Drying Time and Residual Moisture Content
The initial moisture content of MSK varied between 36 and 38% (wb). The MSK were dried to final moisture content ranged from 6.29±0.17 to 9.00±0.35% (wb). The time required for drying of MSK varied significantly with an increase in hot air-drying temperature. For instance, the time taken for drying of seed kernels to a desired final moisture content at 70 °C and 80 °C drying temperature were 472 and 390 min, respectively. The lowest drying temperatures (50 °C and 60 °C) took more time to dry MSK to reach equilibrium moisture content (data not shown).
The result indicated that an increase in drying temperatures noticeably reduced the time required for drying of seed kernel till moisture reaches to the equilibrium. The increased drying temperatures rapidly decreased moisture from produce, which further accelerates the moisture migration out of the food (Mphahlele et al. 2019). Similar findings have been obtained concerning drying of various agricultural by-products such as olive cake (Vega-Gálvez et al. 2010), mango seed kernels (Ekorong et al. 2015), pomegranate peel (Mphahlele et al. 2019), and prickly pear seed (Motri et al. 2013) using different temperatures. Further, the average drying rate (data not shown) was more significant at the beginning of the drying process, conceivably due to evaporation of moisture from the seed surface, which subsequently declined with falling moisture content for all the drying temperatures.
The effect of different drying temperatures on the total carbohydrates and starch content of MSK was evaluated and results are represented in Table 1. The increasing drying temperatures has no significant effect (P<0.05) on carbohydrates content of MSK. The carbohydrate content varied between 71.32±3.30 and 79.20±2.04%. The starch content in MSK increased significantly (P<0.05) after subjecting to drying with increasing temperatures. Drying at higher temperatures (70 and 80°C) had significant positive effect on starch content of MSK as compared to lower drying temperatures (50 and 60 °C). The highest starch content (72.98±5.55%) was observed in MSK samples dried at 80 °C, while the lowest value (44.41±6.82%) was recorded in MSK samples dried at 50 °C.
The experimental results indicated that the seed kernel of mango (Cv-chausa) is a good source of carbohydrates and starch. The obtained values of total carbohydrates in powdered MSK after drying at different hot air temperatures were comparable with previous reported findings. For instance, carbohydrates content in the air-drying (soaking for 30 min, followed by boiling for 15 min in water, then finally drying at 65 oC for 16 h) and pretreated flour of dried seed kernel of mango was found to be 72.07% (Das et al. 2019). Similarly, carbohydrate content of 73% was reported in mango seed kernels, which were subject to blanching and drying (at 85 oC for 24 h) (Uzombah et al. 2019). The composition of dietary fibers in the mango processing by-products depends on both the cultivar and the fruit ripening stage (Ajila et al. 2008). In the present study, the obtained values of starch content (ranged from 44 to 72%) in seed kernel of mango after was reported similar to the reported by previous researchers (Patiño-Rodríguez et al. 2020; Ferreira et al. 2019; Tesfaye et al. 2018). It is evident that starch content in seed kernel manly depends on the genotype and growing climatic conditions. The categorization of MSK starch is chiefly based on the size of its granules, shape, and proportion of amylase and amylopectin (Mwaurah et al. 2020). Digestibility studies on starch have revealed the presence of more of resistant starch than readily and slowly digestible starch (Sandhu and Lim 2008). Resistant starch can be absorbed in the small intestines, hence; it gets fermented by the microbiota in the large intestines (Patiño-Rodriguez et al. 2020). The extraction starch can be used as stabilizers, thickeners, production of alcohol, and in the cosmetic, paper, and textile industries. Concerning reducing sugar, to the best of knowledge, there is no report on how reducing sugar from MSK get affected by different drying temperatures. However, the extraction of reducing sugar from the extract of seed kernel has been reported (Bangar et al. 2021). The amount of reducing sugar recovered decides the efficiency of pretreatment method and will aid in making microbial fermentative metabolite production more economical (Premjet et al. 2018).
Nitrogen and protein characteristics
Nitrogen content (%), hence the protein content of dried MSK samples varied non-significantly across the drying temperature conditions. Nitrogen content in the studied MSK samples ranged from 0.97±0.15% (at 60 °C) to 1.01±0.19% (at 80 °C). The highest protein content (6.30±1.18%) was found in the MSK dried at 80 °C (Table 1). The analyzed powdered samples of MSK showed a reasonable amount of nitrogen and protein content. However, different drying temperatures had no significant (P<0.05) impact on nitrogen and protein (%) in MSK flour. In the case of the effect of drying on protein, the more or less similar value of protein has been reported in MSK dried using different hot air temperatures. For instance, MSK showed the protein values of 8.3% (dried at 65 °C for 16 h), 7.76% (drying at 50 °C), and 6.20% (drying at 85 °C for 24 h) (Das et al. 2019; Ashoush and Gadallah 2011; Uzombah et al. 2019). Previous studies indicated that MSK flour contains reasonable quantities of proteins (from 6 to 7.76%) (Nzikou et al. 2010; Olajumoke 2013).
Protein quality and essential amino acid index of MSK are high, showing the standard quality of the proteins. The bioavailability of MSK protein can be positively compared with the standard protein obtained from eggs (Abdalla et al. 2007). Protein content in mango by-products such as peel and MSK can be correlated with pectin modification during the fruit maturity stages. Besides use as ingredients in functional food development, MSK, being a good source of nitrogen and protein, can be used as feedstock increased production of microbial polyhydroxybutyrate polymer (McAdam et al. 2020).
Effect of different drying temperature on different physicochemical characteristics
Physicochemical characteristics of MSK powder prepared after drying 50-80 °C temperatures are given in Table 2. Significant change in ash content of MSK was not observed after drying at 50-70 °C, however, the ash content was significantly lower for MSK dried at higher temperature. The pH of MSK powder solution changes due to the leaching of MSK compounds to the solution. Specific trend was not observed in surface pH of MSK dried at increasing temperature, however the values were significantly (P<0.05) different at each treatment level. Highest pH value of 5.44 was observed for MSK dried at 70 °C. Ascorbic acid content (mg/100 g sample) was lowest in samples dried at lower temperature an increased significantly (P<0.05) after drying at 60 °C and remained unaffected by increasing drying temperature till 80 °C. Similar trend was also observed in water holding capacity of the MSK powder which remained unaffected by increasing drying temperature after 60 °C. Significant (P<0.05) reduction was observed in OHC of MSK powder as hot air temperature increased from 50 °C to 60 °C. The OHC values increased significantly (P<0.05) with drying temperature increasing from 70 °C to 80 °C.
Surface pH provides information about the change in pH of water when a known amount of MSK is added to it. The changes in pH occur due to the leaching of seed kernel compounds to the solution. In foods, ash content denotes the amount of mineral content as an inorganic portion (Kaur and Srivastav 2018) and as an incombustible solid material. The level of ash (0.70-1.5%) obtained in the present study is comparable to the previous research findings (Okpala and Gibson-Umeh 2013; Ashoush and Gadallah 2011). In the present study, ascorbic acid (2.45-3.27 mg/100 g) values in seed kernels dried using different temperatures are comparatively less than those previously reported (Sogi et al. 2013; Mwaurah et al. 2020). Sogi et al. (2013) studied the effect of drying on the ascorbic acid content of MSK. They reported a significant difference in the ascorbic acid content of MSK subjected to hot air (at 60 °C), vacuum (at 60 °C), infra-red, and lyophilized drying. The drying methods also affect the ascorbic content, and higher drying temperatures reduce the ascorbic acid content (Somsub et al. 2008). At higher drying temperatures, ascorbic acid is rapidly oxidized to dehydroascorbic acid, converted to 2, 3-diketogulonic acid, and finally, polymerized to other nutritionally inactive compounds (Nath et al. 2022). WHC and OHC are the important functional properties of mango kernel. WHC is mainly relies on the amount and types of the hydrophilic constituents, to some extent on the pH and nature of the protein (Owuarnanam et al. 2013). Additionally, several other factors such as porosity charge dependency and pectin structure can influence the WHC of food (Shivamathi et al. 2022). In a similar way, OHC of food is due to its hydrophilic and overall charge density constituents (Bayar et al. 2018). Similar to our findings, the water retention (1.22 g/g) and oil retention capacity (0.94 g oil/g) was reported in the thermal pretreated (Soaking for 30 min followed by boiling for 15 min and drying at 65 °C for 16 h) flour of MSK (Das et al. 2019).
Total phenolics and antioxidant activity
The effect of different HAD temperatures on phenolics and antioxidant content of powdered seed kernel is presented in Figure 2. The total phenolic content of MSK decreased gradually from 8.33±0.23 mg GAE/g to 4.98±0.03 mg GAE/g with increasing drying air temperature from 50 to 80°C. The reduction was significant (P<0.05) at all the drying temperature levels except at highest drying temperature of 80 °C. The drying temperature had significant (P<0.05) negative impact on antioxidant activity of the MSK powder. The antioxidant activity measured in terms of DPPH % scavenging assay significantly (P<0.05) reduced from 80.69±2.76% (at 50 °C) to 61.15±0.76% (at 80 °C).
The phenolics and antioxidant metabolites are the groups of bioactive compounds that perform specific biological actions besides being used as functional food ingredients (Granato et al. 2020). Natural antioxidants act against oxidative stress, reactive oxygen species, and free radicals produced by the body during diverse metabolic processes (Ma et al. 2011, Pateiro et al. 2021). MSK is a good source of phenolic compounds and antioxidants (Castro-Vargas et al. 2019). In our findings, both phenolics and antioxidant activity of MSK were reduced with an increase in HAD temperatures. Similar results on the negative effect of drying on phenolics and antioxidants action of MSK have been reported. For example, hot air oven drying reduced the total polyphenolic content in MSK from 1.20 mg/g (at 40 °C) to 0.20 mg/g at 80 °C (Ekorong et al. 2015). A similar trend was also noticed in the case of total antioxidant activity (Ekorong et al. 2015). In mango by-products, phenolic compounds such as xanthones and flavonoids are susceptible to degradation at higher temperatures (Ancos et al. 2018). At higher temperatures, phenolic compounds are reduced in MSK, probably due to their degradation caused by chemical and enzymatic action and thermal decomposition. Additionally, the possible explanation for the reduction in phenolics content at higher temperatures is a gradual inactivation of polyphenol oxidase (Dibanda et al. 2020). The antioxidant activity is correlated positively with their bioactive compounds, namely with phenolic compounds (Dorta et al. 2012). Thus, MSK subjected to higher drying temperatures has reduced antioxidant activity and phenolic content (Dibanda et al. 2020).
Mineral elemental profile affected by different drying temperatures
The results on the effect of different drying temperatures on the major and micro minerals of MSK powder are tabulated in Table 3. The observed concentrations of major nutrients studied were much higher than micronutrients. Though, the specific trend in terms of concentrations of minerals was not visible. The result indicated that concentrations (mg/kg of the sample) of most analyzed nutrients increased with increasing temperature. For instance, MSK samples dried at 50 °C showed the highest concentrations of K (3,438.5±65.50 mg/kg sample) and Ca (726.50±11.30 mg/kg sample). The highest concentrations of P (1,810.50±34.50 mg/kg sample), Mg (1,395.00±15.00 mg/kg sample), and S (888.75±14.15 mg/kg sample) was observed in samples dried at 70 °C. While the maximum concentrations of all micronutrients such as Fe (80.62±2.58 mg/kg sample), Mn (11.23±0.04 mg/kg sample), and Zn (8.01±0.18 mg/kg sample) was observed in MSK dried at 80 °C. The observed concentrations of nutrients in MP in decreasing order were as follows: K>P>Mg>S>Ca>Fe>Mn>Zn.
The ICP-OES investigation revealed the presence of macro (K, P, Ca, Mg, and S) and microelements (Fe, Mn, and Zn) in all MSK subjected to drying at different HAD temperatures. These nutrients have been detected in the seed kernel of various cultivars of mangoes (Mwaurah et al. 2020). MSK is an acceptable source of minerals and could be used to develop functional foods to alleviate micronutrient deficiency. These nutrients have a pivotal role in human body metabolism. Additionally, MSK with varied concentrations of mineral nutrients may be exploited to design and standardize the fermentation media for higher production of specific microbial metabolites such as polyhydroxy butyrate. The current outcome of the study supports the findings related to the presence of several macro (K, P, Mg, S, and Ca) and micro (Fe, Zn, Mn, etc.) minerals in the MSK (Lasano et al. 2019). However, the reported values for some analyzed mineral nutrients were lower than the findings of the present investigation. The differences in these results could be attributed to the distribution of vascular tissue, sink characteristics, and metabolic rate of the plants (Lasano et al. 2019). The increase in mineral concentration in the by-products of the dried fruit following drying treatment has been reported (Rafiq et al. 2019; Mohammed et al. 2020). This could be due to the excessive desiccation and the substantial increase in dry matter of the dried produce (Mohammed et al. 2020). In addition, Suna et al. (2014) reported that the dry matter and mineral nutrients in dried produce correlated positively.
Effect of different drying temperature on color profile of MP powder
As seen from the colour profile of MSK powder (Table 4) the L* values of mango stone kernels showed non-significant (P<0.05) marginal increase with increasing drying temperatures. Browning index was significantly higher for kernels dried at 80 °C as compared to the lower drying temperatures. Whiteness index of MSK powder bears great importance as it may directly affect the color of starch being extracted as end product. Whiteness index of powder was significantly higher for MSK dried at lower temperature of 50 and 60 °C and decreased significantly with increase in drying temperatures till 70 °C. WI values showed significant (P<0.05) decrease with increasing drying temperature beyond 70 °C. Browning index of powder was significantly (P<0.05) higher for MSK dried at 80 °C as compared to lower drying temperatures owing to higher L* values and lower a* and b* values. The obtained color values are more or less similar to that of reported by Das et al. (2019). Slight deviation in the results may be due to the varietal difference, determination process and error.
Optimization of dilute acid treatment for recovery of reducing sugar from dried stable powdered peel
The suitability of using dilute acids for optimum extraction of reducing sugar from MSK (dried at 70 °C) is expressed in Figure 3. Use of increasing concentrations (from 1% to 3%) of HCl has negative correlation with extraction of reducing sugars from MSK wherein, the values decreased from 398.80±12.49µg/mL to 348.17±13.75µg/mL. However, as the HCl concentration increased to 4 and 5%, the observed values for extraction were significantly (P<0.05) higher (416.22±34.71 µg/mL and 415.78±21.81µg/mL, respectively). The ability of HCl at 4 and 5% concentration to assist higher extraction of reducing sugars from MSK did not vary significantly (P<0.05), and 4% HCl solution was found optimum for the task. Varying concentrations of dilute H2SO4 has non-significant (P<0.05) effect on extraction of reducing sugars from dried MSK. Different levels of dilute H2SO4 were able to extract reducing sugars in the range from 364.41±93.53 µg/mL to 374.72±50.50 µg/mL.
The powdered MSK samples pretreated with varying concentrations of dilute acids improved recovery of reducing/fermentable sugars. To the best of our knowledge, there are no scientific studies dealing with the recovery of reducing sugar from hot air dried MSK powder. However, few researchers have attempted to analyze reducing sugar obtained from fruit processing waste using either acid or alkaline pretreatment. Reddy et al. (2011) also tried to recover the fermentable sugar from mango peels. In the present study, dilute acid hydrolysis pretreatment was used to break down the hemicellulosic and lignocellulosic components of MSK. Dilute acid as a pretreatment can enhance simple sugar release from lignocelluloses biomass. Dilute acid is a promising technique to convert hemicelluloses into monomeric sugars by modifying the chemical structure of lignocelluloses biomass. This pre-treatment yields better extraction of fermentable sugars from fruit processing by-products and other agro biomass (Fernandes et al. 2021). The preliminary treatment of biomass achieves pre-fermentation bioconversion, enabling the availability of sufficient substrate for the activity of fermentative microorganisms. Furthermore, more cellulose in the biomass is made more accessible to the microbial/enzymatic action (Chaudhary et al. 2021), which eliminates the need for cellulase/hemicellulase enzymes mixtures required for the breakdown of cellulose and hemicelluloses. The recovered fermentable/reducing sugar efficiently increases microbial metabolism to produce the surplus amount of desired metabolites of industrial importance. Bacteria and yeast efficiently metabolize fermentable monosaccharides (such as glucose and fructose) as a carbon source. Reducing sugar is a good carbon source in fermentation to synthesize several industrial metabolites from microbes (Fabricio et al. 2022).
Microbial analysis/safety of MSK powder
After 30 days of ambient storage, only a bacterial population (total plate count, CFU/g) was observed among the microbes under consideration (Table 5). A bacterial population, within the safe limit, was observed only in samples dried using 50 and 60 °C. On the contrary, population count of other microbes such as fungi (CFU×103/g), Salmonella spp. (CFU×102/g), and E. coli (CFU×102/g) was nil for all MSK samples dried at different temperatures.
Mango processing wastes are generally susceptible to microbial attack, probably due to high moisture content and bioactive compounds, limiting their reuse in the food industry (Ajila et al. 2007; Sogi et al. 2013). The drying process can overcome this limitation as pretreatment. Drying limits, the activities of microbes and enzymes accountable for degrading MSK and improves safe storage and transportation. The presence of antimicrobial activities in the MSK has been reported. In the current study, MSK samples dried at higher temperatures (60-80 °C) were devoid of any significant microbial presence. Their absence could be linked to the higher drying temperature, lack of moisture, or substrate's antimicrobial nature. The potent antimicrobial activity in South African MSK against Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Candida albicans has been demonstrated (Ahmed et al. 2005). The antimicrobial and antibacterial property of MSK and plant-based compounds is probably due to polyphenolic, flavonoids, tannins, terpenes, and coumarin compounds (Mutua et al. 2016). The capability of different phenolic compounds in inhibiting rot-causing fungi in multiple foodstuffs has been documented (Dukare et al. 2020a). Further, convective HAD can significantly limit the activity of mesophilic bacteria and bacterial pathogens growing on processed products of fruits and vegetables (Alp and Bulantekin 2021). In food processed using higher drying temperatures, microbial growth is restricted probably due to the cell wall damage and protein denaturation (Alp and Bulantekin 2021).