D CONTROL (concentrated milks)

D CONTROL (concentrated milks)

Summary

Significant hazards a r Cl. botulinum and bacilli.

Control measures

Initial level (H 0 ) r Low levels of pathogens in unprocessed milk are likely (see section on

initial microflora of raw milk).

Reduction (ΣR) r Heat treatments applied are sufficient to destroy vegetative

microorganisms. r Process milk according to HACCP principles with special attention to

the control of heating and aseptic filling. r Cleaning and disinfection after processing are essential.

Increase (ΣI ) r Post-process contamination may occur during filling and packing and during distribution if packaging materials are damaged.

r Growth of contaminating microorganisms during storage and

distribution.

Testing

r Routine testing for pathogens is not recommended. r Incubation tests to detect of deviations from Good Hygienic Practice.

Spoilage

r Spoilage may occur due to the growth of contaminating microorganisms (mostly mesophilic and facultative thermophilic bacilli) during

evaporation, reverse osmosis or ultrafiltration. Parameters are off-taste, off-odor, curdling/coagulation.

a In particular circumstances, other hazards may need to be considered.

Hazards to be considered Apart from the theoretical presence of Cl. botulinum, surviving bacilli in the product are of concern. Control measures Initial level of hazard (H 0 ). Low levels are likely, however quantitative data are missing. Reduction of hazard (ΣR). The reduction effect on microorganisms is very similar to the UHT-milk

procedure.

MICROORGANISMS IN FOODS 6

C, if not run under hygienic conditions, may lead to increases in numbers of spore-forming bacteria, which may affect stability of products. The sticky nature of sweetened condensed milk increases the difficulty of cleaning and residues may become a problem throughout the entire line, and in particular in the crystallization tanks or in the complex fillers, which are inherently difficult to clean adequately. Cans and lids, tubes, and other packaging material should be cleaned and sterilized before filling, as no heat treatment is applied to the product after filling. Critical to control is maintenance of the water activity below that allowing growth of surviving spores and post-processing contamination.

Increase of hazard (ΣI). Evaporators running at temperatures around 40–50 ◦

In the case of reverse osmosis and ultrafiltration, an important aspect in terms of microbial growth are the large surfaces of membranes offering an ideal support for the development of biofilms. Increases in cell numbers in the liquid phase may be due to the constant release of organisms or of fragments of the biofilm reflected in sudden increases in populations of microorganisms. Cleaning and disinfection are thus very important and must be adapted to the type of membranes used, bearing in mind that bacteria embedded in biofilms are more resistant to cleaning and more tolerant of sanitizing agents (Defrise and Gekas, 1988; Carpentier and Cerf, 1993).

Testing Routine testing for pathogenic bacteria is not recommended. In case of suspicion (e.g. incorrect

sealing of containers), the detection of vegetative bacteria may be useful to identify the defect. Integrity testing of containers may be part of the testing procedure.

Spoilage Microbiological spoilage concerns for concentrated and condensed milks, and for sweetened con-

densed milk with low sugar content are similar to pasteurized or UHT milk. The control measures in particular the integrity control of the containers are also similar.

VII Dried dairy products

Many milk products, including whole milk, skim milk, whey, buttermilk, cheese, and cream, may be dried by the application of heat. The manufacturing processes of dried dairy products are described in detail in Masters (1985), Knipschildt (1986), Caric (1994), Anonymous (1995a), Spreer (1998), and Bylund (2001).

Dried milk may be rehydrated and consumed directly, but more commonly, dried dairy products are used as ingredients in a number of products such as bakery, chocolate and confectionery, baby foods, ice cream, animal feeds, and culinary products.

A Effects of processing on microorganisms Before drying, milk is submitted to preliminary treatments such as clarification, standardization, and

heating (see Section III-A, IV-B-E). Two types of dried milk powder are manufactured by spray drying or roller (drum) drying. The spray-dried product has the better solubility. A further differentiation can

be made according to the intensity of the heat treatment applied. In the case of low-heat milk powders, the heat treatments correspond to a pasteurization; in the case of medium-heat powders, temperatures of 85–95 ◦

C for up to 30 s are used to obtain high-heat powders. Drying does not provide a controlled killing effect. The extent of microbial destruction during drying

C for 20–30 s are applied and temperatures above 120 ◦

673 drying or on the drum temperature and retention time for drum drying. Various vegetative bacteria,

MILK AND DAIRY PRODUCTS

including Gram-negative Enterobacteriaceae, survive the drying process (Daemen and van der Stege, 1982). Doyle et al. (1985) determined that L. monocytogenes also survives a typical spray-drying process. Therefore, dairy products for drying must be given a heat treatment equal to or greater than pasteurization, and the product must be protected against contamination between the pasteurizer, the dryer, and the packaging operations. After dehydration, the products will not support microbial growth.

Instant dry milk evolved from the desire for an easily soluble dried milk for use in beverages. Dried milks do not wet or disperse easily when added to water. The instantizing process rewets surfaces of dried particles in steam or atomized water droplets, causing them to agglomerate in clusters. The product is then redried to 5% moisture or less. The principal microbiological problems associated with instant dry milk occur upon accidental contamination during rewetting or after it is reconstituted, since the water activity of the dried product is too low to permit growth.

Subsequent processing steps such as cooling of the powder, intermediate storage, mixing, and pack- aging may influence the microbiological quality by recontamination from the line or the environment. During storage of dry milks, some vegetative microorganisms present may slowly die-off (Thompson et al., 1978) but others may survive over prolonged periods. Spore-formers, being the most resistant, retain viability indefinitely.

B Spoilage Due to the extremely low water activity (0.3–0.4) of dried dairy products, development of spoilage

organisms is not possible. However, presence of water derived from condensation on wet packaging material, may allow development of molds or bacteria and must be avoided. Spoilage may occur in reconstituted products similar to pasteurized milks (see Section IV-F).

C Pathogens Salmonella. Several outbreaks of salmonellosis have been traced to dry milk products. One of the first

nationwide outbreaks occurred in the United States during 1964–1965 (Collins et al., 1968; ICMSF, 1980) and was associated with non-fat dry milk that was produced in one plant and instantized in other factories. After an investigation in 156 plants located in 23 states, milk and environmental samples were found to contain salmonellae, initiating a number of improvements in processing and sanitation measures. Non-fat dry milk contaminated with S. Typhimurium and S. Agona occurred in Oregon (USA) in 1979 (Anonymous, 1979). Recommendations concerning preventive measures from different authors (e.g. IDF, 1991b; Burgess et al., 1994) and the implementation of HACCP have greatly contributed to improvements. Nevertheless, outbreaks in different countries are reported from time to time, including dairy based infant formulae, which are manufactured in a similar way (Becker and Terplan, 1986; Rowe et al., 1987; Gelosa, 1994; Usera et al., 1996; Anonymous, 1997a; Threlfall et al., 1998; Bornemann et al., 2002; Forsyth et al., 2003). These outbreaks demonstrated that failures in preventive systems, such as presence of water allowing multiplication of salmonellae, or zones that are difficult to maintain and to clean (isolation from a drying tower), were the origin of contamination, and that salmonellae with particular characteristics (lactose positive) were involved. Another small outbreak occurred in Canada in 1992 from infant formulae, produced in the United States, and resulted in various recall of products containing several lots of dry milk manufactured by the same company (Anonymous, 1993). Other outbreaks were reviewed by Mettler (1994).

Listeria monocytogenes. There have been no outbreaks of listeriosis linked to dry dairy products. Doyle et al. (1985) examined the fate of L. monocytogenes during spray drying and ambient storage

MICROORGANISMS IN FOODS 6

process and noted that, although the organism progressively died during storage, some samples tested positive for L. monocytogenes for up to 12 weeks. L. monocytogenes is, however, readily killed by the heat treatments applied before drying and presence in finished products is unlikely.

Proper heat treatments and prevention should preclude the presence of Listeria spp. in dry zones of powder processing plants. The difference in the incidence of L. monocytogenes or Listeria spp. can

be seen in the investigation of Gabis et al. (1989) in 18 plants manufacturing dry dairy products, all samples from the dry zones showed negative results. Surveys of the incidence of L. monocytogenes over more than 10 years have not detected it in dry milk products (Pak et al., 2002).

Staphylococcus aureus.

A very large outbreak causing more than 13 000 cases has recently been reported in Japan (Asao et al., 2003) due to preformed staphylococcal enterotoxin in the powder. This was traced back to poor hygienic and manufacturing practices during processing of liquid milk, in particular the storage conditions. Other cases of illness have been due to contamination and abuse of reconstituted products (Umoh et al., 1985; El Dairouty, 1989).

Bacillus cereus. The presence of B. cereus at low levels in dried milk has been reported (Becker et al., 1989, 1994), with over 60% of milk powder supplied in the U.S. positive for B. cereus. Although outbreaks of food poisoning due to B. cereus have not been directly attributed to dry dairy products, temperature abuse of the reconstituted product is of major concern. A review and assessment of the risks related to the presence of B. cereus in infant formulae has been published by the Food Standards Australia and New Zealand (2004).

Enterobacter sakazakii. Enterobacter sakazakii has been implicated in sporadic outbreaks causing neonatal meningitis and several of these cases have been associated with the consumption of con- taminated infant formulae. In several case studies, environmental contamination in the hospital and temperature–time abuse has been underlined as one of the main factors contributing to the outbreaks. Outbreaks caused by E. sakazakii have been reviewed by Nazarowec-White and Farber (1997), Lai (2001), and Iversen and Forsythe (2003).

The presence in milk powder of the potentially pathogenic Pandoraea norimbergensis was reported by Moore et al. (2001) but no outbreaks have been reported.

Mycotoxins. Occasionally, dried milk has been found to contain aflatoxin M 1 (Galvano et al., 1996). Although the amount of toxin present in fluid milk is reduced somewhat by the drying process, a significant percentage of it appears to survive the process and will survive for extended periods in the dry product (Marth, 1987). Other mycotoxins have not been found at any significant level in dried milk products and appear unlikely to occur.