capable of developing successively to crystal nuclei and to crystal is a function of degree of super saturation and that the existing crystal centers will increase with the degree of super saturation. In the absence of crystal nucl ei, or because of difficulty of migration of particles to existing nuclei, lactose even in highly super-saturated solution may refuse to crystallize or will crystallize only very slowly.

In addition, seeding at optimum temperature for mass crystallization with properly prepared seed lactose, and in the presence of vigorous agitation, yields crystals of uniform size. Literally born at or near the same time, they get their start under identical conditions of time and degree of super saturation with respect to lactose and of viscosity of milk and they are checked in their growth at the same time that is when super saturation has been dissipated and scattered. Crystals of larger size will be few in numbers, being limited largely only to crystals that were already present in the hot milk in the vacuum pan and at any other time before the seeding period.

8.4.5 Seed material

The seed material commonly used is powder of lactose hydrated. Lactose should be of smallest possible size preferably 200 mesh ( <10 µ) having sharp crystal edges. To avoid contamination of product, the lactose used for seeding should be sterilized under vacuum at 93°C which will convert lactose hydrate to anhydrate. Then it is ground to fine particles in a perfect pulverized mill, filling in the can and sealing it. Then sealed can is sterilized at 130°C for 1-2 hours.

The smaller size and sharper particle edge of the seed material have advantages such as:

  • Greater incentive to the formation of crystals centers, and crystal nuclei
  • It brings about mass crystallization
  • More numerous and smaller permanent crystals and
  • Smoother texture of the finished product.

8.4.6 Amount of lactose to be used

It is always desirable not to use more seed material than necessary for optimum mass crystallization. When using ground lactose of 200 fine mesh, than 37 – 50 gm of lactose per 100 kgs of original fluid milk will be sufficient.

8.4.7 Adding the seeding material

The seed material is not added in dry form to batch. Precaution should be taken that it should not form lump into large aggregate which will not disaggregate. In that case the seed material is incapable to initiate mass crystallization. To obtain the desired result, it is usually dissolved in sweetened condensed milk in a suitable sterilized container. Stir the product till all the lumps disappear and form homogeneous mixture. This mixture is added slowly to the whole batch while stirring vigorously and keeping temperature constant Agitation is continued for 50 minutes. Agitation plays on important role because it promotes and increases the rate of crystallization. Such agitation facilitates active migration of the particles to the crystal nuclei in this product. In addition, vigorous agitation is necessary from beginning to the end of the cooling process in order to cool all parts of the batch as much as possible to the same temperature. The usual seeding temperature is 27 – 39°C for cow’s milk and for buffalo milk it ranges from 30 – 36°C.

After crystallization is over, lower down the temperature to room temperature. This cooling is done by several methods. After cooling, it is again stirred for 20 minutes because, due to lower temperature, solubility decreases and there is possibility of crystallization.

8.4.8 Packaging sweetened condensed milk

Packaging in cans is common. The cans are then covered with a lid and the seams are sealed. Cans and lids are first sterilized. The packaging room is supplied with air purified through bacterial filters. Sweetened condensed milk reaches the market in the form of bulk good and also in different size retail packs. It is important to fill the cans full in order to exclude as much of the air from the container as possible. For sanitary reasons the filling machines should be emptied and washed after each day’s use. It is important to close the tins immediately after filling . If they are left open their contents are exposed to air and light and prolonged exposure may cause the surface to crust over and to develop a tallowy flavor. It may also invite contamination with insets. The seal must be air tight and sufficiently rigid to withstand a rough handling.

Last modified: Thursday, 4 October 2012, 5:33 AM