Lesson 18. AGE THICKENING AND GELATION OF CONDENSED MILK
Module 7. Physico-chemical properties of condensed milk
AGE THICKENING AND GELATION OF CONDENSED MILK
When evaporated milk is kept, its viscosity may initially decrease slightly. This may be explained in terms of the casein micelle aggregates formed during sterilization which changes from an irregular to a spherical shape, and as a result the effective volume fraction decreases. Subsequently, the viscosity increases, and it becomes strongly dependent on the shear rate, the milk displays a yield stress, and a gel is formed that firms rapidly.
The mechanism involved in the age thickening or gelation is not quite clear. However, in most cases, gelation is not caused by proteolytic enzymes or Maillard reactions, although the latter parallel the gelation. Also, gelation is not related to heat coagulation. It does not depend significantly on the pH, and its rate increases rather than decreases after lowering of the calcium content.
Electron microscopy reveals that thread-like protrusions appear on the casein micelles, which eventually form a network. It is likely that a slow change in the micellar calcium phosphate is at least partly responsible for the changes observed, but a definitive explanation is still lacking. Age thickening and gelation occur faster in UHT-evaporated milk. It may then be due to proteolysis caused by enzymes released by psychrotrophs, but also if such enzymes are absent, fast age gelation occurs. A more intense sterilization after evaporation delays gelation whereas it is faster in more concentrated milk and at a higher storage temperature.
It is observed that addition of sodium polyphosphate delays gelation considerably. Addition of citrate or orthophosphate often accelerates gelation, presumably because of binding of calcium. Polyphosphates may be hydrolyzed to yield orthophosphate, especially during heating. Consequently, addition of polyphosphate does not counteract gelation of in-bottle-sterilized evaporated milk.
Conventional evaporated milk only gels if kept for a long time at a high temperature. Extensive Maillard reactions also occur. Rapid gelation can occur if the evaporated milk before its sterilization is kept refrigerated at 4°C for a few days.
18.3 Sweetened Condensed Milk
The sweetened condensed milk is highly concentrated. Its mass concentration ratio is very high and because of this and the high sugar content, the product is highly viscous, i.e., η a is approximately 2 Pa . s, about 1000 times the viscosity of milk. The product is somewhat glassy in appearance because the fat globules show little light scattering as the refractive index of the continuous phase is almost equal to that of fat. The turbidity of the product is largely due to lactose crystals. Most of the lactose crystallizes because of its supersaturation.
18.4 Recombined Concentrated Milks
Viscosity of recombined EM is one of the important aspects in consumer acceptance which is decided by original powder and conditions of processing and stabilization. Age thinning is a problem when high storage temperatures are employed. High total solids and HTST method of sterilization give age thickening or gelation defect.
Viscosity, the most important property of recombined SCM, should be ~ 35 poise in fresh product which can increase up to 3 times, yet giving good acceptance. Too low viscosity poses air incorporation at filling and fat separation problems. Too high initial viscosity accelerates rate of age thickening.
Means to control viscosity of recombined SCM, in order of importance are:
2. Selection of milk powder,
4. Pasteurization and
5. Storage temperature.
The color of recombined EM varies with use of anhydrous milk fat, powder and level of heat treatment. Colour and flavour of recombined SCM are affected by raw materials used and severe pasteurization conditions employed. Homogenization is found to lighten the colour.
Flavour of the product can be improved by use of buttermilk powder due to higher phospholipids content. Holding the product for some period, at least for 7 days after production and before release, allows post manufacturing hydration of dry ingredients and normalization of flavour characteristics.
The most preferred flavour in recombined WMP results when fresh whole milk, commercial additives and fresh buttermilk powder contributing number of flavour compounds lost during the preparation of anhydrous milk fat are included in the mix. Incorporation of distilled monoglycerides or buttermilk powder provides best flavour stability on extended storage of recombined WMP. Powders with moisture level below 3% provide best flavour initially and during storage. Moisture content >5% leads to crystallization of lactose resulting in lumpy powder, increased free fat and increase in colour development within 2-3 months.
18.4.4 Fat separation
The efficient homogenization, absorption of casein on fat globule, product viscosity and minimum fat globule clustering controls the fat separation in recombined EM. High ambient temperature increases the defect. Addition of carrageenan gives best control of fat separation but have greater tendency towards gelation. Formation of a stable fat globule from anhydrous milk fat and SNF is very important for recombined evaporated milk.
Homogenization reduces fat globule size as well as increases viscosity thereby reduce fat separation tendency. Commonly employed pressures are up to 10 MPa, about 3.5 MPa being most common. Homogenization of reconstituted skim milk with milk fat (at 50 kg/cm2 or 150 kg/cm2) increases viscosity, while recombined SCM made using recombined cream (0.7-0.9 % protein, 20.2-20.9 % fat) age thickens slowly. Age thickening is also accelerated by higher storage period and temperature as well as by higher proteins and lower whey protein nitrogen with changed casein fraction. The pasteurization conditions can be used to control viscosity but is less convenient to use.
18.4.5 Lactose crystallization
The second important quality determining factor for recombined SCM is lactose crystallization which can be controlled by storage temperature and using dry seeding procedure.
18.4.6. Other factors important for quality aspects
• Fortification of recombined SCM with 1000-3000 IU of Vit-A and 30 mg of ascorbic acid is recommended.
• Holding period of up to 2 weeks before release of recombined SCM in the market is desirable to allow post manufacturing hydration of dry ingredients and normalization of flavour characteristics.
High temperatures preheat treatment of milk, as part of manufacture of full-cream milk powder; can reduce the susceptibility of product to oxidation during storage as a result of development of sulfhydryl compounds. However, such heat treatment is not always compatible with the development of required specific functional properties.