Effect of UV-C treatments followed by storage on the water activity, fungal growth and aflatoxin content of sesame seeds
Water activity (aw) is an essential measure of viable water for chemical reactions, microbial growth and shelf-life stability of a product. The changes in the water activity (aw) of the UVC treated sesame seeds alone or followed by storage are shown in Table 1. The aw for untreated sesame seeds was found to be 0.196 and was significantly (P < 0.05) decreased to 0.184, 0.182 and 0.177 when the sesame was treated with 2.5, 5.0 and 10.0 kJm-2, respectively. During the storage, the seeds' aw was found to be significantly higher than that of unsorted seeds. It was increased as the storage time was increased for each UVC dose.
The fungal growth of the UV-C treated sesame seeds before and after storage was illustrated in figure 1. Before the treatment, the fungal load in the sesame seed was found 4.56 log cfu/g. Exposition of the seeds to the UV-C causes a significant (P < 0.05) reduction in the fungal load of the seeds. It was decreased (P < 0.05) as the dose was increased. The lowest fungal load value 4.1 log cfu/g was recorded when the seeds were treated with 10.0 kJm-2. After storage, the fungal load was decreased for untreated and treated samples, particularly those stored for 12 months. The lowest fungal colonies were observed among the stored seeds when the seeds were treated with 10.0 kJm-2. It was found to be 4.07 and 3.8 log cfu/g for the storage period of 6 and 12 months, respectively.
The reduction of the fungal load in the treated samples might result from the UV-C's deadly effect on fungal DNA transcription and replication, which diminishes spore multiplication (Braga et al., 2015). Moreover, UV-C treatment motivates enzymes that respond to the synthesis of phenolic compounds, which acts directly as a defensive response or indirectly by strengthening the cell wall (Shenga et al., 2018, Wu et al., 2016). Although fungi can repair DNA damage caused by UV-C radiation over the storage period (Goldman and Kafer, 2004, Wen et al., 2019), our finding revealed that the UV-C radiation was able to efficiently decrease fungal load in sesame seed up to 12 months of storage particularly at 5.0 and 10.0 kJm-2.
Table 2 describes the aflatoxin content (B1, B2, G1, and G2) in control and UVC treated sesame seeds before and after storage. It was clear that the aflatoxin content of the control samples is lower than Method Quantification Limit (MQL) even after storage. The aflatoxins were also not detected in the UVC treated samples even after storage for 12 months. Although the untreated seeds and UVC treated seeds were stored at optimal conditions for fungal growth, the level of aflatoxin in the seeds was not raised. This might be attributed to controlling postharvest processing practices essential to maintaining the safety and quality of agricultural products.
Effect of UV-C treatments followed by storage on the colour of sesame seeds
Table 3 describes the effect of the UV-C treatments followed by storage times on the colour of the sesame seeds. The values of the lightness (L*), Redness (a*) and yellowness (b*) of the seeds showed significance to the UV-C doses as well as to the storage time. Before UV-C treatment, the L* value was found to be 64.6. The lightness of the seeds (L*) was progressively decreased after the UV-C treatment. It was reduced (P < 0.05) to 62.2m 61.8 and 60.6 when seeds were treated with 2.5, 5.0 and 10.0 kJ-2, respectively. On the other hand, during the storage of the control and UVC-treated seeds for 6 and 12 months, the lightness of the sesame seeds was significantly (P < 0.05) decreased for each UV-C dose.
The sesame seeds' a* and b* values were increased by increasing the UV-C doses (Table 2). However, there was no significant (P < 0.05) impact of the UV-C amounts on the a* values of the sesame seeds. Untreated seeds with UV-C exhibited lower a* values (0.5) and b* value (23.0) than sesame treated with UV-C between 5.0 to 10.0 kJm-2 at each storage time. Nevertheless, (P < 0.05) a* values were found for the stored seed treated with UV-C, the stored samples showed higher a* values than those without storage for each UV-C dose. The highest a* and b* values 5.6 and 29.3 were recorded in the seeds treated with 10.0 kJm-2 and stored for 12 months.
The total colour difference (ΔE) in sesame seeds treated with 0.0, 2.5, 5.0 and 10.0 kJm-2 was described in figure 2. It was clearly observed that the ΔE in the seeds significantly (P < 0.05) increased as the doses was increased. The ΔE was found to be 2.6, 3.06 and 4.28 when the seeds treated with UV-C doses of 2.5, 5.0 and 10.0 kJm-2. Similar observation was also recorded in each storage time. Also, theΔE was found to increase significantly (P < 0.05) during the storage of UVC-treated sesame seeds. For each UVC dose, the ΔE value significantly increased as the storage time was increased.
Our findings demonstrate that the UV-C application at 2.5, 5.0 and 10.0 kJm-2 resulted in perceived colour changes by visual observation. The changes in colour due to the UV-C treatments in chicken meat and goat meat are also reported by Lázaro et al. (2014) and Degala, Mahapatra, Demirci, and Kannan (2018). Change in colour after UV-C treatment might be due to Lipid oxidation and protein denaturation of seeds leading to exposure of hydrophobic groups and increased free water changing meat surface reflectance (Koutchma et al., 2009, Canto et al., 2016). Moreover, this colour change might be due to the effect of UV-C on the main groups of pigment that contribute to the seeds' colour (Naradisorn, 2021).
Effect of UV-C treatments followed by storage on the free fatty acid of sesame seeds
Figure 3 represents the effect of UV-C treatments followed by storage on the free fatty acid of sesame seeds. It was observed that there was a dramatic increase in the FFA levels in sesame seeds with the rise of the UV-C dose. It was increased from 2.6 mg/g to 2.8, 2.9 and 2.9 mg/g when sesame seeds exposure to the UV-C at 2.5, 5.0 and 10 kJm-2, respectively. Likewise, for each UV-C dose, the content of the FFA was increased when the seed was stored for 6 and 12 months. Interestingly, the higher increasing level of the FFA (11.5 & 15.4%) was observed in the untreated seeds when they were stored for 6 and 12 months. However, the level of increase of FFA content was ranged between 3.6 to 7.7% in the treated samples. Said et al. (2020) stated that the destruction of lipase activity might reduce FFA formation in treated samples during storage due to periodic UV-C irradiation.
The FFA content is an index of rancidity and contributes to the development of off-flavour and off-odours in oil during storage. Hence, as an effective method to destroy the enzymes and, in this way, prevent the formation of free fatty acid in stored seeds and stabilise their shelf life.
Principal component analysis and Partial Least Squares regression analysis
The Principal Component Analysis (PCA) shows the interrelationships between UV-C doses and storability and quality characteristics of a sesame seed (Figure 4A). The axes contribution of the principal components F1 and F2 is explained as 68.42% and 25.34%, which resulted in high variability (93.76%) of the plotted components. Moreover, the strongest positive correlation was also observed between UV-C treated sample and seeds storability and quality parameters rather than between control samples and parameters. According to Yan and Fregeau‐Reid (2008), the variable eccentricity and observation that appear <90° angle is positively correlated, while that of angels > 90° is related with negative correlation, and those with a 90° angle do not show a correlation in the biplot. Consequently, the UV-C doses alone or followed by storage periods were grouped into three groups in the biplot according to their impacts on the storability characteristics of the sesame seeds. The control (0 kJm-2) showed greater values of the fungal load and L value of the sesame seeds. These observations revealed that UV-C treatments of sesame seeds could improve their storability and nutritional characteristics.
The Partial Least Squares regression analysis (PLS) was illustrated the interactive effects of the UV- C (x variables) on the measured parameters (y variables) of the sesame seeds (Fig. 4B). According to this model, the UV-C doses were grouped into four groups regarding their effect on the fungal growth, FFA, colour values (L*, a* & b*), changes in colour ∆E and water activity (aw). The PLS model revealed that the UV-C treatments alone or followed by 6 or 12 months' storage of sesame seeds showed a positive validation score for most of the studied parameters. However, the PLS specified that the application of UV-C at 5.0 kJm-2 followed by 6 months' storage reflects the most proper treatment for functional food applications, which might consider for food industry applications.