Effect of some weak acids and Moringa oleifera leaf extract powder on the physicochemical and storage properties of dried Granny Smith apple

Abstract

This study aimed to find alternatives to sulphite as a preservative for dried fruits which provided a quality product with a reasonable shelf life. Granny Smith apples were sanitised in a 200 ppm sodium chlorite solution, de-cored, peeled and cut into slices. The sliced apples were pre-treated/dipped in a water solution containing the three weak acids namely; ascorbic acid, citric acid and potassium sorbate as well as Moringa oleifera leaf extract powder (MOLEP). A screening fractional factorial experiment consisting of five independent variables (ascorbic acid, citric acid and potassium sorbate, time and temperature and MOLEP) constrained at their upper and lower levels (ascorbic acid: 0.5% to 2.0%, citric acid: 0.3 to 2.0%, MOLEP: 0.1% to o.2%, time: 7 h to 16 h and temperature: 57°C to 70°C) were evaluated for their effect on the colour of the dried sliced apples. Ascorbic acid and potassium sorbate did not have an impact on the lightness of the dried sliced apples. A dipping solution of citric acid at 2.0%, Moringa oleifera leaf extract powder at 0.1% (CMO) and drying time of 7 h at 70°C reduced the browning of the dried sliced apples. Storage stability of the dried sliced apples were done using the optimum dipping solution with citric acid at 2.0%, MOLEP at 0.1% and another dipping solution with citric acid at 2.0%, MOLEP at 0.1% and potassium sorbate at 0.2% (CMOP). Microbiological testing (osmophilic yeast, Escherichia coli and yeast and moulds) and total acidity were done as well as physical tests such as moisture analysis, water activity, texture analysis and colour at day 0, day 60 and day 120. The optimum combination of 2% citric acid and 0.1% Moringa oleifera extract powder (CMO) effectively minimized the increase in the redness and the yellowness of the dried apple slices over the storage period. The osmophilic yeasts, E.coli, yeasts and moulds results of the dried apple slices pr-treated with the water solution of 2.0% citric acid and 0.1% Moringa oleifera leaf extract powder (CMO) were within the limits as prescribed by Codex Alimentarius Commission (2003) at the start and end of the shelf life study. The water activity of the dried apple slices pre-treated with CMO was below the limit (<0.6%) conducive to microbial growth during the storage time. Moisture increased over the shelf life period which affected the extensibility of the pre-treated dried sliced apples negatively. A 3 month (120 days) shelf life was accomplished, using the optimal combination of 2% citric acid and 0.1% Moringa oleifera leaf extract powder. A consumer acceptability test was done using the 9-point hedonic scale. The dried apple slices pre-treated with the optimal combination of 2% citric acid and 0.1% Moringa oleifera leaf extract powder water solution were acceptable to consumers with regards to colour, texture and taste. The CMO pre-treatment was effective in reducing browning and inhibiting microbial growth of the dried apple slices.