18 Technology of concentrated milk products –Evaporated milk, Sweetened condensed milk

INTRODUCTION

In order to preserve the milk constituents in a concentrated form, condensed milk/concentrated milk is manufactured from whole or skim milk by evaporating part of water under vacuum. Its main purpose is to diminish the volume and to enhance the shelf life. But this form of milk still has limited shelf life. Attempts to prolong the shelf life of this product have been made through theaddition of sugar (sweetened condensed milk) or byheat processing (evaporated milk) to destroy the microorganisms.

HISTORY

• The evaporation of milk has been known for many years, even as early as in the year of 1200 when Marco Polo described the production of a paste like milk concentrate in Mongolia.
• The present day concept of preservation of milk by concentration and by application of heat had the beginning with the pioneering work of Frenchman Nicholas Appert around 1793.
• Gail Bordon of United States of America was first to commercially exploit the method of concentration of milk under vacuum. He was awarded U.S. Patent in the year 1856 for processing milk in a semi fluid state after evaporating under vacuum. Commercial production of sweetened condensed milk by Gail Bordenstarted in1856.By 1858 Borden’s milk, sold as Eagle Brand, had gained a reputation for purity, durability and economy.
• In 1864, Gail Borden’s New York Condensed Milk Company constructed the New York Milk Condensery in Brewster, New York.
• The basic principle of the process of sterilization by heat was introduced by Mr. John B. Meyenberg – a native of Switzerland. Jhon. B. Meyenberg migrated to United States was awarded a patent in 1884 for a process of sterilization of milk by steam under pressure. In the year 1885 evaporated milk was manufactured on commercial scale in United States based on the patent of Meyenberg.

LEGAL DEFINITION

The condensed and dried milks are manufactured by the commercial plants as per the legal standards prescribed by FSSA, 2006 (The Food Safety and Standards Act, 2006; Rules and Regulations, 2011)

Evaporated Milk– means the product obtained by partial removal of water from milk of cow and/ or buffalo by heat or any other process which leads to a product of the same composition and characteristics. The fat and protein content of the milk may be adjusted by addition and/ or withdrawal of milk constituents in such a way as not to alter the whey protein to casein ratio of the milk being adjusted. It shall have pleasant taste and flavour free from off flavour and rancidity. It shall be free from any substance foreign to milk. It may contain food additives permitted in these regulations including Appendix A. It shall conform to the microbiological requirements prescribed in Appendix B. It shall conform to the following requirements:

Table 1:The Food Safety and Standards Act, 2006 Requirements for Evaporated Milk

Sweetened Condensed Milk– means the product obtained by partial removal of water from milk of Cow and/or Buffalo with the addition of sugar or a combination of sucrose with other sugars or by any other process which leads to a product of the same composition and characteristics. The fat and/ or protein content of the milk may be adjusted by addition and / or withdrawal of milk constituents in such a way as not to alter the whey protein to casein ratio of the milk being adjusted. It shall have pleasant taste and flavour free from off flavour and rancidity. It shall be free from any substance foreign to milk. It may contain food additives permitted in these regulations including Appendix A. It shall conform to the microbiological requirements prescribed in Appendix B. It shall conform to the following requirements:

COMPOSITION

The gross composition of different types of condensed milk is given in Table 3.

PRINCIPLE OF EVAPORATION

Evaporation is a unit operation which involves simultaneousheat and mass transfer. Evaporation means the removal of solvent as vapour from asolution or slurry under such conditions that no attempt is made to separatecomponents of the vapour. In majority of evaporation process the solvent is waterand latent heat for evaporation is supplied by condensing steam and heat from thesteam is transmitted to the solution indirectly through the metallic surfaces.The unit in which heat transfer takes place is called heating element or calandria.An effect is one or more bodies boiling at the samepressure. A multiple effect evaporator is one in which vapour from one effect isused as heating medium for another effect boiling at a lower pressure.

BASICS OF EVAPORATORS

Vacuum Pan

This type of evaporator is employed in the majority of condenseries for the manufacture of condensed milk. Batch operating type evaporator is preferred when relatively small quantities of milk are to be handled in a batch operation for product like sweetened condensed milk.

The pansare internally fitted with steam coils and the lower portion is steam jacketed.The operation is usuallycarried out at 54-63°C, the milk boils at this temperature owing to reducedpressure (63.5 cm of mercury) which is maintained in the pan. A vacuumpump is used to maintain the vacuum in the pan. When the water vapour, which rises fromboiling milk inside the pan, comes in contact with cold metal surface orcold spray of water in the condenser, it is condensed and carried off aswater by means of pumps or other means.

Falling Film Evaporators

This type of evaporator consists of verticaltube bundles heated by condensing steam on the outside. Theliquid to be evaporated is fed to the top of the tube and allowed to flowdownwards on the inside of the tubes. The liquid will flow downwards forming a thin film, from which the boiling/evaporation will take place because of the heat applied by the steam.The resulting vapourand concentrated liquid flow into a separator at the foot of the evaporator.The vapour is drawn off from the top of the separator and liquid from thebottom by a pump.This type of evaporator is desirable from a product point of view, as it offers a short holding time.

As vapour, from the evaporated milk contains almost all the applied energy, it is obvious to utilize this to evaporate more water by condensing the vapour. This is done by adding another calandria to the evaporator. This new calandria – the second effect – where the boiling temperature is lower, now works as condenser for the vapours from the first effect, and the energy in the vapour is thus utilized as it condenses.

Multiple effects

In the first evaporation unit, steam is introduced and part of the water is evaporated from the milk. The steam condenses, and vapour is separated from the concentrated milk. The vapour is used to evaporate water from the concentrated milk in the second effect, etc. The flows of milk and vapour are co-current. The number of effects varies from 3 to 7. Thus greateconomies of operation may be made in large installations with multiple- effect evaporator. The vapour coming from the last effect is condensed in a special condenser; the temperature of the water in the condenser determines the boiling temperature in the last effect. The boiling temperatures in the other effects are determined by the pressure drop of the vapour when being transported to the next effect.

SEPARATORS

As the vapours generated from the evaporation are used as heating media in the “next” calandria, any product must be separated, since it would otherwise contaminate the condensate and further represent a loss. The majority of the concentrate is discharged from the bottom of the calandria below the tube bundle. The separation is done in a separator with tangential vapour inlet, connected to the calandria below the tube. To reduce space requirements a new development has taken place with the design of the Wrap-Around Separator. It is integrated into the base of the calandria. It has the same high efficiency as the classical separator with a low pressure drop.

SWEETENED CONDENSED MILK

Sweetened condensed milks are milk products which can be obtained by the partial removal of water from milk with the addition of sugar, or by any other process which leads to a product of the same composition and characteristics. It may sometimes contain added refined lactose, calcium chloride, citric acid and sodium citrate, sodium salts of orthophosphoric acid and polyphosphoric acid not exceeding 0.3 per cent by weight of the finished product.

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.Different steps involved in the manufacture of Sweetened Condensed Milk are given in the Figure 1.

Receiving Milk:Raw milk received for the manufacture of condensed milkshould be of good quality. This is important because the quality and marketabilityof final product depends upon the initial quality of raw milk.As far as possible, milk should be fresh, sweet and should be free from off-flavors. It should not have sediments.

Fig.1 Manufacturing steps for sweetened condensed milk Filtration/Clarification:Milk after weighing should be clarified and filtered to remove visible foreign matter.Simple filtration may remove suspended particles by straining process while clarification removes the same by centrifugal sedimentation.

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 (9:22) 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.

Standardization of milkinvolves addition of sufficient quantity of cream or skim milk to ensure theresultant product has the correct fat: SNF ratio to yield the final product ofdesired composition.

Forewarming/Pre-heating:The standardized milk is Forewarmed / Pasteurized prior to condensing. Pathogens and potential spoilage organisms must be killed. Among the enzymes, milk lipase should primarily be inactivated; because if present, it can cause severe rancidity.

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. Forewarming also helps in dissolving the sugar which is added during thenext stage of operation. Further, evaporation takes place more rapidly if heated milk is fed into vacuum pan or evaporator during concentration.The temperature-time of forewarming 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 trendis towards high temperature short time heating such as 115-118°C withoutholding. Tubular heat exchangers are commonly used for fore warming orpreheating; either a double-tube or shelf-and-tube heat exchangers are preferred.Plate heat exchangers may also be used.

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.

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 sugar-in-water concentration of condensed milk.

The following formula is used for this purpose.

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 condensedmilk by % total milk solids in fresh milk. Then divide the percentage of sugar incondensed milk by ratio of concentration.

%TMSin condensedmilk

Ratio of concentration = ————————————

%TMSin freshmilk

%Sugar in condensedmilk

Sugar in milk =            ————————————-

Ratio to concentration

Condensing:Sweetened condensed milk is concentrated to a greater degree than is the case with the unsweetened variety.Concentration for manufacture of sweetened condensed milk is usually done by evaporation. 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.

Striking the Batch:The term refers to the end of concentration operation in thevacuum pan as determined by specific gravity or density tests. The precise point atwhich the batch should be “struck” depends largely upon the experience of vacuumpan operators. Samples are taken at regular intervals by means of the samplingcocks. Baume’ hydrometer or a viscometerisused to determine the progress of concentration. Baume’Hydrometer test is mostcommonly used for density tests of vacuum pan samples.

The hydrometer scale may record the density either directly or indirectly. Forcondensed 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’) isreached, the batch of milk is ready for removal from the pan. This operation iscalled striking the batch. The steam is shut-off, the vacuum broken and the milk runoff to cooling tank.

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.

Cooling of Sweetened Condensed Milk:Cooling of condensed milk is essential to control the texture of condensed milk. It is absolutely essential to cool the product promptly to desired temperature because in these steps, formation of large lactose crystals must be avoided. 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.To avoid this defect it is necessary to create conditions favorable for maximum crystallization of lactose. 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.

Crystallization:The purpose of this is to produce mass crystallizationof lactose. It is the period in the cooling process when sweetened condensed milkreaches a temperature, which is most favorable for rapid crystallization of lactose.The relative smoothness of product depends 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. For sweetened condensed milk of average composition the temperature of maximum rapidity of crystallization is around 30°C.

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. 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.Other alternative is to use spray dried skim milk powder as seed material. Seeding refers to the introduction of lactose in a very finepowder form during cooling process to provide nucleic for crystallization. Thepurpose is to give lactose, present in the super saturated state, an added incentive tocrystallize. Seeding at an optimum temperature for mass crystallization with properlyprepared lactose and with vigorous agitation yields large number of small crystals of uniform size. The recommended amount of finally pulverized lactose for seedingpurpose is 375-500 gm per 1000 kg of initial milk taken or 0.1 to 0.3 percent ofcondensed milk. 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.

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.

EVAPORATED MILK

Evaporated milk is sterilized, concentrated, homogenized milk. The product can be kept without refrigeration and has a long shelf life; it is completely safe for the user. 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. A major problem with sterilization is the heat stability; the higher the concentration of the milk, the lower its stability.

Manufacture of evaporated milk

Various steps involved in the manufacture of evaporated milk are depicted in Figure 2. Receiving milk/filtration/clarification: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. Variation of the quality of the milk has a direct effect on heat stability of evaporated milk and thus indirectly also influences the viscosity of the finished product. Standardization of Fat to SNF desired:The milk that has passed inspection is then dumped into the weigh tanks and after sampling and recording weights; it is dropped into supply tank from which it is pumped over or through a cooler into holding tanks. There, it is tested for fat and TS and standardized to the ratio of Fat to SNF desired in the evaporated milk.

Forewarming of Milk for Evaporated Milk:Preheatingserves to enhance the heat stability of the evaporated milk, inactivate enzymes, and kill microorganisms, including a significant proportion of the bacterial spores present. Here the destruction of biological agencies injurious to health or damaging to keeping quality, that might be present in the raw milk, is the function of the process of sterilization by steam under pressure. However, aside from this, the forewarming is indispensable for satisfactory vacuum pan operation also.

Temperature of Fore-warming:The heating temperature–time relationship is usually selected on the basis of heat stability. Formerly, a long heat treatment (e.g., 20 min) at a temperature below 100°C was often applied. Currently, UHT treatment is generally preferred. It reduces the number of spores in the milk considerably, and therefore a less intensive sterilization suffices.

Condensing of Evaporated Milk:The forewarmed milk is condensed to desired concentration in the vacuum pan or evaporator in a similar manner as done in the case of SCM. Evaporated milk boils at somewhat more vigorously and needs somewhat closer watching because of absence of the stabilizing influence of sugar.

Ratio of Concentration:On the bases of fluid milk with an average total solids content of 12.25%, the ratio of concentration of evaporated milk is applied as follows:

26.00

FSSA Standards Evaporated Milk =———— = 2.1224:1

12.25

Striking the Batch:The striking or sampling and testing for density of evaporated milk is more easily accomplished than that of viscous syrup SCM. When evaporated milk approaches the desired concentration, its consistency resembles that of rich milk or thin cream, its fluidity, therefore, ensures ready equilibrium of the hydrometer. The Baume’ hydrometer, with a scale of range from 0 to 15°Be’ subdivided into 1/10 degrees is commonly used for testing the pan sample. Establishment of standard striking temperature helps to avoid misleading results. A convenient temperature is the pan temperature which near the finish of the batch usually ranges from approximately 48.9 – 54.4°C.

Homogenization:Homogenizationserves to prevent creaming and coalescence during storage. It should not be too intensive because the heat stability becomes too low. Product should be homogenized at 49°C or above to avoid fat clumping and action of lipase enzyme. Two stage homogenizer is advisable at first stage 140 – 175 kg/cm2 and at second stage 35 kg/cm2.

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. 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 (e.g. 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.

Pilot Sterilization Test: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, forewarming, 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 is disturbed the heat stability may be affected. 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.

Canning: Packagingin 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.

Sterilization of Evaporated Milk Cans:In-bottle or in-can sterilization can be applied batch wise (in an autoclave) at 115°C for 15 – 20 min or continuously by UHT processing (135°C/1-3 sec). Storage:After sterilization cans of evaporated milk are unloaded from the sterilizer, and cooled by water spray. As evaporated milk is a sterilized product it may be stored at or below room temperature.

SUGGESTED READINGS

• Caric´, M. (1994). A general overview on an introductory level: Concentrated and Dried Dairy Products, VCH, New York.
• Compendium of Lectures delivered at refresher Course on “Drying of Milk and Milk Products” published by Sheth M.C. College of Dairy Science, Gujarat Agricultural University, Anand – 388 110 during 5 to 15 June, 1989.
• De Sukumar (1980). Outlines of Dairy Technology, Oxford University Press, Delhi.
• Hall, C.W. and T.I. Hedrick (1971). Drying of Milk & Milk Products – The AVI Publishing Co., Inc., Westport, Connecticut, USA.
• Hunziker, O.F. (1920). Condensed Milk and Milk Powder 3rd Edition, La Grange, Illinois, USA.
• Masters, K. (1991). General aspects of spray drying: Spray Drying Handbook, 5th ed., Longman, Harlow.
• Robinson, R.K. (1986). Modern Dairy Technology Vol-1. Advances in Milk Processing. Pub: Elsevier applied Science Publishers, London & New York.
• Vagn Westergaard (2004). Milk Powder Technology Evaporation and Spray Drying., Niro A/S, Copenhagen, Denmark, http://www.niro.com
• Walstra P, Jan T. M. Wouters and Tom J. Geurts (2006). Dairy Science and Technology, Second Edition Pub. Taylor & Francis Group, LLC, Wageningen, The Netherlands.