**2.2 Biological structure**

Grains have biological structures which prevent the penetration and growth of microorganisms. The testa of seeds and shell of nuts are examples of such structures. Some physical structures/barriers may exert antimicrobial potential. Intact biological structures prevent the entry of microbes, subsequent growth and production of mycotoxins in grains. However, these structures are destroyed during harvesting, transporting, or processing of the grains. Insect infestation could pave way for microbial proliferation of grains [46, 47]. Extract of Peanut testa was reported to exhibit pronounced antifungal activities against *Penicillium* sp., *A. niger*, and *Actinomucor* sp. The cardinal and purple peanut testa produced a significant zone of inhibition at concentrations of 0.8 and 2.0 g/L, respectively. It was concluded that the fungicidal potentials of the testa depend on the type of peanut [48]. Nevertheless, the environment, variety, type of farming system adopted, duration of storage, etc., may affect the fungicidal potency of these peanut testae.

The biocidal activities of *Dacryodes edulis* and *Garcinia kola* testae have been reported [49]. The antimicrobial activities of these testae are associated with the presence of phytochemicals (alkaloids, saponins, etc.), and was confirmed in experimental studies [50, 51]. The methanolic extract of *Simmondsia chinensis* testa (Link) C.K. Schneid exhibited no fungicidal activities against *Candida albicans* [52], indicating that not every grain testa could inhibit microbial growth.

All the studies mentioned above support the fact that the biological structures of the grains may have the potential to prevent microbial proliferation. These

**107**

aw levels.

*The Potential Application of Nanoparticles on Grains during Storage: Part 1 – An Overview…*

claims cannot be guaranteed when the structures covering the seeds are destroyed during harvesting or drying. Therefore, care should be taken to minimize the destruction of these structures on grains during or after harvest. Busta et al. [53] reported that pathogens lack the enzyme necessary to break down the protective

The oldest method of preserving food is controlling the MC. It is applicable during grain storage since the moisture influences the growth of microorganisms and subsequent production of mycotoxins. The water requirement of microbes is known as the water activity (aw) of the food or environment and is defined as the ratio of the water vapor pressure of the food substrate to the vapor pressure of pure water at a constant temperature [47]. The aw of grains describes the degree to which water is bound in the grains, its availability to participate in chemical/biochemical reactions, and its accessibility to facilitate the growth of microorganisms [53] which

Cereals have an aw between 0.10 and 0.20 when adequately dried, making it difficult for microbes to reproduce. Although the optimum MC for growth and subsequent toxin production for the various aflatoxigenic fungi varies, many achieve the best growth and toxin synthesis at an MC of 17.5% [53, 54]. *Aspergillus* requires about 13% moisture or a relative humidity of 65% (aw, of 0.65) for growth and toxin

The highest *A. flavus* population was observed at aw = 0.95. Aw significantly altered the AFB1 produced and the expression of *aflR* at aw 0.90 and 0.95 respectively. The optimum expression of the *nor-1* gene was at aw 0.95 and 0.90, whereas deficient expression occurred in the driest treatment (aw 0.85) [56]. Molds were unable to germinate when the aw of the grains remained below 0.60. Also, when molds are allow to flourish, they could predispose the stored grain to mite and insect infestation [3, 57] because mites feeds on molds. Co-culturing *A. parasiticus* with *S*. *lactis* and *Lactobacillus casei* suppressed aflatoxin synthesis [54]. In a similar study, Faraj et al. [58] reported a significant reduction in total aflatoxins synthesized when fungi (*A. niger* and *Rhizopus oryzae*) were co-cultured with a bacterium (*Bacillus stearothermophilus*). Since aflatoxins synthesis was minimal at 40°C and high between 8°C and 40°C, the authors associated the findings to the temperature differential between the strains [59]. However, mycotoxins such as rubratoxins from *Penicillium purpurogenum*, cerulenin from *Cephalosporium caerulens*, and *Acrocylindrium oryzae* inhibited fungi growth at the same time enhance aflatoxin synthesized [45, 60]. The growth of *Trichoderma asperellum* (strains PR10, PR11, PR12, and 659-7) was reported being sensitive to aw reduction [61]. Therefore, lowering aw could inhibit the growth of fungi. According to [62], grains stored for a year, 8–9 months, and weeks should have MC about 9%, 13%, and 14%, respectively. A low MC could curb problems like molds infestations, discoloration, respiration loss, insect dam-

Adequate drying of grains (produce) to lower moisture levels is critical to create unfavorable conditions to inhibit microbial and insect proliferation. It is recommended to dry harvested produce to safer moisture levels of 10–13%. Low moisture help keep grains longer without losing nutrients and other vital bioactive compounds [63, 64]. Water activity in stored grains could increase depending on climatic conditions, cellular respiration of microorganisms, or urine from rodents. Improper drying, especially during winter or autumn, could also elevate

*DOI: http://dx.doi.org/10.5772/intechopen.91005*

layers covering grains.

synthesis [55].

**2.3 Moisture content (MC)**

leads to the synthesis of metabolites.

age, and moisture absorption.

claims cannot be guaranteed when the structures covering the seeds are destroyed during harvesting or drying. Therefore, care should be taken to minimize the destruction of these structures on grains during or after harvest. Busta et al. [53] reported that pathogens lack the enzyme necessary to break down the protective layers covering grains.
