On the Effect of Microwave Heating on Quality Characteristics and Functional Properties of Persimmon Juice and Its Residue

Abstract

In this study, it was investigated whether integration of microwave-heating into the pretreatment step of persimmon juice processing allows the concomitant production of both functional juice and added-value solid residue from the Diospyros Kaki "Jiro" cultivar. In this direction, persimmon pulp was treated under three different microwave-heating conditions (0.7, 4.2, and 8.4 kJ/g) prior to enzymatic maceration and compared to the non-heated material. Irrespective of microwave energy employed, the proposed hybrid treatment was highly efficient in terms of juice yield (70% w/w). The mildest heating conditions resulted in juice and residue that were both of inferior quality. Intensification of the microwave energy reduced the microbial load of the juice up to 2-log without compromising the content in total soluble solids, sugars, and L-ascorbic acid. Under the most drastic conditions, the juice was enriched in gallic acid, polyphenols, and potent DPPH^● scavengers, but its orange color faded and was more acidic. In parallel, the solid juice residue retained pro-vitamin A carotenoids (~278 µg retinol activity equivalents) and low-methoxy pectin (9 g/100 g DW). Overall, our findings can assist the efforts of the local juice processing industry to utilize persimmon fruits through energy-efficient technologies in a sustainable approach.

Keywords: DPPH● scavenging activity; Diospyros kaki Jiro; carotenoids; color; juice; microwave heating; pectin; persimmon; polyphenols; solid residue.

Figure 1

Flow diagram of persimmon juice processing without (black line) or after persimmon pulp treatment at different microwave (MW) heating conditions (red line) (the inset table shows the tested MW-heating conditions) (* the inset table shows the tested MW-heating conditions).

Figure 2

Changes in the population (log₁₀CFU/mL) of aerobic mesophilic counts (AMCs) and yeasts and molds counts (YMCs) in persimmon juice produced without (PJ-C) and after persimmon pulp treatment at different microwave heating conditions (PJ-M30, PJ-M60, and PJ-M120). Error bars represent the standard deviation (sd) of the mean value (n = 4). Different lowercase letters in the same substrate indicate significant differences (p < 0.05).

Figure 3

Changes in L-ascorbic acid content (mg/L) of persimmon juice produced without (PJ-C) and after persimmon pulp treatment at different microwave heating conditions (PJ-M30, PJ-M60, PJ-M120). Error bars represent the standard deviation (sd) of the mean value (n = 4). Different lowercase letters indicate significant differences (p < 0.05).

Figure 4

Antioxidant potential of persimmon juice without (PJ-C) and after persimmon pulp treatment at different microwave heating conditions (PJ-M30, PJ-M60, and PJ-M120 using the Folin–Ciocalteu (F–C) and DPPH^● assays. Error bars represent the standard deviation (sd) of the mean value (n = 4). Different lowercase letters in the same test indicate significant differences (p < 0.05).

Figure 5

Smoothed, baseline-corrected FT-MIR spectra of pectin isolated from persimmon juice residues obtained from juice production process without (PJR-C) and with pulp treatment at different microwave heating conditions (PJR-M30, PJR-M60, and PJR-M120). The spectrum of commercial low-methoxy pectin from citrus is also shown.

Figure 6

RP-(C30)-HPLC carotenoid profile at (A) 450 and (B) 402 nm. Chromatographic traces from bottom to the top represent untreated persimmon pulp (PP) (a) and persimmon juice residues (PJR) after microwave heating of the pulp for (b) 30 s: PJR-M30; (c) 60 s: PJR-M60; and (d) 120 s: PJR-M120. Each trace was divided in 62 sub-regions based on the elution time intervals of every carotenoid peak detected in the studied extracts. Further information about the elution order and spectral characteristics of the peaks is provided in Appendix A (Table A1).

Figure 7

PCA scatterplots of RP-(C30)-HPLC-DAD data at 450 nm representing (A) the distribution of persimmon juice residue (PJR) samples according to t1/t2 score values and (B) the contribution of individual carotenoids to the scattering of PJR samples. Persimmon pulp (PP) was included as a reference for monitoring the changes in the native carotenoid forms of persimmon pulp.