D CONTROL (cereals)

D CONTROL (cereals)

Summary

Significant hazards

r Trichothecenes, DON and NIV, in wheat outside of Australia. r Ochratoxin A in barley in northern North America and Europe. r Fumonisins, trichothecenes, and aflatoxins in maize.

Control measures

Initial level (H 0 ) r Harvest grain from areas with minimal crop stress if possible. r Dry crops rapidly and completely.

Increase (ΣI) r Store and transport at <12% moisture. r Avoid temperature fluctuations during storage. r Fumigation and other pest management practices minimize damage that

promote subsequent fungal growth.

Reduction (ΣR) r Usually not possible.

Testing

r Visual observation for fungal growth, product damage, and insect infestation. r Monitoring of temperature changes, or carbon dioxide production during

storage as an indicator of fungal growth. r Use of an electronic nose for the same purpose where this is technologically

feasible. r After harvest, chemical analysis of mycotoxin concentrations and

segregation of lots with higher and lower concentrations can be of value.

Spoilage

r Control measures that prevent mycotoxin production control most spoilage

fungi and bacteria.

Comments. The traditional way to avoid microbial growth in grains is to dry them thoroughly and keep them dry. Adequate ventilation in storage bins will remove moisture, prevent condensation, lower and equilibrate temperatures and prevent heating. Bag stacks and manual handling of grain has given way to bulk handling and storage, with great improvements in control of insect and fun- gal damage, even in tropical areas (Champ and Highley, 1988). Drying technology has shown great advances, with sophisticated computer control of drying rates and temperatures now in use in developed countries.

Monitoring grain storage for increase in temperature or carbon dioxide production provides effective warning systems. Thermocouples placed not >2 m apart are an effective means of detecting hot-spots where spoilage is occurring. Grain is a good heat insulator, so that even a minor rise in tempera- ture at the position of a thermocouple may indicate a nearby hot spot (Christensen and Kaufmann, 1977a,b).

More modern approaches to grain storage rely on fumigation, and sealed storage under controlled atmospheres, especially in tropical and subtropical regions where insect damage is a major problem (Champ et al., 1990). Fumigants are gases added specifically to kill insects, and in some cases they also destroy fungi. Fumigants are usually used as a rapid method for killing insects, and then subse- quently removed by ventilation. A variety of gases have been used as fumigants, either singly or in combination, including ethylene dichloride, carbon tetrachloride, carbon disulfide, ethylene dibromide, chloropicrin, hydrogen cyanide, ethylene oxide, methyl chloride, carbon dioxide, methyl bromide, and phosphine. For a variety of reasons, only methyl bromide and phosphine are in widespread use

409 Table 8.8 Suggested target dosages for gaseous treatments

CEREALS AND CEREAL PRODUCTS

of grain at 25 ◦ C Gas

Time (days) a Concentration b Concentration×time Carbon dioxide

7 100 mg/m 3 –

Methyl bromide c 1–2

150 g h/m 3

Hydrogen cyanide

From Annis, 1990a. a

In cases of slow gas introduction or poor gas distribution, increased exposure times may be necessary. b c Minimum concentration achieved at end of exposure. d Regulartory prohibition in certain regions owing to safety concerns. Concentration necessary not well defined.

(Annis, 1990a). Recommended concentrations for use are given in Table 8.8. Environmental consid- erations are resulting in the phasing out of methyl bromide, and the search for alternative fumigants continues.

Controlled atmospheres may be used for grain storage. This technique relies on continuous applica- tion of atmospheres low in O 2 or with high CO 2 concentrations. The recommended approach is to add such gas mixtures to sealed storage and to maintain the grain in totally sealed systems. Where this is not practicable, continuous flow of such gas mixtures may be possible (Annis, 1990a). Recommended

O 2 and CO 2 concentrations are given in Table 8.8. Fumigation and controlled atmospheres help control fungal growth on grains by direct destruction of spores, by inhibition of growth or by killing insects that damage kernels. Fumigants, which merely destroy insects, have no lasting effect (Vandegraft et al., 1973); however, methyl bromide destroys fungi as well as insects (Majumder, 1974), and phosphine has some fungicidal properties (Hocking and Banks, 1993). Use of methyl bromide is restricted in the EU and the US owing to toxicological concerns. Modified atmospheres controlling insects may also have a substantial effect in controlling fungi (Hocking, 1990).

Control of insect and fungal damage in grain stores is of particular importance in the tropics, where most grains are stored in sacks in warehouses unsuitable for sealing and fumigation. Recent approaches to sealing stacks of bags in such stores, fumigating and then maintaining the sealed stacks under controlled atmospheres have shown the potential to reduce grain losses markedly (Annis, 1990b; Graver, 1990).

Many investigators have suggested using heat or chlorine to destroy microorganisms in grains. However, this technology has been little used, with the recognition that such remedial processes do not destroy mycotoxins and are not a substitute for clean grain.

In the same way, ionizing radiation at a level of 2–3 kGy destroys fungi that commonly spoil rice (Iizuka and Ito, 1968; Ito et al., 1971). Such a process is not yet permitted, or is unacceptable to consumers, in many countries.

IV Flours, starches, and meals

A Effects of processing on microorganisms Flour made from wheat is the predominant flour product in international trade; however, flours are

also produced from rice, maize, and other cereals. Grains for milling are subjected to a number of cleaning and aspiration steps before tempering. These steps reduce the microbial level of the grains

MICROORGANISMS IN FOODS 6

as they enter the milling operation. However, analysis of samples from five Australian flour mills revealed that neither traditional wheat cleaning nor scouring changed the microbial load appreciably, nor did scouring affect the microbial counts of flour (Eyles et al., 1989). Some contamination of wheat occurred during tempering. After tempering for a predetermined time, the grain then passes through a milling and sifting sequence that separates the hulls (bran) and the germ from the endosperm, which is crushed into flour. Maize is sometimes ground to flour, meal or grits without a tempering step.

Sound, clean grains, especially those properly screened and tempered, contain few microorganisms. However, contact with mill machinery introduces contamination of a quantity and variety that is affected by the degree of cleanliness of the mill equipment. A correlation exists between microbial levels in the

product and mill sanitation. High mold levels exist in poorly maintained factories, up to 3.4 × 10 6 /g of flour residues on equipment (Christensen and Cohen, 1950) and up to l0 8 /g in mill dust (Semeniuk, 1954).

Flour with ≤12% water will not support microbial growth (Hesseltine and Graves, 1966). However, water from any source will encourage microbial growth in flour and in residues on mill machinery. Such residues can become damp from high atmospheric humidity, from condensate on cool surfaces, from improper clean-up procedures (Hesseltine and Graves, 1966; Graves et al., 1967) or from insect activity (Thatcher et al., 1953).

Wheat flour may be treated with oxidizing agents such as chlorine dioxide or chlorine, and may also

be bleached by treatment with benzoyl peroxide (Thatcher et al., 1953). Bleaching reduces microbial numbers to some degree, but spores are little affected. The bacterial flora of flour is much more diverse than that of the wheat from which it was made. Both wheat and the flour made from it contain many psychrotrophs, flat sour bacteria, and thermophilic spore-forming bacteria (Hesseltine, 1968). These are of particular interest to processors of canned and chilled foods in which flour is an ingredient. Rope bacilli originate from both insects and insanitary equipment; they are basically soil bacteria.

Aerobic colony counts in flour from different geographical regions range from 10 2 to 10 5 /g, (Table 8.9) with most samples having populations in the range of 10 4 /g (Eyles et al., 1989; Legan, 1994; Richter et al., 1993). Milling conditions and weather during growth and harvesting influ- ence microbial levels. Although the coliform group is sometimes used as an indicator of sanitary conditions, the use of this group is of questionable value for flour. Coliforms are part of the natu- ral flora of flour and were isolated from 72% to 100% of flours in the UK from the 1988 to 1994 harvest years (Legan, 1994). Interpretation of reported coliform levels in the literature must con- sider the methodology used, as results generated using most probable number determinations are frequently lower than those obtained using newer methods such as Petrifilm TM . Unpublished data suggest that confirmation media (Brilliant Green Bile Broth) may inhibit certain lactose-fermenting, Gram-negative bacteria in flour (Swanson, personal communication). Escherichia coli was present in 28–56% of flour from the UK (Legan, 1994) and 13% of flour in the United States (Richter et al., 1993).

Table 8.9 Total aerobic colony count and mold count reported in wheat flour from various countries

No. of

Total aerobic

Yeast and

Reference Australia

Country Period

samples

colony count/g

mold count/g

Eyles et al., 1989 UK

Legan, 1994 US

Richter et al., 1993

411 Fungal counts (yeast and mold counts) commonly approach 1 000/g (Table 8.9). The fungi in finished

CEREALS AND CEREAL PRODUCTS

flours are mostly Penicillium and Eurotium species and Asp. candidus. The genera of fungi found in flours differ markedly from those found in the wheat from which the flour was made, demonstrating again the importance of the mill as a source of the microorganisms. Yeasts become important only if flour becomes wet.

Flour and starch form the foundation for many dry mix products. Flour is mixed with other dry ingredients such as powdered egg, dry milk, spices and seasons and leaven agents. These products are sold in the dry state for subsequent use by foodservice establishments and consumers. The dry mixing process has little impact on numbers of microorganisms. Maintenance of dry conditions is important to prevent incidental contamination.

B Saprophytes and spoilage For flour and maize meal, 12% water is a critical level, below which no microbial growth will occur.

Above 12%, some xerophilic fungi can grow; and at 17% some bacteria also can grow (Hesseltine and Graves, 1966). The rate of growth is proportional to the water activity and temperature (Kent-Jones and Amos, 1957). If the moisture level is high, as in a flour and water paste, bacteria will predominate because they grow faster than fungi. Lactic acid bacteria will begin acid fermentation, followed by alcoholic fermentation by yeasts and then oxidation to acetic acid by Acetobacter. This sequence is less likely in stored flour because of reductions in viable counts during storage. In the absence of lactic acid bacteria, micrococci may acidify damp flour, and in their absence Bacillus spp. may grow, producing lactic acid, gas, alcohol, acetoin, and small amounts of esters and aromatic compounds. It is characteristic of most flour pastes to develop an odor of acetic acid and esters.

Canners often set low specifications for sporulating organisms that can survive canning procedures and spoil the product. Good hygienic practices are necessary to control spore levels.

C Pathogens and toxins Mycotoxins can be an important health hazard in flours and meals as mycotoxins present in grain

will carry through into the flour, surviving heating steps or other procedures designed to kill fungi. In addition, moist flour and maize meal (>14% water) will support fungal growth in the same way that grains do, and mycotoxins can be produced (Seeder et al., 1969; Bullerman et al., 1975).

Of 70 molds isolated from flour and bread, 16 were Aspergillus, 48 Penicillium and six were other genera. Fifteen of the 48 Penicillium species and one strain of Asp. ochraceus produced mycotoxins on laboratory media (Bullerman and Hartung, 1973). Mycotoxins do not present a health hazard in dry mixes when mycotoxins are controlled in flour and meal, because fungal growth cannot occur at the low water activities of these products.

Salmonellae also present a health hazard in flours, meals and dry mixes. Salmonellae were present in 0.3–3.0% of wheat flour, with variation due to harvest season and wheat type (Richter et al., 1993). Salmonellae will remain viable in dry flour for several months (Dack, 1961). They are, however, quite

sensitive to heat. When maize flour at 10–15% moisture was spray inoculated with 10 5 salmonellae/g and held at 49 ◦

C, 99.9% of cells were inactivated in 24 h (van Cauwenberge et al., 1981). Normal cooking of flour-based products inactivates the salmonellae.

B. cereus is common in wheat flour, but is usually present in very low numbers (Eyles et al., 1989). Kaur (1986) reported that B. cereus levels in flour were low and only occasionally >10/g. The organism cannot grow in the dry flour and thus does not represent a hazard at this stage.

MICROORGANISMS IN FOODS 6