CONDENSED AND DRIED MILKS

Module 8. Defects in condensed and evaporated milks

Lesson 19
DEFECTS IN SWEETENED CONDENSED MILKS

19.1 Introduction

The major defects that are characteristics of unmarketable sweetened condensed milk are discussed here under:

19.2 Sandiness

The relative smoothness of texture of sweetened condensed milk is determined mainly by the size of the lactose crystals it contains. Sandiness is due to the presence of relatively large, coarse crystals. For a product of the desired smoothness, the crystal should average ≤10µ in length.

19.2.1 Sandiness due to sucrose

When the sucrose ratio of finished product exceeds 64.5%, it approaches the status of a saturated solution of sucrose. When such condensed milk is subjected to low temperature such as in cold storage or otherwise on its journey from factory to consumer, some of the sucrose that is present in excess of saturation will crystallize. They give the condensed milk a coarse, sandy texture. Incomplete solution in the pan has also been found to be a contributing factor of sandiness.

When sweetened condensed milk that has been allowed to become too cold for packaging is re-warmed in the presence of agitation, the development of sandiness is unavoidable. Vigorous agitation while warming tends to cause the smaller lactose crystals to redissolve. This increased super-saturation promotes diffusion of the dissolved lactose. The numbers of crystals present is thus much reduced, so the remaining crystals have relatively much dissolved lactose to grow, forming sufficiently large aggregates to give the milk an objectionable sandy character as can be seen from Table 19.1.

Table 19.1 The quality of sweetened condensed milk and the numbers and size of lactose crystals

19.2.2 Sugar sediment or sugar down

Deposits of sugar sediment on the bottom of can or barrel of sweetened condensed milk are not an uncommon occurrence. The microscopic examination as well as chemical analysis shows such sediments to consist predominantly of lactose crystals. The main cause is due to the difference in specific gravity between the crystallized lactose and the remainder of the condensed milk. The specific gravity of lactose hydrate at 68°C is 1.5453. The average specific gravity of sweetened condensed milk is 1.3085.

It is reported that sugar sediment in sweetened condensed milk of a given viscosity increases or decreases respectively with an increase or decrease of the crystal size. Condensed milk of normal viscosity in which the lactose crystals do not exceed 10µ in length generally remains free from objectionable sugar sediments.

19.2.3 Effect of viscosity on sugar sediment

The greater the viscosity, greater is the resistance to the force of gravity. Hence lesser will be the ability of the lactose crystals to drop to the bottom. In many cases of sediment trouble, the finished product is definitely too thin to hold the crystals in permanent suspension. In such cases reasonable increase in concentration may avoid sediment, provided that crystal size is dependably controlled.

19.2.4 Effect of homogenization

Homogenization increases the viscosity of milk. This in turn decreases the tendency for the finished product to drop the lactose crystal. This effect increases with increasing pressure.

19.2.5 Effect of Super Heating

Due to super heating, there is swelling of casein which gives the product more body and tends to retard the sugar sediment.

19.3 Age Thickening

It is a common defect of sweetened condensed milk. It may be due to:

1. Bacterial Activity

2. Physico - chemical reactions

Bacterial thickening is caused by extremely heavy bacterial contamination with the causative species of micro-organisms, which results from lack of efficient sanitary supervision in manufacture. It can be prevented by proper sanitation and efficiently controlled forewarming temperature. In the presence of causative organism; it can be controlled by increasing the sucrose in water ratio to nearly 64.5 %.

The exact reactions involved in Sweetened condensed milk age thickening due to physico-chemical causes have not yet been fully explained but it may be due to:

1. Hydration of proteins mainly the casein which adsorbs water and thickening is increased

2. The influence of season on age-thickening is very marked.

3. Proper adjustment of fore-warming temperature prevents the defect age thickening.

19.3.1 Effect of sucrose content

An increase of the sucrose in water ratio decreases the tendency to age thickening. An increase in addition of sucrose to the fresh milk used in the manufacture of sweetened condensed milk reduces the viscosity of the concentrated milk and retards or inhibits age thickening. The ratio should be adjusted according to the seasonal stability or instability of the product.

19.3.2 Effect of time of adding the sucrose

Fore warming the milk and sugar syrup separately, cooling to pan temperature (55°C), then mixing sugar syrup and milk and drawing the mixture to pan decreases the tendency of age thickening. Adding sugar near the end of the condensing period tends to cause age thinning and may cause fat separation.

19.3.3 Effect of pan temperature

Condensing the milk at high temperature tends to destabilize the product and increase the tendency to age thickening. Lowering of the pan temperature near the end of the condensing period is effective in lessening the viscosity and thus decreasing the tendency to age thickening.

19.3.4 Effect of SNF on age thickening

An increase in the % of SNF and especially in the ratio of SNF–in-water increases the tendency to age thickening. So, SNF content should be adjusted before processing.

19.3.5 Effect of ratio of fat to SNF

The reasonable ratio should be provided such as 8.5% fat to 19.5% SNF provided by the U.S. Standard which has tendency to ameliorate the effect of influences that cause colloidal swelling of the proteins. The fat lessens the intensity of reaction. It acts as a softener and dilutant of the protein suspension, yielding product with plain and smoother texture than the product made from skim milk. The age thickening tendency is more in case of sweetened condensed skim milk.

19.3.6 Effect of Concentration

High concentration – high viscosity – more age thickening. Too low concentration – Fat separation and sugar deposits.

19.3.7 Effect of acid reaction

Fresh and good sweet milk has good stability and good keeping quality of condensed milk. If acidity is lowered by the addition of an alkali, there is improvement in storage stability of the product. Addition of ~ 0.2 – 0.4 kgs of NaHCO₃/100 kgs of raw milk is sufficient to stabilize the milk that is unstable towards age thickening.

19.3.8. Effect of storage temperature

In case of unstable milk, the age thickening tendency increase with the storage temperature. At 15°C, if the product is stored, there is decrease in age thickening. At 21°C the tendency of age thickening increases. It is observed that the rate of thickening approximately double with a rise in temperature of every 6°C.

19.4 Flavour Defects

19.4.1 Rancid flavour

It is not a frequently occurring defect of sweetened condensed milk. It can be controlled by taking following precaution:

(1) Fore warming the fresh milk at temperature above 76°C.

(2) Avoid leaks of raw milk in to the batch during fore warming and between the forewarmer and vacuum pan.

(3) Do not add sugar during fore-warming process.

(4) Use unpolluted water and keep all the equipments in sanitary condition.

19.4.2 Metallic flavour

It may be caused due to the use of copper equipment. It may give pickery coppery taste of copper salts. Use stainless steel equipments.

19.4.3 Tallowy flavour

It is due to the autoxidation of milk fat. Now-a-days it does not occur, but it may occur if the finished product is exposed for a considerable period of time to air and light and in plants that are still using copper vacuum pans and copper forwarmer.

19.4.4 Fat separation

It is the result of abnormally low viscosity and thin body. In such milk at rest, there is definite tendency of some of the fat to rise to the surface forming a layer that has deeper yellow colour than fresh milk. This defect can be controlled by proper processing and proper concentration of total solids.

19.4.5 Dark colour / Brown discolouration

The immediate causes of the discoloration are the intense heat treatment during fore warming and high storage temperature. At low temperature, the colour is not noticeably affected by aging.