CONDENSED AND DRIED MILKS

Module 9. Microbiological qualities of Sweetened condensed and evaporated milks

Lesson 21
MICROBIOLOGY OF SWEETENED CONDENSED MILK

21.1 Introduction

Sweetened condensed milk is not sterile. It contains living microbes and spores. The low water activity (~0.83) or, rather, the high sugar content prohibits growth of most but not all microorganisms.

Deterioration usually occurs by osmophilic yeasts, most of which belong to the genus Torulopsis . The yeasts often cause gas formation (bulging cans), a fruity flavor, and coagulation of protein. Coagulation may result from ethanol production. As a result, the product becomes unacceptable. The yeasts do not start easily, especially if the sugar concentration is high. It may thus take several weeks for incipient growth to be perceptible.

Some micrococci may grow in sweetened condensed milk, although slowly, especially if water activity and temperature are high. Presumably, the presence of oxygen is required. It may happen that they grow to reach a colony count of, say, 10⁵ ml ^{− 1} and then stop growing, without causing noticeable defects. If they keep growing, coagulates eventually form and several off-flavors develop.

Some molds, especially strains of Aspergillus repens and A. glaucus , can grow as long as oxygen is present. If so, fairly firm colored lumps are formed and an off-flavor develops. One spore in one air bubble can cause such a lump.

Obvious remedies for microbial spoilage include the killing of all saprophytes and mold spores in the milk and in the sugar. Bacterial spores cannot germinate in sweetened condensed milk. Growth of harmful microorganisms in the dairy plant should be rigorously avoided. No sugar and residues of the milk should be left about. Satisfactory hygienic standards must therefore be maintained, especially in the packaging room. Harmful microorganisms cannot grow during concentrating, but the machinery must be thoroughly cleaned, immediately after evaporation. Mold spores can be removed by air filtration. The packaging machine should fill the cans very accurately with a safety margin of 1 g. Too little condensed milk in the cans means that more air is left, which increases the chance of growth of molds and micrococci. If the cans are overfilled, the milk may spill over the side and encourage growth of osmophilic yeasts.

21.2 Spoilage Due to Bacteria

Sweetened condensed milk has better keeping quality than plain condensed milk because high sucrose concentration retards quality damaging bacterial development. The bacterial count has been found to increase with age in some cases and decreases in others. In normal milk, these changes are very gradual, depending considerably on the temperature of the storage. In the case of heavy contamination leading to definite bacterial defects however, the rate of increasing of the causative organism is greatly expended. The chief bacterial defects observed in sweetened condensed milk are:

21.3 Mold Button Formation

This defect is generally absent in fresh product. The number and size of button will depend upon the type of causative organisms, age of the product and temperature of the storage and time. The button formation is mostly due to the mold Aspergillus repens. Buttons in sweetened condensed milk have also been traced to the molds.

21.3.1 Aspergillums glalicus

The growth of mold is stopped as soon as oxygen is removed or decreased. But the size of the colonies will go on increasing because the enzyme produced by mold continues activity after the death of the mold. Due to this defect in condensed milk we may find lumps of variable size, cheesy consistency and whitish yellow to reddish brown colors. They constitute firm, self contained units that do not emulsify into the body of the milk. The buttons themselves have a stale cheesy taste and they develop in the milk a stale odor. These molds do not survive the customary range of forewarming temperatures used. The appearance of buttons is due to the contamination after forearming.

Control

Proper sanitation and following precautions in manufacturing and packing avoids recontamination of the product.

1. Keep as minimum air space as possible in tins

2. Seal the product in vacuum of 51cm, below this mold will not grow.

3. Store the product at 15°C or below.

4. Inversion of condensed milk tins at regular intervals.

21.4 Bacterial Age Thickening

It is very common defect in storage. In most cases, this may be due to physico-chemical changes. However, there are certain micro-organisms which may cause age-thickening. These problems arise when raw milk is of very high count. When sweetened condensed milk has age thickening defect due to micro organisms, then taste of the product is changed and often acidity is increased. It is due to micro organisms of bacillus type, which may survive on heat treatment. There are both acid producers and enzyme producers which involve in age thickening. The thickening is invariably due to cocci type of organism. They are:

1. Micrococcus lactic albidus

2. Micrococcus freudenreichii

This defect is usually accompanied by a high germ count, a disagreeable stale and often cheesy flavor and taste. When the thickened milk is diluted with water and heated, the curd separates. Large number of S. pyogenes, Staphylococcus aureus, B. subtilis and Bacillus megentricus and yeasts are also present in such milk. It is observed that the acidity factor is non-important and the thickening is due to the production of rennin like enzyme.

Control Measures

1. Elimination of sources of contamination by cleaning and sterilization.

2. Selecting milk of good quality

3. Raising sucrose concentration. The sugar ratio should not exceed 64.5 %, other-wise it may involve problem of sucrose crystallization.

4. Storing at low temperature

21.5 Bloat

This defect is mainly due to gaseous fermentation and this defect is considered from consumer’s point of view. It is a severe defect, people will not like to have even if little defect is developed. In case of canned milk, resulting pressure is so much that it may cause ends to bulge out and in extreme cases, it may burst out. In case of barrel, the head may be blown off. In case of sweetened condensed milk the aerogenous type of bacteria and certain type of yeast mainly named as Torula lactis condensi ferments the sucrose forming alcohol and CO₂. Many investigators found the causative micro organisms to be budding yeast capable of fermenting sucrose in concentrated solution.

21.5.1 Mechanism of gaseous fermentation

The sucrose is inverted by the enzyme invertase contained in and secreted by yeast cells. In this inversion, the molecule of sucrose takes on one molecule of water and breaks down into 2 molecules of monosaccharide. The monosaccharides are readily fermented by this yeast.

Yeasts cells require O₂ for metabolism and growth. They must be present in invigorated form and in sufficient numbers to expedite the air utilization in the condensed milk and in the sealed container before this limited O₂ supply is exhausted by micro-organism other than sucrose fermenting yeast. Contamination must be sufficiently heavy to make available enough invertase to initiate inversion at once. When once started, inversion and fermentation proceed rapidly at or above room temperature.

Concentrated sucrose solution because of its relatively high osmotic pressure, tend to retard, if not prevent, the activity of microorganism. This effect is intensified with increasing concentration. A minimum sugar - in water ration of 62.5% will hold in check most of the species of micro organisms that are normally present in sweetened condensed milk produced under reasonable average attention to temperature control in manufacture, factory sanitation and quality of supplies. Under such conditions, danger of gaseous fermentation is remote. But not so in the presence of heavy contamination with invigorated sucrose fermenting yeast. Here, the sucrose concentration fails to prevent the development of bloats even when the sucrose is present in saturated solution. As the yeasts are generally insufficiently resistant to the pre-heating temperatures applied, their presence must be due to contamination. It has been found that sugar may be source of contamination, especially when moist.

Controls

1. The most likely and most direct cause of bloat of sweetened condensed milk is contaminated sugar. Keep the sugar sealed in its original package until needed.

2. Use barrels in preference to sacks. Keep it protected from dampness and insects such as bees, flies cockroaches. Insects plus dampness cause inversion and fermentation.

3. Adding the sugar to the milk in the pan in the form of concentrated syrup in boiling hot water (~ 65% sugar-in-water) is a dependable safe guard.

4. Sanitation of all equipments, from hot well to filling mechanism, and protecting the condensed milk from prolonged exposure to air by filling into the final container as soon as possible after manufacture, are important additional precautions. Container should be filled as full as reasonable allowance for heat expansion will permit.

5. Keep less air space.

6. Store below room temperature.

21.6 Flavour Defects

The usual bacterial flavor defects encountered are rancid, stale, tallowy etc. Generally it is due to hydrolysis of fat.