Harvested grain carries a wide range of bacterial and fungal contaminants. Depending on effectiveness of storage conditions, and the climatic region of the world the level and type of contamination will vary. Grain itself and the microbial contaminants respire slowly when stored dry. However, if the water availability is increased to 15-19% moisture content (= 0.75-0.85 aw) predominantly spoilage fungi, particularly Eurotium spp., Aspergillus, and Penicillium species grow resulting in a significant increase in respiratory activity, resulting in increased temperature and sometimes spontaneous heating that results in colonization by thermophilic fungi and actinomycetes (Fleurat-Lessard 2002; Lacey and Magan 1991). The chemical process involved in heat generation is predominantly aerobic oxidation of carbohydrates such as starch. The energy is released by the following equation:
Heating occurs when this energy is released faster than it can escape from the cereal substrate. In contrast, little energy is released in anaerobic respiration and little or no heating occurs in the absence of oxygen. The requirement for oxygen increases with temperature to a maximum of 40°C but does not decrease greatly until the temperature exceeds 65°C. At this temperature, microbial growth is largely inhibited and heating results from exothermic chemical oxidation. Thus, the respiratory quotient may be 0.7 to 0.9 up to 65°C but less than 0.5 at higher temperatures.
By utilizing the respiratory quotient CO2 production can be translated into dry matter loss. Typically, complete respiration of carbohydrates gives a respiratory quotient, i.e., ratio of O2 consumed to CO2 produced of 1.0, and it has been calculated that 14.7 g CO2/kg grain will be released for every 1% loss of grain dry matter. During anaerobic fermentation, only about 0.493 g CO2 is evolved from a kilogram grain for every 1% dry matter loss. Alternatively, a respiratory quotient below 1.0 may result from lipid or protein metabolism. For example, tripalmitim has a quotient of 0.7. The higher the CO2 production, the shorter the safe storage period without loss of dry matter. Studies by Jonsson et al. (2000) utilized respiratory rates over a wide range of aw levels and temperatures to examine development of molds such as P. verrucosum in stored grain and also effects on germinability, fungal biomass, and maximum safe storage times in days. They suggested that the maximum storage time without mold growth was probably halved if moisture content at harvest was increased by 1-3% (= 0.05 aw) or if storage temperature was increased by 5°C. Fleurat-Lessard (2002) in his excellent review has suggested that for the modeling and prediction of global quality changes the rate of CO2 production can be used as a "storability risk factor."
The ratio of contribution of spoilage molds and grain to total respiration has been argued for many years. A range of studies has demonstrated that grain deterioration and by implication dry matter loss is predominantly determined by fungal activity. Wheat deterioration has been measured and models developed based on germination rates, visible mold growth, or respiration of grain and microorganisms (Fleurat-Lessard 2002). Kreyger (1972) pointed out, from previous work with wheat that dry matter loss was unimportant, provided there was no visible molding. However, his own work showed that barley of 24% water content (= 0.94 aw) stored at 16°C for 10 weeks lost 2% dry matter with visible molding. With maize showed that fungal invasion and aflatoxin content could be unacceptable before the grain had lost 0.5% dry matter and mold growth became visible (Seitz et al. 1982). Latif and Lissik (1986) proposed a model for respiration based on the rate of dry matter breakdown, but was not related to important environmental factors such as aw and temperature. This worked suggested that Kreyger's assumptions were not completely accurate. White et al. (1982) noted that 0.1% was unacceptable for first grade wheat and proposed an absolute level of0.04%. However, when 55 days safe storage was predicted for grain stored at 18.4% mc (= 0.86 aw) visible molding occurred after 23 days of storage. Recently, Karunakaran et al. (2001) determined deterioration rates in wheat stored at constant or step decreases in temperature. Deterioration rates at 17% mc for wheat were 5, 7, and 15 days at 35, 30, and 25°C, respectively. Interestingly, they found that respiration rates of 17-19% mc wheat at 25°C increased while germination percentages decreased with storage time. Dry matter losses were about 0.05% and visible mold was observed when dry matter loss was about 0.1% at these w.c.s.
There are problems with the use of visible molding as a criterion of deterioration (Hamer et al. 1991; Lacey et al. 1994). While Kreyger (1972) used this extensively, a clear definition was not produced. Many workers have questioned this subjective index of the safe storability of grain (Hamer et al. 1991; Lacey and Magan 1991). Magan (1993) pointed out as an early indicator, microscopic growth may be a more effective measurement of fungal activity than visible molding.
et al. 1991). Cahagnier and Richard Mollard (1991) suggested that the ergosterol content in storage fungi was not significantly affected by environmental factors such as aw. They thus suggested that ergosterol could be used as an "index" of the level of fungal biomass and the length of storage of the grain. Tothill et al. (1993) showed that there was a significant positive correlation between ergosterol content and total CFUs at 0.95 aw, while in grain of 0.85 aw there was no significant correlation. Grain inoculated with individual species (Alternaria alternata, E. amstelodami, and P. aurantiogriseum) at 0.95/0.85 aw and 25°C showed a significant correlation between CFUs and ergosterol although the content for an individual species varied considerably. Overall, levels of <5-6 mg/g can be found in fresh wheat grain, with that having microscopic growth containing about 7.5-12 mg/g. This correlates with a grain as a spoilage threshold indicator. Fleurat-Lessard (2002) has suggested that perhaps modeling of ergosterol production rates under different environmental conditions using sigmoid curves similar to those used for insect population dynamics may enable the use of an ergosterol index in the future when correlation models become available. It may also be possible to use both ergosterol and the production of mycotoxins in predicting potential environmental factors over which spoilage/mycotoxins may be produced. Two-dimensional models for growth and fumonisin production have already been developed (Marin et al. 1999a,b,c) and such information may be useful in further development of predictive models for risk assessment of spoilage and toxin contamination. Perhaps, modeling of cumulative ergosterol production by spoilage fungi and associated mycotoxins in relation to aw, temperature, time, gas composition (modified atmosphere storage), and time may allow more effective and precise risk assessment of mold contamination and mycotoxin occurrence to the consumer.
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