Isochoric freezing and isochoric supercooling as innovative postharvest technologies for pomegranate preservation

Authors: Bilbao-Sainz, Cristina; Chiou, Bor-Sen; Takeoka, Gary; Williams, Tina; Wood, Delilah - De; POWELL-PALM, MATTHEW - University Of California Berkeley; RUBINSKY, BORIS - University Of California Berkeley; Wu, Vivian; McHugh, Tara

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/16/2022
Publication Date: 8/24/2022
Citation: Bilbao-Sainz, C., Chiou, B., Takeoka, G.R., Williams, T.G., Wood, D.F., Powell-Palm, M., Rubinsky, B., Wu, V.C., McHugh, T.H. 2022. Isochoric freezing and isochoric supercooling as innovative postharvest technologies for pomegranate preservation. Postharvest Biology and Technology. 194. Article 112072. https://doi.org/10.1016/j.postharvbio.2022.112072.
DOI: https://doi.org/10.1016/j.postharvbio.2022.112072

Interpretive Summary: Pomegranate is a highly valuable fruit due to its sensory and nutritional properties as well as potential health promoting benefits. However, pomegranate has a short two to three months harvest season. Pomegranate is also very sensitive to climatic conditions, such as heat, cold, water scarcity, heavy rain and hail, causing disorders such as sun-burnt husk, husk scald or splits and cracks in the husks. Externally damaged whole fruits could be used for “ready to eat” pomegranate arils. However, extracted pomegranate arils have a greatly reduced postharvest life compared with whole fruit. In this study, we investigated two innovative technologies, isochoric supercooling and isochoric freezing, to extend the shelf-life and preserve the quality of whole pomegranate and fresh-cut arils after harvesting. We found out that Isochoric supercooling of whole pomegranates at -2.5°C maintained aril quality in terms of mass, appearance, color and texture properties. Isochoric supercooling was also useful in maintaining characteristics that contributed to organoleptic quality, such as total soluble solids, titratable acidity and pH. In addition, isochoric supercooling increased ascorbic acid and anthocyanin contents. On the other hand, isochoric freezing at -2.5°C of whole pomegranates proved to be more beneficial in increasing ascorbic acid content by pressure-induced impregnation, and to inhibit microbial growth while preserving total soluble solids, titratable acidity, pH, color and anthocyanin content. However, the hydrostatic pressures developed during isochoric freezing adversely affected the texture of the arils. Isochoric supercooling at -2.5°C was the most effective preservation technology for fresh-cut arils. Isochoric supercooling minimized microbial growth and better maintained color and texture properties while increasing ascorbic acid content. Cold storage caused visible microbial decay and quality loss in terms of mass, color, texture and phytochemical contents. Isobaric freezing caused the greatest mass loss, leading to significant deterioration in the arils’ color and texture. In comparison, isochoric freezing at -2.5°C inhibited microbial growth, but adversely affected the color of the arils when compared with isochoric supercooling.

Technical Abstract: In the present study, the effect of isochoric supercooling at -2.5 °C and isochoric freezing at -2.5 °C/15 MPa on the qualitative attributes of arils from whole pomegranates (cv. “Wonderful”) and fresh-cut arils preserved for 30 days was investigated and compared with cold storage at 5 °C/95% RH and isobaric freezing at -2.5 °C/0.1 MPa. Mass, total soluble solids (TSS), titratable acids (TA), pH, color, texture, microstructure and phytochemical components including anthocyanin and ascorbic contents were determined. Isochoric supercooling of whole pomegranates led to color and texture retention, and increased levels of total anthocyanins and ascorbic acid. In addition, isochoric supercooling inhibited mesophilic aerobic bacteria growth and led to significantly lower counts of yeast and molds in comparison with cold storage at 5 °C. On the other hand, isochoric freezing was the most effective technology to inhibit bacterial, yeast and fungal growth and to increase ascorbic acid content. However, this technology caused some loss in texture. Isochoric supercooling was also the most effective preservation technology for fresh-cut arils to minimize loss in textural and color properties. In addition, mesophilic aerobic bacteria, yeast and mold counts were significantly lower in comparison to cold storage at 5 °C. These findings suggested that isochoric supercooling can be beneficial as a postharvest technology to extend shelf life and maintain physicochemical quality of whole pomegranates and fresh-cut pomegranate arils.