Enzyme inactivation and its kinetics in a reduced-calorie sapodilla (Manilkara zapota L.) jam processed by thermal-assisted high hydrostatic pressure
Graphical abstract
Introduction
Sapodilla (Manilkara zapota L.) is a climacteric fruit found in various American, Asian, and European countries. Kalipatti is one among the 41 varieties of sapodilla with excellent taste and aroma, soft and mellow flesh, high productivity, and less number of seeds. Sapodilla fruit is rich in sugars, proteins, bioactive compounds, and minerals. It is a potential source of antioxidants attributing to its various health benefits including the anticancer activity (Leong and Shui, 2002; Srivastava et al., 2014). However, it has a limited shelf-life of up to nine days at ambient temperature and it is also vulnerable to cold injury at lower temperatures (Diaz-Perez et al., 2000). Sapodilla fruit possesses the oxidative enzymes which are responsible for the degradation of fruit color, flavor, and nutritional value (Chakraborty et al., 2014b; Matoba et al., 2011). Polyphenol oxidase (monophenol monooxygenase; EC 1.14.18.1) is responsible for the color degradation in sapodilla fruit by forming brown compounds – quinones, from interaction with the phenolic compounds (Cortez et al., 2013). Peroxidases are also found in considerable amount in sapodilla fruit and contribute to enzymatic browning (Alia-Tejacal et al., 2007). Therefore, sapodilla needs to be preserved for off-season where jam is an excellent option as sapodilla is rich in sugars (∼25°Brix) as well as fiber (41% on dry basis), required for the development of jam texture, particularly in high-pressure processed jam.
Fruit jam is a widely consumed intermediate-moisture food product mainly composed of fruit pulp/puree, sucrose, citric acid, and pectin. According to the international standards, the total soluble solids (TSS) content of jam should be more than 65%, required for gel formation and jam shelf-stability (Codex stan 296–2009). The low water activity (achieved by high sugar content) and acidic pH make the jam shelf-stable. However, the thermal processing has a major role in jam shelf-stability. But, a massive loss of bioactives take place while thermal processing due to their heat-labile nature (Shinwari and Rao, 2018b). Moreover, a huge difference between fruit and jam with respect to color and aroma is observed due to the thermal processing of jam which necessitates the external addition of color and flavoring agents. On the other hand, high pressure processing (HPP) has minimal effect on nutritional and sensory properties of a food product while achieving the microbial and enzymatic inactivation in food (Basak and Ramaswamy, 1996, 1998; Cheftel, 1995; Gimenez et al., 2001; Igual et al., 2013; Knorr, 1995; Shinwari and Rao, 2018a, 2018b, 2020a, 2020b). The HPP is one of the novel food processing technologies among the others like pulsed electric field, cold plasma, irradiation and ohmic heating (Shinwari and Jindal, 2015; Tiwari et al., 2009). The plant bioactive compounds viz. polyphenols, flavonoids, etc. have covalent bonds, which are not affected by pressure processing (Chakraborty et al., 2016; Shinwari and Rao, 2018a; Shinwari, 2021). However, HPP enhances hydrogen and hydrophobic interactions required for gel formation by volume reduction (Shinwari and Rao, 2018c). Therefore, HPP favors the gel network formation in the jam with reduced sugar content (Shinwari and Rao, 2020a, 2020b).
The enzyme inactivation by HPP has been widely studied in fruit juices and purees, but scarcely reported for the fruit jam. Only few studies have been reported on the application of high-hydrostatic pressure for the jam processing (Dervisi et al., 2001; Gimenez et al., 2001; Igual et al., 2013). The inactivation of only pectinmethylesterase and peroxidase enzymes has been studied in HPP jam, and limited to grapefruit jam (Igual et al., 2013). Moreover, the kinetics was applied for enzyme inactivation during the whole pressurization cycle including the compression. Therefore, the aim of the study was to evaluate the impact of high pressure processing parameters on the residual enzyme activity including polyphenol oxidase (PPO) and peroxidase (POD) enzymes in sapodilla jam, which has not been attempted so far. Secondly, the enzyme inactivation kinetics was applied after the segregation of the pressure cycle, which is yet to be established in case of a fruit jam. The pressure and temperature sensitivity of the inactivation rate constant was also explored.
Section snippets
Chemicals and reagents
Anhydrous citric acid; catechin; dipotassium hydrogen phosphate; disodium hydrogen phosphate; hydrogen peroxide; polyvinylpolypyrrolidone (PVPP); potassium dihydrogen phosphate; sodium chloride; and sodium hypochlorite were purchased from Merck (India). Table sugar purchased locally was used for jam making. High methoxyl pectin (degree of esterification = 65–70%), Triton X100, and all culture media were purchased from Himedia (Mumbai, India). Deionized water, obtained through a Millipore
Microbiological quality of high-pressure processed samples
All the samples having the pH 3.3 ± 0.1 were microbiologically safe immediately after the treatment in terms of yeast and mold count, aerobic mesophiles, psychrotrophs and coliforms. The microbial count in the treated samples was under the detection limit (10 cfu·mL−1), which might be due to the low initial microbial load and the hygiene maintained during handling of the samples. Chakraborty et al. (2015) also reported yeast and mold count below the detection limit in pineapple puree processed
Conclusion
The high-pressure processed jam samples were microbiologically safe. However, the enzyme inactivation by HPP within the studied domain was partially achieved because of the lower water activity of the jam matrix compared to fresh fruits or juices. But, all the process parameters (temperature, pressure, and dwell time) showed a significantly positive effect on the inactivation of both PPO and POD. A good fit of the inactivation kinetic model was observed after segregating the whole
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The monthly stipend during the research period was provided by the Ministry of Human Resource and Development, Government of India through the Indian Institute of Technology Kharagpur.
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