1. Introduction

Mushroom resources have been exploited in most developed economies because of their huge agro-industrial, medicinal and commercial benefits ^[1]. Nigerians utilized mushroom-forming fungi only for food and folk medicine for many decades. Auricularia auricular Judae (Bull.) Quél, Lentinus squarrosulus Mont., Pleurotus tuberregium (Fr.) Singer and Volvariella volvacea (Bull.) Singer was some of the common edible mushrooms that were successfully cultivated in Nigeria on small-scale basis. The mushroom resources in Nigeria are grossly under-studied and their attractive potentials under-exploited for addressing economic and industrial development. Resourceful biotechnological approach in the application of mushrooms in agriculture, medicine, industry and environment is inchoate and uncommon in the country.

Functionality of food protein has been defined as those physical and chemical properties which affect the behaviour of proteins in food systems during processing, storage, preparation and consumption ^[2]. Functional properties constitute the major criteria for the adoption and acceptability of proteins in food systems. Many factors influence the functional properties of proteins, including moisture, temperature, pH, enzymes concentrations, reaction time, chemical additives, mechanical processing, ionic strength and amount, sequence, rate and time of additives had been studied ^[3].

Protein functionality is dependent on hydrophobic, electrostatic, and steric parameters of the proteins, which are essential for defining the protein structure ^[4]. Functional properties may be classified according to the mechanism of action on three main groups. These groups are (1) properties related with hydration (absorption of water/oil, solubility, thickening, wettability); (2) properties related with the protein structure and rheological characteristics (viscosity, elasticity, adhesiveness, aggregation and gellying), and (3) properties related with the protein surface (emulsifying and foaming activities, formation of protein-lipid films, whippability) ^{[5, 6]}. These properties vary with pH, temperature, protein concentration, protein fraction, prior treatment, ionic strength and dielectric constant of the medium as well as other treatments such as interactions with other macromolecules in the medium, processing treatments and modifications, physical, chemical and enzymatic methods ^[5].

The addition of salt may increase the total water content of the protein system at specific water activity value, although it may decrease the preferential binding of water to the protein. These effects are marked by dependence on anion and cation components ^{[7, 8, 9]}.

Far more work must be carried out on those characteristics which will result in products which are more convenient to distribute, easier to process, and have the physical, chemical and organoleptic properties required by the target markets. For those foods such as mushrooms that do not have the native functional characteristics that are desired by the target market, an additional effort must be made to value-add or modify them so that they can compete internationally.

The aim of this research work is to determine the effect of NaCl, NaSO₄, NaNO₃, NaNO₂ and CH₃COONa on protein solubility of seven mushroom samples found in southwest of Nigeria.

2. Materials and Methods

The mushrooms; Lentinus subnudus Berk (M₁), Chlorophyllum molybditis (M₂), Volvariella esculenta (M₃), Coprinus tramentarius (M₄), Pleurotus ostreatus Jacq (M₅), Termitomyces microcarpus (M₆) and Pleurotus pulmonarius (M₇) were collected from Federal College of Agriculture campus, Akure, Ondo State, Southwest part of Nigeria. The bad or rotten samples were sorted out. The samples were oven dried at 65°C for 72 hours [ref?] and were then pound into powdered form using porcelain pestle and mortar. The milled samples were then sieved with a 2 mm mesh sized sieve [ref?] and stored in waterproof polyethylene bags at room temperature for further analysis.

0.2 g of each flour sample and 10 ml of a particular salt concentration were thoroughly mixed with a magnetic stirrer (1000 rpm) at room temperature (30°C). Insoluble materials were removed by centrifuging at 3500 rpm for 30 min. The supernatant was dissolved by the Biuret method. The salts used were sodium nitrite, sodium nitrate, sodium chloride, sodium acetate, sodium sulphate with their concentrations ranging between 2 and 12%). The nitrogen obtained was converted to crude protein by multiplying by 6.25 [ref?]. The results were recorded as means of triplicate determinations.

Data obtained were generated in triplicates and analyzed using Mean, Standard deviation and one-way analysis of variance with Duncan Multiple Range test at 95% confidence or p < 0.05.

3. Results and Methods

The results in Table 1 showed the effect of NaCl on the PS of the mushroom samples in percentage. The PS results varied from 9.05 ± 2.00 to 56.31 ± 2.70% and significantly differed from one another at p > 0.05. The PS of M₁, M₃, M₆ and M₇ decreased with increased salt concentrations while the PS of M₂, M₄ and M₅ increased with increased salts concentrations.

The results obtained in this work are in agreement with the works of Mepa et al., ^[10] on the maximum solubility (50.2%) using 1.0 M NaCl solution on coconut protein concentrates. They also observed that at high NaCl concentration, solubility decreased drastically. According to researchers, protein solubility has been recorded to be a useful indicator for the performance of protein isolates incorporated in food systems and to determine the extent of protein denaturation because of heat or chemical treatment at line with those reported by others ^[11].

Table 1. The Results of the Effect of NaCl on Protein Solubility (%) of Mushroom Samples

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Table 2. The Results of The Effect of Na₂SO₄ on Protein Solubility (%) of Mushroom Samples

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The results in Table 2 showed the effect of Na₂SO₄ on the PS of the mushroom samples in percentage. The PS results varied from 6.27 ± 2.06 to 43.25 ± 2.04% and significantly differed from one another at p > 0.05. The PS of M₁, M₂, M₄, M₅ and M₇ decreased with increased salt concentrations while the PS of M₃ and M₆ increased with increased salts concentrations.

Our results are not comparable with results obtained by Masood and Rizwana ^[12] on legumes protein isolates. They recorded between 65 and 82%, which are far above our results. This higher solubility of legumes protein isolates as compared to mushroom samples may be due to the presence of low number of hydrophobic residues and the elevated charge. The solubility of a protein is usually affected by its hydrophilicity or hydrophobic balance, depending on surface active agents, can form and stabilize the amino acid composition, particularly at the protein surface ^[6].

The results in Table 3 showed the effect of NaNO₃ on the PS of the mushroom samples in percentage. The PS results varied from 6.27 ± 2.06 to 43.25 ± 2.04% and significantly differed from one another at p > 0.05.The PS of M₁, M₂, M₄, M₅ and M₇ decreased with increased salt concentrations while the PS of M₃ and M₆ increased with increased salts concentrations.

According to Mwasaru et al. ^[13], the calculated results obtained for protein solubility of pigeon pea and cowpeas were 53.4 and 61.8% respectively. The higher solubility of legume protein isolates as compared with our results on mushrooms may be due to the presence of low number of hydrophobic residues and the elevated charge. The identification of some functional properties of these mushroom varieties is essential in determining their potential uses in the formulation of foods.

Table 3. The Results of Effect of NaNO₃ on Protein Solubility (%) of Mushroom Samples

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The results in Table 4 showed the effect of CH₃COONa on the PS of the mushroom samples in percentage. The PS results varied from 6.00 ± 2.00 to 45.00 ± 2.00% and significantly differed from one another at p > 0.05. The PS of M₁, M₂, M₄, M₅ and M₇ decreased with increased salt concentrations while the PS of M₃ and M₆ increased with increased salts concentrations. The results obtained using NaNO₃ had a similar PS when compared to these results, but PS of NaCl was quite higher than these results. In this work, CH₃COONa may have decreased the preferential binding effect of water to the mushrooms’ protein.

Table 4. The Results of the Effect of CH₃COONa on Protein Solubility (%) of Mushroom Samples

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The results in Table 5 showed the effect of NaNO₂ on the PS of the mushroom samples in percentage. The PS results varied from 6.20 ± 2.00 to 54.28 ± 2.06% and significantly differed from one another at p > 0.05. The PS of M₁ and M₂ decreased with increased salt concentrations while the PS of M₃, M₄, M₅, M₆ and M₇ increased with increased salts concentrations. The results obtained here were in agreement with results of Mepa et al. ^[10]. The coconut protein in their work showed solubility of about 54.6% and 53.2% in FFC-PC and DFC-PC, respectively.

Table 5. The Results of The Effect of NaNO₂ on Protein Solubility (%) of Mushroom Samples

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4. Conclusion

From the results obtained in this work, it is clear that salts have effects on the protein solubility of the different mushroom varieties. In comparing the different salts used it was observed that NaCl was better in comparisons with other salts concentrations. It had higher protein solubility at the least and higher concentrations.

Recommendation

It is recommended that in food formulations, different concentrations of NaCl should be harnessed.

References

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