CONDENSED AND DRIED MILKS

Module 19. Microbiological quality of various dried milks including infant foods

Lesson 47
MICROBIOLOGICAL QUALITY OF VARIOUS DRIED MILKS INCLUDING INFANT FOODS

47.1 Introduction

Microbial and enzymic deterioration are rare in milk powder. However some changes related to these might be observed in dried milks.

47.2 Effect of Water Activity

Growth of microorganisms can strongly depend on water content of a food. For microbial deterioration to occur, a _w should increase to over 0.6 (and for the majority of microorganisms much higher); such a high a _w can only be reached if the powder is exposed to fairly moist air. Deterioration then is often caused by molds. Enzymatic hydrolysis of fat has been observed at a _w ≥ 0.1, although extremely slow. Accordingly, whole milk powder must be free of lipase. Milk lipase will always be inactivated by the intense pasteurization of the milk as applied in the manufacture of whole milk powder. This is by no means ensured, however, for bacterial lipases. Hence, not too many lipase-forming bacteria should occur in the raw milk. Proteolysis in milk powder appears highly improbable and has not been reported.

However, enzymic deterioration of liquid products made from the milk powder can occur if enzymes are present before the drying, as drying usually does not cause substantial inactivation of enzymes. Several authors relate the possibilities of growth of microorganisms to the water activity, and the lowest values at which organisms can grow are shown in Table 47.1.

Table 47.1 Lowest values of water activity at which organism grows

A simple explanation is that a low value of a _w implies a high value of the osmotic pressure. The organism is unable to tolerate high osmotic pressure as it will cause water to be drawn from the cell, damaging its metabolic system, generally because the internal concentration of harmful substances becomes high. However, microorganisms have acquired various mechanisms to neutralize the effect of a high osmotic pressure and the effectivity of these vary greatly among organisms:

• A fairly small difference in osmotic pressure between t he cell and the environment can often be tolerated. Bacterial spores can tolerate a considerable difference in osmotic pressure.

• Some solutes, e.g., most alcohols, can pass the cell membrane unhindered; hence, these do not cause an osmotic pressure difference. If such a solute is, moreover, compatible, which means that it is not harmful to the cell at moderate concentration, the organism can survive and possibly grow.

• The organisms may produce and accumulate low-molar-mass compatible substances that keep the internal osmotic pressure high; it often concerns specific amino acids.

• Specific harmful substances, e.g., lactic acid, may be to some extent removed from the cell.

Consequently, it often makes much difference what solute is involved; in other words, the lowest value of a _w tolerated strongly depends on the composition of the medium.

47.3 Effect of Hygienic Aspects

The requirements for the bacteriological quality of milk powder partly depend on its intended use and, in connection with this, also on the manufacturing process. For example, whether the powder is meant for direct consumption or whether it is subjected to a heat treatment after reconstitution (e.g., for recombined milk) is important. The heat treatment during the manufacture of (skim) milk powder, classified as ‘low heat’, usually is not more intense than the heat treatment during low pasteurization (say, 72°C for 20 s); consequently, many bacteria may survive the manufacturing process. The causes for milk powder to be bacteriologically unacceptable or even unsafe can be of three kinds:

• In the fresh milk, bacteria are present that are not killed by the heat treatments to which the milk is subjected before and during drying.

• Conditions during the various process steps until the product is dry do allow growth of some bacterial species.

• During manufacture, incidental contamination may occur. The level of contamination is generally low and remains low if the bacteria involved cannot grow.

In establishing the bacteriological quality of the powder, the species of bacteria involved should be considered. Then the cause of the contamination may be deduced, as may the measures that must be taken to improve the quality.

47.4 Sampling and Checking for Microbiological Quality

Bacteria originating from contamination or growth prior to drying will usually be homogeneously distributed throughout the powder and cause no problems with sampling. This is different for bacteria originating from incidental contamination of the powder, which may be distributed quite inhomogeneously. It is not possible to devise sampling schemes that guarantee detection of incidental contamination. To ensure that a product is bacteriologically safe, not only should the powder be sampled but samples must also be taken at sites that are potential sources of contamination.

47.5 Growth of Bacteria During Manaufacture

• Temperature and water activity during successive steps in manufacture are such that some thermophilic bacteria can readily grow and they are not or insufficiently killed during drying. The type of bacterium often is characteristic of the cause of the contamination.

• In the regeneration section of pasteurizers and thermalizers (and possibly in the part of the evaporator plant where the milk is heated in counterflow), S . thermophilus, in particular, can develop. The bacterium grows fastest at 45°C but scarcely multiplies at temperatures over 50°C. It generally does not grow in the evaporator, because the temperature is too high in the first effects, whereas in the later effects a w will be too low. Because S.thermophilus is moderately heat resistant, relatively high counts may result, especially in low-heat milk powder and in whey powder. In medium and high-heat powder the bacterium is killed during manufacture. Determination of the count of S .thermophilus in the milk just before the preheater may give a good indication of the fouling of the heating section of plant and of the moment at which it should be cleaned.

• In some drying plants that employ a wet washer to recover powder fines, the outlet air is brought into contact with a film of milk rather than water, thereby preheating the milk and saving energy; this implies that the preheated milk acquires the wet bulb temperature (~ 45°S . thermophilus . C), which leads to ideal growth conditions for

• The conditions in the second half of the evaporator and in the balance tank are not optimal for S . thermophilus . Enterococci ( E . faecium ), in particular, will start to grow. If the milk is properly preheated and the plant is satisfactorily cleaned and sterilized, it will take a rather long time, however, before substantial counts of E . faecium have developed. In actual practice, these prerequisites are not always met and in milk powder with a relatively high count, E . faecium often is the predominant species.

• Likewise, the conditions in the second part of the evaporator and in the balance tank are favorable for growth of Staphylococcus aureus. The bacterium generally is killed by pasteurization, and strains of S.aureus in milk powder have been shown to have phage characteristics different from the strains in raw milk. Presumably, they originate from direct or indirect human contamination. Heat stable enterotoxins can be formed and the amounts formed at counts of 107 to 108 per ml may cause the powder to be a health hazard. Although S . aureus is not heat resistant, the conditions during drying appear to be such that complete killing is not achieved. Roughly 10 ^{− 5} to 10 ^{− 1} of the initial count of these bacteria have been found to survive under various practical conditions. This means that S . aureus can at least be found in 1 g of fresh powder if its count before the drying was so high that production of enterotoxins could have occurred.

• Bacillus stearothermophilus (ssp. calidolactis in particular) can readily grow at higher temperatures. Its growth range is from 45°C to 70° C, with an optimum near 60°C. It can also grow in concentrated milk and therefore throughout the equipment between preheater and drier. Moreover, the bacterium can form spores under these conditions, which further limits its killing during drying. Obviously, some growth of B . stearothermophilus will always occur during manufacture, even in a well-cleaned and disinfected plant. However, under most conditions this will not cause problems.

47.5.1 Measures to control the growth of bacteria

With regard to the growth of bacteria during the manufacturing process, a number of measures will have to be taken to prevent the equipment from becoming a kind of fermentor for bacteria:

• Special attention should be paid to the temperature and the time for which the product stays in the equipment, and to matching of the capacities of the various parts of the plant. Multiple-effect falling-film evaporators are more satisfactory in this respect than are flash evaporators.
  

• It is recommended that the volume of the concentrate balance tank be kept as small as possible. Mostly, there are two tanks, making possible a change every 2 h or so, and the cleaning of one while the other is in use.
  

• Contamination of the product flow from outside should be prevented, with particular reference to S . aureus .
  

• The concentrated milk may be pasteurized just before it enters the drier, and this is increasingly being done. It is especially successful with regard to the killing of E . faecalis and E . faecium , provided that the temperature applied is high enough. Heating for 45 s at 72°C has little effect; 45 s at 78°C causes a considerable reduction.

47.6. Incidental Contamination

A distinction can be made between contamination before drying (wet part) and during or after drying (dry part). Bacteria involved in these types of contamination generally do not grow during the process and contribute little to the count of the powder.

Contamination after preheating and before drying can readily occur if the equipment has been insufficiently cleaned. This is only important if the bacteria involved can survive the drying (and the pasteurization before drying, if carried out). In spite of the high inlet and outlet temperatures of the air, the concentrate droplets usually will not attain a high temperature; moreover, the heat resistance of the bacteria increases markedly with the dry-matter content. About 70% of E . faecalis and E . faecium survive during drying, whereas survival of S . aureus varies widely. About 10 ^{− 4} to 10 ^{− 5} of the initial count of Salmonella spp. and E . coli will survive. Based on these facts, and due to the relatively low level of contamination, the powder may be expected to contain no appreciable counts of enterobacteria immediately after leaving the drier. This is confirmed in actual practice.

Contamination of the powder can occur at many places — in the spray drier, during fluid bed drying, and during packaging. The species of contaminating bacteria can vary widely, but it usually concerns species that can grow in wet remnants of milk powder in the drier or in the surroundings of the manufacturing line. Contamination via (in) direct human contacts should also be considered (e.g., S . aureus ). Bacteria can easily survive in dry powder, and undesirable bacteria can start to grow if the water content increases to over 20%. The supply of cooling air into the spray drier and into the fluid bed drier can be a source of direct contamination. It may also be responsible for indirect contamination because it gives, at certain sites, better conditions for survival and growth of bacteria in remnants of not fully dried powder. Special precautions are needed if the drier and its accessories have been wet-cleaned. To restrict such incidental contamination, the plant and its surroundings should be rigorously freed of remnants of (wet) powder.

Among the kinds of bacteria found in this type of contamination, enterobacteria are of special interest. It generally concerns coliforms; this is possibly caused by lactose being the only carbon source in this environment. Nevertheless, the use of coliforms as an indicator organism is of restricted value for milk powder: If coliforms are absent, salmonellae or other pathogenic bacteria may still be present.