Development of Membrane Filtration Process Protocol for Reduction of Astringency and Retention of Bioactive Compounds in Cashew Apple (Anacardium occidentale L.) Juice

Abstract

India is one of the largest producers of cashew fruits in the world. The fruit consists of cashew apple and the nut. The cashew nut has high commercial value. However, despite a high concentration of bioactive compounds, the cashew apple is still an underutilized by- product of the cashew processing industry. The cashew apple contains a high amount of tannin that gives an astringent taste and has low consumer acceptability. The traditional method of removing tannin and further processing of cashew apple juice involves thermal treatment, which results in the loss of many bioactive compounds, including polyphenols and ascorbic acid. The heat treatment changes the colour and aroma of the juice. It also imparts a 'cooked flavour' to the juice and the value-added products. Hence, it is necessary to develop a non-thermal process protocol to reduce the astringency and retention of the bioactive compounds in the juice. For this purpose, different properties of the cashew apple fruit have been determined. The result showed that this fruit has a high moisture content of 85.62% (w.b.), which is the main reason for its high softness and perishability. The fruit resembled an oblate-ellipsoid shape (sphericity: 0.85) with an average dimension of length (50.34 mm), width (42.78 mm), and thickness (36.08 mm). The pH, total sugar, phenolic content, protein and ascorbic acid of this yellowish colour fruit were 4.37, 10.57%, 365.30 mg/100 g GAE, 1.13% and 218.93 mg/100 g, respectively. Later, juice from the cashew apple was extracted using the enzymatic hydrolysis technique. The effect of pectinase, cellulase, and tannase enzymes on the yield and physicochemical quality of juice was studied to find the most suitable enzyme for enzyme-assisted juice extraction. After considering the optimum processing condition and juice quality, the cellulase enzyme with an optimum incubation time of 27 min, incubation temperature of 34 ºC, and cellulose concentration of 0.091% was the best enzyme for cashew apple juice extraction. Subsequently, technology has been developed with optimum process conditions for clarifying cashew apple juice using centrifugation, microfiltration, and ultrafiltration. At the optimum conditions of 7532 rpm rotational speed and 52.6 min time, the centrifugation removed large suspended particles and improved the clarity of the juice. Similarly, at the optimum conditions of microfiltration (0.2 μm membrane pore size and 138 kPa transmembrane pressure) and ultrafiltration (30 kDa molecular weight cut-off and 138 kPa transmembrane pressure), the steady-state flux obtained was 74.87 L/m2h and 16.55 L/m2h, respectively. The membrane clarification removed all microorganisms, and reduced turbidity, viscosity and total tannin content of the juice. It also increased the colour intensity, improved the clarity, and preserved the ascorbic acid content of the juice. The clarified juice was later concentrated using a reverse osmosis membrane. The optimum process condition for concentration was achieved as 30 bar transmembrane pressure with a steady-state flux of 25.79 L/m2h. The juice concentrated at this condition had a 97% reduction in turbidity, 87% reduction in total tannin, 15% reduction in viscosity, 94% reduction in colour intensity, and 97% increase in clarity. The concentrated juice retained 89% of the ascorbic acid content. A storage study of the concentrated juice at 4 ºC in glass bottles was taken up. The analyzed quality parameters, including the sensory and microbial qualities, confirmed that the juice concentrate could be stored for 12 weeks. In addition, the cost analysis revealed that the manufacturing cost of the developed cashew apple concentrate is ₹1050.31 per L, which is comparable to the current market price of the commercial concentrated juices. The results suggest that the membrane filtration process is suitable for developing astringent-free, clarified cashew apple concentrate and could be scaled up for industrial use.