**2.3 Moisture content (MC)**

*Mycotoxins and Food Safety*

**2.1 Nutrient content**

**2.2 Biological structure**

physical factors surrounding the grains whereas those attributed to the characteristics of the grains are intrinsic factors (pH, acidity, nutrient composition, biological structure, moisture content/water activity, redox potential, naturally occurring and added antimicrobial factors). Details on how these factors contribute to or promote

Every organism requires essential nutrients for growth and maintenance of metabolic functions. Hence, the type and concentration of nutrients needed depends on the class of microorganism. A source of energy, water, nitrogen, vitamins, minerals, and other compounds provide these nutrients. The growth of *Aspergillus flavus* on grains was significantly affected by the concentration of soluble sugars. Low sugar levels retarded its growth, whereas concentrations between 3.0 and 6.0% resulted in rapid growth, and the subsequent production of aflatoxin B1. Nevertheless, aflatoxin B1 production was significantly promoted due to the bioavailability of amino acids (arginine, glutamic acid, aspartic acid) and zinc in the grains [43]. In a similar study, Li et al. [44] reported different concentrations of mycotoxins (aflatoxin B1 (AFB), deoxynivalenol (DON), zearalenone (ZEA) and ochratoxin A (OTA)) on numerous swine feeds. These outcomes could be attributed to the nutritional composition of the feeds. The nutritional requirement of pigs depends on the state (gestating, finisher, grower, starter, etc.) hence varied feed rations are given which contain different nutrient concentration; as a result influence fungi growth and subsequent mycotoxins production. The bioavailability of nutrients in most grains would support the growth of a wide range of microorganisms. Although each strain of mold has the genetic potential to produce a particular mycotoxin, nutrient

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

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

microbial contamination of grains are examined below.

bioavailability could influence their levels significantly [45].

**106**

peanut testae.

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 leads to the synthesis of metabolites.

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 synthesis [55].

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 damage, and moisture absorption.

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 aw levels.

### **2.4 pH, acidity and redox potential**

For centuries, people have learned to increase the acidity of food either through fermentation, or by adding weak acids in the form of preservatives. These techniques have proven successful. Organic acids are effective preservatives in their undissociated state. pKa is the term used to illustrate the dissociation of an acid. Therefore, lowering the pH of grains increases the effectiveness of organic acids as preservatives [39, 53].

Naturally, grains in the field are undried and possess high pH; however, drying decreases the MC and subsequently the aw, thereby reducing the pH. Adadi and Obeng [65] reported that the lower the pH value the higher the total acidity (TA), which inhibits the growth of microorganisms. The pH of grains could interact with other parameters (aw, salt, temperature, redox potential) in the food to inhibit microbial growth. The general rule of food microbiology states that pathogens do not grow, or grow slowly, at pH below 4.6- but there are exceptions. For instance, at pH 4.2, an organism was able to survive and synthesize a mycotoxin [66].

Rice and maize have pH about 6.02 ± 0.01 and 6.53 ± 0.01 during the rainy season and 6.20 ± 0.20 and 6.42 ± 0.12, respectively, in the dry season [67]. The season seems to influence the aw and the TA, thus altering the pH of the grains. The rainy season is defined by continuous rain, resulting in the elevation of the MC of the grains, which affects the pH. The pH range of beans (string and lima) is between 4.6 and 6.5 [53].

According to [68], fungi can secrete butyrate, oxalate, maleate, citrate, gluconate, and succinate into their environment, thereby changing the acidity of the ecological niche. *Sclerotinia sclerotiorum* and *Botrytis* sp. secrete oxalic acid while *Penicillium* spp., and *Aspergillus* spp., synthesize mainly gluconic and citric acids [69–71]. Fungi can grow comfortably in pH above 8.5; however, below pH 2.2, their growth was inhibited. Microorganisms can modify the pH of the environment in which they reside, making it challenging for farmers to control the pH of stored grain. A phenomenon like this could lead to significant economic loss due to microbial proliferation. The synthesis of ochratoxin A was maximized at lower pH [72]. Different fungi strains (*Trichoderma harzianum*, *Trichoderma aureoviride*, and *Trichoderma viride*) can grow over a broader pH range (from 2.0 to 6.0), with optimal growth at pH = 4.0 [73]. Hence, adjusting the pH is a great way of inhibiting the germination of any fungi spores on stored grains.

The redox potential (Eh) of a substance is the ratio of the total oxidizing (electron-accepting) power to the whole reducing (electron-donating) energy of the material. It is quantified in millivolts (mV) at pH 7.0. Eh correlates to the pH of a substrate [47]. Generally, aerobes, facultative anaerobes, and anaerobes grow well at Eh between +500 to +300 mV, +300 to −100 mV, and + 100 to less than −250 mV, respectively [74]. Some microorganisms require an Eh of less than +60 mV for growth; nevertheless, slower growth rates were observed at higher Eh values [53]. The Eh values of wheat (whole grain), wheat (germ), and barley (ground) is within −320 to −360, −470, and +225, respectively [46]. Oxidants such as KMnO4, NaClO4, or Fe2O3 can influence the Eh of a material [75]. The growth of *Fusarium oxysporum* and *Rhizoctonia solani* were suppressed when decomposable organic material was introduced [76, 77]. pH and Eh can impact a wide range of fungal physiological processes (regulation and expression of genes) [78–80] thus complicating the storage process. Therefore, controlling the Eh and pH of grains is necessary to manipulate fungi growth during storage.

#### **2.5 Temperature**

All microorganisms have a defined temperature range within which they can grow and synthesize toxins which cause food poisoning. Therefore, understanding the

**109**

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

temperatures range, coupled with other intrinsic and extrinsic factors, are crucial to selecte the proper storage conditions for grain storage. Temperature has a dramatic impact on the growth and lag period of an organism. The growth rates of most microorganisms are favored at low temperatures, though there are exceptions. Reaction rates for specific enzymes in an organism become slower at lower temperatures. Also, low temperatures minimize the fluidity of the cytoplasmic membrane, thus interfering with transport mechanisms in the cell [46, 53]. The expression of proteins are temperature regulated. A slight change in temperature can influence bacterial and archaeal community structure. 16S rRNA genes were altered due to changes in temperature [81, 82]. A wide range of temperatures play a vital role in the growth and synthesis of toxins in fungi. For instance, *Penicillium* and *Cladosporium* were able to grow below 20°C whereas the growth of *Aspergillus* species were inhibited. However, at a temperature above 20°C, the growth was maximized [55]. Virulent *A. niger* has optimal growth between 30–35°C [83], thus, rendering stored produce susceptible to a toxin secreted by these fungi. The growth rates of *Phoma* spp. 1, *Phoma exigua*, *Mortierella gamsii*, and *Mortierella* sp. 1 was high at 4°C [84]. Warmer (33°C) and more humid conditions may increase aflatoxin prevalence. However, the opposite scenario is expected in tropics, since most aflatoxigenic fungi will not survive the

The knowledge of optimal temperature for microbial growth and mycotoxin synthesis gives more accurate assessment of the potential risk to human health [72]. Molds can grow over a broader range of temperatures, from below freezing to temperatures over 50°C. For a given substrate, the rate of mold growth decrease with decreasing temperature and water availability. Below 17°C grains are susceptible to insect infestation; however, mite infestations can occur between 3 and 30°C [86]. Degradation of fungi mycotoxins can occur at 40°C [58]. Therefore, keeping the temperature of the storage room elevated could be of valuable aid in detoxification

Mycotoxins are considered a significant health and economic problem. Mycotoxins can find their way to the human body by way of contaminated food, skin contact, or inhalation [87, 88]. The most common form of exposure is through

The level of exposure and the type of mycotoxins which one is exposed to determine the nature of adverse effects on the human, either in the form of an allergic reaction, infections, or a toxic disease [90]. The seriousness of mycotoxins depends on the toxicity of the mycotoxin involved, the age, wellbeing of the exposed individual, and the length of exposure [91]. Mycotoxicosis is the disease caused by mycotoxins. Mycotoxins such as aflatoxins have been documented causing liver cancer [92]. Other serious conditions, such as chronic interstitial nephropathy, Balkan endemic nephropathy, and urothelial tumors, as well as testicular cancer in men, have also been linked to mycotoxins [93]. Acute diseases, namely abdominal pains, headache, dizziness, throat irritation, and nausea, have also been associated with mycotoxin exposure in humans [94]. It is, therefore, important to ensure that

The hazardous effects of mycotoxins on humans and animals had called for the development of rapid methods for their detection and quantification in cereals

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

expected 40°C [45, 85].

and probable killing of stored microorganisms.

**3. Effects of mycotoxins on human health**

oral ingestion of contaminated food [89].

grains are free of mycotoxin contamination.

**3.1 Methods of detecting and analyzing mycotoxins**

*The Potential Application of Nanoparticles on Grains during Storage: Part 1 – An Overview… DOI: http://dx.doi.org/10.5772/intechopen.91005*

temperatures range, coupled with other intrinsic and extrinsic factors, are crucial to selecte the proper storage conditions for grain storage. Temperature has a dramatic impact on the growth and lag period of an organism. The growth rates of most microorganisms are favored at low temperatures, though there are exceptions. Reaction rates for specific enzymes in an organism become slower at lower temperatures. Also, low temperatures minimize the fluidity of the cytoplasmic membrane, thus interfering with transport mechanisms in the cell [46, 53]. The expression of proteins are temperature regulated. A slight change in temperature can influence bacterial and archaeal community structure. 16S rRNA genes were altered due to changes in temperature [81, 82]. A wide range of temperatures play a vital role in the growth and synthesis of toxins in fungi. For instance, *Penicillium* and *Cladosporium* were able to grow below 20°C whereas the growth of *Aspergillus* species were inhibited. However, at a temperature above 20°C, the growth was maximized [55]. Virulent *A. niger* has optimal growth between 30–35°C [83], thus, rendering stored produce susceptible to a toxin secreted by these fungi. The growth rates of *Phoma* spp. 1, *Phoma exigua*, *Mortierella gamsii*, and *Mortierella* sp. 1 was high at 4°C [84]. Warmer (33°C) and more humid conditions may increase aflatoxin prevalence. However, the opposite scenario is expected in tropics, since most aflatoxigenic fungi will not survive the expected 40°C [45, 85].

The knowledge of optimal temperature for microbial growth and mycotoxin synthesis gives more accurate assessment of the potential risk to human health [72]. Molds can grow over a broader range of temperatures, from below freezing to temperatures over 50°C. For a given substrate, the rate of mold growth decrease with decreasing temperature and water availability. Below 17°C grains are susceptible to insect infestation; however, mite infestations can occur between 3 and 30°C [86]. Degradation of fungi mycotoxins can occur at 40°C [58]. Therefore, keeping the temperature of the storage room elevated could be of valuable aid in detoxification and probable killing of stored microorganisms.

## **3. Effects of mycotoxins on human health**

Mycotoxins are considered a significant health and economic problem. Mycotoxins can find their way to the human body by way of contaminated food, skin contact, or inhalation [87, 88]. The most common form of exposure is through oral ingestion of contaminated food [89].

The level of exposure and the type of mycotoxins which one is exposed to determine the nature of adverse effects on the human, either in the form of an allergic reaction, infections, or a toxic disease [90]. The seriousness of mycotoxins depends on the toxicity of the mycotoxin involved, the age, wellbeing of the exposed individual, and the length of exposure [91]. Mycotoxicosis is the disease caused by mycotoxins. Mycotoxins such as aflatoxins have been documented causing liver cancer [92]. Other serious conditions, such as chronic interstitial nephropathy, Balkan endemic nephropathy, and urothelial tumors, as well as testicular cancer in men, have also been linked to mycotoxins [93]. Acute diseases, namely abdominal pains, headache, dizziness, throat irritation, and nausea, have also been associated with mycotoxin exposure in humans [94]. It is, therefore, important to ensure that grains are free of mycotoxin contamination.

#### **3.1 Methods of detecting and analyzing mycotoxins**

The hazardous effects of mycotoxins on humans and animals had called for the development of rapid methods for their detection and quantification in cereals

*Mycotoxins and Food Safety*

**2.4 pH, acidity and redox potential**

For centuries, people have learned to increase the acidity of food either through fermentation, or by adding weak acids in the form of preservatives. These techniques have proven successful. Organic acids are effective preservatives in their undissociated state. pKa is the term used to illustrate the dissociation of an acid. Therefore, lowering the pH of grains increases the effectiveness of organic acids as preservatives [39, 53]. Naturally, grains in the field are undried and possess high pH; however, drying decreases the MC and subsequently the aw, thereby reducing the pH. Adadi and Obeng [65] reported that the lower the pH value the higher the total acidity (TA), which inhibits the growth of microorganisms. The pH of grains could interact with other parameters (aw, salt, temperature, redox potential) in the food to inhibit microbial growth. The general rule of food microbiology states that pathogens do not grow, or grow slowly, at pH below 4.6- but there are exceptions. For instance, at

pH 4.2, an organism was able to survive and synthesize a mycotoxin [66].

ing the germination of any fungi spores on stored grains.

Rice and maize have pH about 6.02 ± 0.01 and 6.53 ± 0.01 during the rainy season and 6.20 ± 0.20 and 6.42 ± 0.12, respectively, in the dry season [67]. The season seems to influence the aw and the TA, thus altering the pH of the grains. The rainy season is defined by continuous rain, resulting in the elevation of the MC of the grains, which affects the pH. The pH range of beans (string and lima) is between 4.6 and 6.5 [53]. According to [68], fungi can secrete butyrate, oxalate, maleate, citrate, gluconate, and succinate into their environment, thereby changing the acidity of the ecological niche. *Sclerotinia sclerotiorum* and *Botrytis* sp. secrete oxalic acid while *Penicillium* spp., and *Aspergillus* spp., synthesize mainly gluconic and citric acids [69–71]. Fungi can grow comfortably in pH above 8.5; however, below pH 2.2, their growth was inhibited. Microorganisms can modify the pH of the environment in which they reside, making it challenging for farmers to control the pH of stored grain. A phenomenon like this could lead to significant economic loss due to microbial proliferation. The synthesis of ochratoxin A was maximized at lower pH [72]. Different fungi strains (*Trichoderma harzianum*, *Trichoderma aureoviride*, and *Trichoderma viride*) can grow over a broader pH range (from 2.0 to 6.0), with optimal growth at pH = 4.0 [73]. Hence, adjusting the pH is a great way of inhibit-

The redox potential (Eh) of a substance is the ratio of the total oxidizing (electron-accepting) power to the whole reducing (electron-donating) energy of the material. It is quantified in millivolts (mV) at pH 7.0. Eh correlates to the pH of a substrate [47]. Generally, aerobes, facultative anaerobes, and anaerobes grow well at Eh between +500 to +300 mV, +300 to −100 mV, and + 100 to less than −250 mV, respectively [74]. Some microorganisms require an Eh of less than +60 mV for growth; nevertheless, slower growth rates were observed at higher Eh values [53]. The Eh values of wheat (whole grain), wheat (germ), and barley (ground) is within −320 to −360, −470, and +225, respectively [46]. Oxidants such as KMnO4, NaClO4, or Fe2O3 can influence the Eh of a material [75]. The growth of *Fusarium oxysporum* and *Rhizoctonia solani* were suppressed when decomposable organic material was introduced [76, 77]. pH and Eh can impact a wide range of fungal physiological processes (regulation and expression of genes) [78–80] thus complicating the storage process. Therefore, controlling the Eh and pH of grains is necessary to manipulate fungi growth during storage.

All microorganisms have a defined temperature range within which they can grow

and synthesize toxins which cause food poisoning. Therefore, understanding the

**108**

**2.5 Temperature**

and other foods. However, sampling methods, extraction, and the instrument used could alter mycotoxin quantification. In response, Rahmani et al. [95] compiled a good comprehensive review to address the challenges mentioned above.

The impact of the sampling on sample preparation and analytical instrument contribute to the total variance during the analysis of ochratoxin A (OTA) in flour and aflatoxinB1 (AFB1) in oats was recently reported. The authors suggested that increasing sample weight (size) could potentially reduce the high heterogeneity encountered [96, 97]. For efficient extraction, methods of detection and quantification of mycotoxins, the reader(s) are referred to the following good sources [95, 98–101].
