Manufacture of Sweetened Condensed Milk

Different steps involved in the manufacture of Sweetened Condensed Milk are given in the Figure .

Flow diagram for preparation of sweetened condensed milk

i) Receiving Milk : Raw milk received for the manufacture of condensed milk should be of good quality. This is important because the quality and marketability of final product depends upon the initial quality of raw milk. Great care, therefore, must be taken to ensure that only good quality milk is received for processing into sweetened condensed milk. After the milk has been accepted on the basis of standards platform tests, it is weighed, sampled and further processed.

ii) Filtration/Clarification: The objective is to remove visible foreign matter.Simple filtration may remove suspended particles by straining process while clarification removes the same by centrifugal sedimentation. Filters generally,contain cloth or pad of described pore size, which retain smaller particles,while clarification removes sediments/slime much more efficiently than filtration.Clarifiers remove even finer particles that escape filters. The clarified milk is then chilled and stored in large storage tanks until further processing.

iii) Standardization: The object is to standardize fat and solids-not-fat in milk to obtain a final product, which meets the legal standards. This operation is of considerable importance as the yield and keeping qualities of the final product are very much dependent on it. Further it also helps in maintaining the requirements of legal standards in the final product. Standardization establishes the desired ratio of fat: SNF in milk . Standardization of milk involves addition of sufficient quantity of cream or skim milk to ensure the resultant product has the correct fat: SNF ratio to yield the final product of desired composition. The following steps are followed to know whether cream or skim milk is to be added to milk for standardization purpose.

Find the ratio of SNF/Fat desired in the final product. (e.g., 22/9 = 2.44).

Accurately calculate the Fat and SNF percent of milk in the storage tank.

Find the ratio of SNF/Fat in milk, i.e., 8.9/3.6 = 2.47 or 9.58/6.5 = 1.47

If the ratio in milk is higher than the ratio desired in the final product, SNF is in excess in milk and hence more source of fat (i.e. cream) should be added to milk.

If the ratio is less, fat is more in milk and hence SNF source (i.e. skim milk)should be added to get the final desired ratio.The quantity of cream or skim milk to be added to milk for standardizing purpose may be determined by the following general formula:

To calculate the amount of cream to be added to milk:

(SNF)/F  (Qxs1 + Cxs2)/(Qxs1 + Cxs2)

Where

(SNF)/F=SNF:F ratio desired in condensed milk

Q = Quantity of milk in Kg available for standardization

C = Amount of cream in Kg required

F1 = Percent fat in milk

S1 = Percent SNF in milk

F2 = Percent fat in cream

S2 = Percent SNF in cream

To calculate the amount of skim milk to be added to milk.

(SNF)/F  ((Q*S1)+(S*S3))/((Q*F1)+(S*F3))

Where

(SNF)/F = F ratio desired in condensed milk.

Q = Quantity of milk in Kg required

S = Amount of skim milk in Kg required

F1 = Percent fat in milk

S1 = Percent SNF in milk

F3 = Percent fat in skim milk

S3 = Percent SNF in skim milk

Standardized milk is now ready for further treatment.

iv) Fore warming/Preheating: Fore warming or preheating refers to heating of milk before it is condensed. The object of this process is to reduce or eliminate bacterial and mould contamination and to destroy undesired enzymes present in milk which encourage slow chemical deterioration in the final product during storage. This treatment exerts an important influence in controlling the viscosity to avoid age-thickening or age-thinning in the finished product. Fore-warming also helps in dissolving the sugar which is added during the next stage of operation. Sugar dissolves much more readily in hot milk than in cold milk. Further, evaporation takes place more rapidly if heated milk is fed into vacuum pan or evaporator during concentration. In brief fore-warming helps in the following:

•  Destruction of microbial contaminants and enzymes• Control of storage defects e.g. Age-thickening and Age-thinning.
•  Dissolving sugar
• Un-interrupted boiling during evaporation process.
• Improve the heat stability of condensed milk.

Fore-warming temperature is determined as to provide optimum viscosity in the condensed milk without inducing thickening or thinning during storage. The temperature-time of fore-warming extends over a wide range, such as 82-93°C for 5-15 minutes; or 116-149°C for 0.5 to 5 minutes. The modern trend is towards high temperature short time heating such as 115-118°C without holding. Tubular heat exchangers are commonly used for fore warming or preheating; either a double-tube or shelf-and-tube heat exchangers are preferred. Plate heat exchangers may also be used.

v) Addition of Sugar: Sugar is added for the purpose of preserving condensed milk without resorting to sterilization by heat. Sucrose is generally referred as sweetening agent. Refined cane sugar or beet sugar is used in the manufacture of sweetened condensed milk. Other sweetening agents such as glucose,dextrose and corn syrup have been tried to partially replace sucrose. The disadvantages of these sweetening agents are their reduced sweetening capacity compared to sucrose and their adverse effects on colour and the rate of thickening during storage. Hence they are not suitable.

Amount of Sugar/Sugar ratio: The quantity of sugar added to milk should be enough to preserve the milk. It has been recommended that a sugar ratio of 62.5 would protect the condensed milk from bacterial defects. Generally sugar ratio of 62.5 to 64.5 % is considered adequate to protect condensed milk against bacterial spoilage and to protect against causing sugar crystallization. Sugar ratio is the sugarin-water concentration of condensed milk.

The following formula is used for thin purpose.

% Sugar ratio =( %Sugar in condensedmilk)/( 100 –%Totalmilk solids in condensesmilk)*100

Example:

Condensed milk contains 31.0% total milk solids and 43.1% added sugar. What is the sugar ratio?

Solution: % Sugar ratio =(43.1)/( 100 – 31)* 62.5%

The following formula is used to determine percentage of sugar required in condensed milk for desired sugar ratio.

Percent Sugar in condensed milk =((100 –%TMS)* SR)/100

Where, TMS = Total milk solids in condensed milk

SR = Sugar ratio

Example:

If, TMS in condensed milk = 31.0%

SR = 62.5

% Sugar in condensed milk =( (100 – 31) 62.5)*100 = 43.1

Amount of sugar to be added to milk to get the desired % Sugar in condensed milk.

First determine ratio of concentration by dividing Percent total milk solids in condensed milk by % total milk solids in fresh milk. Then divide the percentage of sugar in condensed milk by ratio of concentration.

Ratio of concentration =(%TMSin condensedmilk)/ %TMSin freshmilk

Sugar in milk = (%Sugar in condensedmilk)/( Ratio to concentration)

Method of adding sugar: The temperature and time of addition of sugar to the milk in the batch has definite effect on keeping quality and physical stability of the finished product. In one method part of fore warmed milk is run into a sugar mixing pan where sugar is mixed in a fine stream with proper agitation. The sweetened milk is then drawn into the vacuum pan. If sugar is added before condensing milk, an increase in viscosity and greater difficulty in the evaporation of water is experienced.Further the presence of added sugar in the fresh milk during fore warming increases the heat resistance and survival capacity of microorganism thereby adversely affecting keeping quality of the product. Another method is to dissolve sugar by boiling in water in a separate tank called sugar well. This mixture is then added to milk in the vacuum pan towards the end of the evaporation process. The disadvantage of this method is additional water has to be evaporated from added sugar solution. But boiling of solution destroys practically all microorganisms, which the sugar may contain and sugar solution can be filtered to remove any extraneous matters.

(vi) Condensing: Sweetened condensed milk is concentrated to a greater degree than is the case with the unsweetened variety. Vacuum pan method being usually employed in most condenseries for condensing milk.

Operation of Vacuum pan: The vacuum pan is sterilized with steam before the operations commence and vacuum pump is operated until 50-55 cm vacuum is obtained. The milk inlet valve of the pan is opened slightly and milk is drawn by the action of reduced pressure. When each section of the heating coil is covered with milk then the steam is gradually admitted. When the milk level is sufficiently high to cover the heating surfaces the milk inlet is partially closed in order to maintain constant level of milk. During operation heat is so applied as to ensure that the milk boils vigorously and a working vacuum 63-65 cm is attained. The speed with which the milk is condensed to the desired consistency depends upon the following factors:

a) Quantity of milk in the pan

b) Area of the heating surface

c) Capacity of the vacuum pump

d) Temperature of condenser

Striking the batch: The term refers to the end of concentration operation in the vacuum pan as determined by specific gravity or density tests. The precise point at which the batch should be “struck” depends largely upon the experience of vacuum pan operators. Samples are taken at regular intervals by means of the sampling cocks with which the pans are provided. Baume’hydrometer or a viscometer is used to determine the progress of concentration. Baume’Hydrometer test is most commonly used for density tests of vacuum pan samples.The hydrometer scale may record the density either directly or indirectly. For condensing milk the Baume’Hydrometer ranges from 30-37° be at 49°C (120°F). When the desired concentration as indicated by the Baume’ reading (32°Be’) is reached, the batch of milk is ready for removal from the pan. This operation is called striking the batch. The steam is shut-off, the vacuum broken and the milk run off to cooling tank.

vii) Cooling: Cooling of condensed milk is essential to control the texture of condensed milk. Sweetened condensed milk is a highly concentrated solution of lactose and sucrose. The quantity of sugar which milk can carry in solution varies according to the temperature of milk. As the condensed milk is cooled, lactose which is less soluble than sugar crystallizes out and if the lactose crystals are small enough these crystals will not be detected but if they are too large they will impart sandy texture to condensed milk. If the crystal size of lactose grows bigger during cooling or during subsequent storage, gritty texture becomes apparent in sweetened condensed milk. To avoid this defect it is necessary to create conditions favorable for maximum crystallization of lactose.

viii) Forced crystallization: The purpose of this is to produce mass crystallization of lactose. It is the period in the cooling process when sweetened condensed milk reaches a temperature, which is most favorable for rapid crystallization of lactose.For sweetened condensed milk of average composition the temperature of maximum rapidity of crystallization is around 30°C. This temperature is optimum for seeding of condensed milk.

Seeding: Crystallization of lactose is usually assisted by the addition of fine powder of lactose or small quantity of condensed milk from previous batch in which the lactose crystals are in minute form. Other alternative is to use spray dried skim milk powder as seed material. Seeding refers to the introduction of lactose in a very fine powder form during cooling process to provide nucleic for crystallization. The purpose is to give lactose, present in the super saturated state, an added incentive to crystallize. Seeding at an optimum temperature for mass crystallization with properly prepared lactose and with vigorous agitation yields large number of small crystals of uniform size. The recommended amount of finally pulverized lactose for seeding purpose is 375-500 gm per 1000 kg of initial milk taken or 0.1 to 0.3 percent of condensed milk. For best result of forced crystallization the hot condensed milk should be cooled from pan temperature as rapidly as possible to the seeding temperature.

Method of Adding Seed lactose: The required amount of seed lactose is blended with small quantity of condensed milk and added to the batch with vigorous agitation. After seeding and forced crystallization, further cooling of the product is continued slowly to 24-25°C for about an hour or longer when the product is ready for packaging. The packed condensed milk is then stored until further use.

Cleaning and Sterilization of Plant: As with all other plants used in the preparation of food products the equipments used in the production of condensed milk should be cleaned and sterilized immediately after use. All internal surfaces of the plant should be washed first with cold water, then with detergent solution, hot water and finally sterilized by means of live steam. If cleaning-in-place (CIP) system is available,recommended procedures should be followed for cleaning and sterilization of the plant.

Sweetened Condensed Skim milk: The manufacture of sweetened condensed skim milk is similar in all respect to that of sweetened condensed whole milk.