3.2. Microbiological Contamination of Yam, Chips, and Flour
Figure 3 shows the microbial counts for the freshly produced chips and flakes. For the total bacteria (
Figure 3a), about 60% of the samples present a count higher than 1 × 10
6 cfu/g and higher than the limit set by the International Commission on Microbiological Specification for Food (ICMSF, 1998). Moreover, 3 (20%) of the 15 tested samples showed a bacteria population as high as 1 × 10
7 cfu/g. It was observed that yam chip samples from batch C (January 2014, the last one collected) and from the three processors had higher levels of contamination than samples from batches A and B (November and December 2013, respectively). However, no particular trend was observed for the yam flake samples. Considering the
Staphylococcus aureus presence (
Figure 3b), this species was detected in all the samples; however, levels were never above 1 × 10
5 cfu/g, which is considered the safety limit [
39]. Total coliforms (
Figure 3c) were detected in 40% of the samples, and in all cases, the colony count was always about 2 × 10
2 cfu/g, well below ICMSF recommended limit (1 × 10
4 cfu/g). For
Escherichia coli (
Figure 3d), on the other hand, only two samples in batch B showed the presence of this species.
Salmonella species was not detected in any of the sample by direct plating. Regarding the fungi (
Figure 3e), all samples showed contamination; the yam flakes were, overall, less contaminated than the chips, although the differences were not always statistically significant.
In
Figure 4, the results for the food items purchased from selected markets are reported. For the total bacterial count (
Figure 4a), a higher level of contamination than the freshly produced samples was observed, and in all the samples, the total count was above the set limit of 1 × 10
6cfu/g. No particular trend was observed among the different sample sources or batches, and a yam flour sample in batch A from supermarket was the sole exception. Higher levels of contamination can also be seen for the
Staphylococcus aureus presence (
Figure 4b), where over 70% of the samples were contaminated. Overall, the highest contamination rate was observed in the batch C yam flour samples, while the lowest was in the batch B samples. For the total coliform count (
Figure 4c), batch C yam flour was found to have the highest contamination, while yam chips showed the lowest contamination rate. Over 70% of batch A and B samples were contaminated with
Esherichia coli (
Figure 4d), while almost 70% of the batch C samples were free of that microorganism; in particular, none of batch C yam chip samples showed contamination. No
Salmonella spp. were detected in the freshly processed samples by direct plating. Considering fungal growth (
Figure 4e), fungi were detected in over 90% of the samples. Yam flakes showed the lowest contamination rate, while overall, batch A showed the highest (maximum count 6 × 10
5 cfu/g).
The high level of microbial contamination in both the freshly processed and market samples could be attributed to unhygienic handling of these commodities during processing and post-processing. Babajide et al. [
14] reported on the microbial contamination of yam chips from some processing sites in southwest Nigeria, while in other studies a high microbial contamination of yam chip samples was observed from some selected markets in Togo [
40] and southwest Nigeria [
41]. Also, according to those previous studies, the high level of total bacterial count in all the market yam chip, flake, and flour samples, and in about 60% of the freshly processed yam chips and flakes, was above the limit set by the International Commission on Microbiological Specification for Food, which could be attributed to exposure of the samples to environmental conditions. These data showed that the sample form (chips, flakes, and flour) and the point of collection were determinant for the contamination.
The commonly used drying conditions (such as rock surfaces, roadside, cemented floor, spreading on farmlands) could cause the commodities to be in contact with insects and toxigenic mycoflora from humans, environment, and soil; such mycoflora could then be inadvertently carried into storage. Mestres et al. [
37] identified the drying stage in the production of yam chips as the critical control point (CCP). Somorin et al. [
41] also identified the milling process of yam chips into flour as one of the means of microbial contamination. Other literature data also reported high levels of microbial contamination in yam chips from some selected processing sites (Oyo and Ogun states, Nigeria) [
14].
The exposure of yam derived products without appropriate packaging in the markets for sale is another possible means of contamination [
28]. Our study, however, shows that samples sold in supermarkets have comparable microbial contamination above the set limit, which might be due to microorganisms already present in the dried yam chips from which the yam flour was obtained or from the milling machine used [
41]. The presence of
Staphylococcus aureus, either in the freshly processed or market samples is indicative of human contamination, which could be from direct human contact, such as fingers, or indirectly through material used for processing [
42]. The organism is a gram positive coccus that is resistant to heat, drying, and radiation, and associated with endotoxin characterized by a short incubation period (1–8 h), violent nausea, vomiting, and diarrhea. Detection of
Esherichia coli in most of the samples assessed in this study suggests direct or indirect fecal contamination, because it is commonly used as a surrogate indicator.
The frequency of occurrence of mold isolates in batches A, B, and C market (yam chips, flakes, and flour) and freshly processed (yam chips and flakes) samples is presented on
Table 3. Various mold species were isolated in the market samples, more specifically
Aspergillus flavus,
Aspergillus niger, Aspergillus spp.,
Aspergillus fumigatus. Penicillium verricosum, Penicillium spp.,
Fusarium spp.,
Alternaria,
Penicilum marneffei, and
Mucor spp. The most prevalent isolates are
Penicillium spp. and
Aspergillus niger. Table 3 also reports the frequency of mold isolates in batches A, B, and C of the freshly processed yam chip and flake samples; the same species detected in the market products were also detected in the fresh samples, with the exception of
Penicillium marneffei, which was not detected.
Differently to what observed for the bacterial contamination, the mold isolates data do not show significant differences between the freshly produced food items and those bought from market; this further signifies that the post-processing steps are not mainly responsible for this contamination. The mold isolates in this study are in line with the reports of some others in Nigeria, Benin Republic, and Togo [
8,
32,
41,
43,
44]. As some molds have been identified as soil fungi, it could be deducted that the primary source of contamination is tuber contact with soil and absence of appropriate washing before processing; the use of bruised tubers or contact of healthy tubers with contaminated ones could also represent a critical factor [
35].
According to Adegoke [
45], the presence in food products of some of these molds, especially
Aspergillus flavus and
Aspergillus niger, is highly undesirable. Some of these molds, in fact, have been reported to have public health significance because of the production of mycotoxins, which have implication on consumers’ health and food shelf-life decreasing. This phenomenon is especially prevalent in developing countries, where
Aspergillus flavus and
Aspergillus parasiticus are mainly responsible for mycotoxin production [
46].
3.3. Aflatoxin Contamination in Yam Chips, Flakes, and Flour
Considering aflatoxins, no contamination at all was found in all of the freshly processed yam chip and flake samples from the processors. For the foodstuffs purchased from the markets, however, aflatoxin B
1, B
2, G
1, and G
2 were detected in some samples (
Table 4). Values obtained for Aflatoxin B
1 ranged from 0.7 to 3.4 μg/kg, 0.5 to 3.4 μg/kg, and 0.3 to 0.8 μg/kg in batches A, B, and C, respectively. The results further show that 50% of the yam flour samples were contaminated with Aflatoxin B
1, out of which 22% were above the set limit of 2 μg/kg for aflatoxin B
1 in groundnuts, oil seeds, and other processed products intended for direct human consumption or use as an ingredient in foodstuffs [
47]. Only 11% and 8% of the yam chips and flakes, respectively, were contaminated with the toxin but at levels below the set limit. For Aflatoxin B
2, only flour samples from batches A and B were contaminated with values ranging from 0.1 μg/kg to 0.7 μg/kg, while no contamination was observed in yam chips and flakes. Considering the aflatoxin G
1, only two chip samples and one flake sample showed contamination, corresponding to 11% and 8%, respectively. Flours, on the other hand, showed more contamination—aflatoxin G
1 was detected in 8 out of 18 samples (45%), while aflatoxin G
2 was also found in three flour samples. Aflatoxins G
1 and G
2 were found in neither chips or in flakes.
Overall, results show higher levels of contamination in unpacked yam flour samples with respect to yam chips and flakes. Yam flour samples from the supermarket (i.e., the ones packed at the point of sale) did not contain toxins, except Batch A from Supermarket 1. Most of the contaminants were found in flour samples collected in batches A and B (November 2013 and December 2013, respectively) and this could be attributed to the sampling period during the rainy season, when the relative humidity (>70%) was high enough to permit or favor the proliferation of toxigenic fungi and the subsequent production of their secondary metabolites. This corroborates the findings of Makun et al. [
43], who recorded higher mycotoxigenic fungal contamination during the rainy season than in the dry harmattan season among produce in Nigeria. The higher level of contamination in yam flour samples could also be attributed to the ability to absorb moisture from the environment due to the large surface area of flour particles. Nonetheless, a poor storage condition of the commodity by the sellers could have also had a role in contamination.
Regarding yam chips, results of this study differ from previous literature data for Nigeria [
8,
12,
27,
44,
48] and Benin Republic [
49,
50], which reported high levels of Aflatoxin B
1 contamination, above the 20 μg/kg total aflatoxin levels recommended by WHO and FAO (Food and Drug Administration of United States). The difference could likely be attributed to the different sampling locations and sampling periods.
As stated above, the freshly processed yam chip and flake samples from the processors did not show any contamination with aflatoxins. These results may not seem to agree with the mold isolates data presented above; it has to be highlighted, however, that although
Aspergillus flavus is a renowned aflatoxin producer, not all of its strains are actually capable of producing aflatoxin [
50,
51]. Indeed the interactions between some variables have to be taken into account; these include competing microflora in the samples for nutrients as well as unfavorable environmental conditions for toxin production. In addition, the presence of some active compounds added during parboiling of yam chips may also have an effect on the fungi growth rate, and subsequently, on mycotoxin production [
52,
35].
3.4. Heavy Metals Detection in Yam Chips, Flakes, and Flours
Concentration of the heavy metals lead, cadmium, and nickel in all the yam chip, flake, and flour samples, from the processing sites and from the selected markets, is reported in
Table 5 and
Table 6.
Lead concentration was found to be from 0 to 0.42 mg/kg and 0 to 0.64 mg/kg for freshly produced yam chips and flakes, respectively; for the market samples, on the other hand, the ranges were from 0 to 1.02 mg/kg, 0 to 0.68 mg/kg, and 0 to 1.56 mg/kg for yam chips, flakes, and flour from the three batches, respectively. Overall, the market flakes were more contaminated than the freshly produced ones; in fact, 33% of the freshly processed flakes were found to be above the recommended limit of 0.2 mg/kg of Pb (as established for vegetables, cereals, and pulses) [
53]. For the market flake samples, however, 58.3% had Pb concentration above the limit; the rate of highly contaminated food items, therefore, was significantly higher. This difference was not observed for yam chips, where comparable (and not significantly different) rates of samples with high Pb concentration was observed (33% versus 27.8%). Comparing all samples, it can be seen that the yam flour had the highest concentration of Pb contamination. This could be attributed to the ability of the yam flour samples to accumulate a higher amount of this metal from the environment due to the larger surface area, as compared with the yam chips and flakes. Considering just chips and flakes, however, flakes appear to have higher Pb content. This difference could be due to the fact that the processors dry the flakes by the road side where there is high vehicular movement. Yam chips, on the other hand, are dried on rocks or on farm lands. Indeed, some studies [
54,
55] have shown that exhaust from vehicles and gasoline combustion is one of the principal sources of Pb contamination in the environment. These results also show that the sellers or market and the period of collection were not determinant of the level of safety of the different commodities.
The Cd concentration in the market samples was observed to be in the ranges of 0.01–0.04 mg/kg, 0.02–0.12 mg/kg, and 0.01–0.11 mg/kg in yam chips, flakes and flour, respectively. For the freshly processed samples, however, Cd was not detected in any of the samples; for this metal, therefore, phases like storage and transport were the only sources of contamination. Cd can be present in air and soil (and in low amount in water) due to natural or anthropogenic activity. Volcanic eruptions, forest fires, rock weathering, and wind-blown dust are among the greatest natural sources of Cd. Nonetheless, the pollution caused by human activities can be crucial, with production of polyvinyl chloride plastic manufacturing, alloys, fungicides, solders, motor oil, and rubber and textile manufacturing being the main causes of Cd release in the environment [
56]. The results of this study could be an indication that the contamination of the market samples with Cd could be as a result of accumulation in the air in the course of selling, as these commodities are usually exposed in the market places without packaging. However, none of the yam chip samples was above the recommended limit of 0.05 mg/kg Cd [
57], while only 8.3% of the yam flakes and 11.1% of the flour were over this value. Lower levels of Cd have been observed in yam flour [
58,
59]; again, the variation could be attributed to difference in study location.
Regarding nickel, levels in the different commodities were found to be in the range of 0.07–0.45 mg/kg, 0.01–0.94 mg/kg, and 0.07–0.85 mg/kg in market samples of yam chips, flakes, and flour, respectively, while values for fresh yam chip and flake samples from processors ranged from 0.01 to 0.25 mg/kg and 0.3 to 0.27 mg/kg, respectively. It was observed that 96% and 80% of the market and processors’ samples, respectively, had level of nickel above the FAO and WHO tolerable limit of 0.05 mg/kg. Shin et al. [
58] reported increased Ni contamination of commercial South Korean yam powder collected in South Korea after grinding. The higher level of contamination in market samples than the processors samples, especially in the yam flour, could be attributed to post-processing effects, such as milling, and during the selling phase where commodities are exposed to environmental pollution. High level of nickel contamination above the FAO and WHO tolerable limit in this study agrees with the report of Iweala et al. [
59]. No mercury contamination was observed in this study in any of the commodities, irrespective of the sample collection point and batch.
Overall, it could be concluded that some, although not all, of the studied food items showed contamination with heavy metals at worrying levels, above the recommended concentration. The effect of the post-processing treatment was significant, especially for metals such as Cd and Ni. Levels of heavy metal should be then taken into account before ingestion of the analyzed commodities owing to adverse effect on human health. Studies have shown that Pb affects practically the whole body; the adverse effects include reduction in intelligence quotient (IQ), increased blood pressure, and a range of behavioral and developmental defects [
60]. Cadmium is a very toxic metal with no known biological function, and higher levels may cause health hazards [
61]. According to Galadima et al. [
62] and Luckett et al. [
63], high levels of exposure to Cd is associated with irritation of the eyes and respiratory passage, damage to brain, liver, bones, and kidneys, and with association with some cancers, such as pancreatic cancer. Just like cadmium, nickel is also associated with damage to brain, liver, bones, and kidneys, and bronchitis, dermatitis, hypertension, rickets, and asthma at a high levels of exposure [
62].