Phenol metabolism and preservation of fresh in-hull walnut stored in modified atmosphere packaging

Walnut green husk extract enhances the effect of chlorine dioxide on kernel quality and antioxidant properties of fresh-eating walnuts during their shelf life

Fresh-eating walnuts are perishable and become mildewed during shelf life, limiting their sales span. The effects of chlorine dioxide (ClO₂) alone and its combination with walnut green husk extract (WGHE) on shelf stored fresh walnuts were investigated to develop a pollution-free preservative for the produce. The initial development of mildew incidence was delayed by both treatments under 25 °C, whereas, WGHE + ClO₂ acted more effectively than ClO₂ under 5 °C. The WGHE + ClO₂ treatment presented superior effects on improving moisture, soluble sugar and total phenol content, alleviating loss of oil and unsaturated fatty acid and delaying peroxide value increase of walnut kernels at both temperatures. Both treatments inhibited the activities of three lipolytic enzymes and two oxidases at 25 °C and 5 °C, WGHE + ClO₂ acted more effectively at 5 °C. The results guide the combined application of WGHE with ClO₂ on shelf preservation of fresh walnut.

Influence of quince seed mucilage-alginate composite hydrogel coatings on quality of fresh walnut kernels during refrigerated storage

The presence of high unsaturated fat content and polyphenols results in the short storage life of fresh walnut kernels. For prolonging their shelf life, edible coatings incorporated with antimicrobial compounds can be used as a tool. Quince seed mucilage was identified as a novel green biomaterial to be explored as a coating substance. Quince seed mucilage and sodium alginate were mixed in five different proportions of 100:0 (QAH1), 80:20 (QAH2), 60:40 (QAH3), 40:60 (QAH4), and 20:80 (QAH5) and the resultant composite hydrogels were studied for different physical properties. These composite hydrogels incorporated with vanillin were coated on fresh walnut kernels while uncoated samples served as control. Composite hydrogel coatings with a higher proportion of QSM retained a higher whiteness index, lightness (L*), DPPH radical scavenging capacity, total phenolic content, and overall acceptability values in walnut kernels during the entire storage period of 35 days. QAH1 showed the lowest weight loss percentage, lipid oxidation, and yeast and mold counts while the control sample showed the highest (P < 0.05) values. The results concluded that quince-based composite hydrogel coatings were effective in retention of quality and prevention of degradation of fresh walnut kernels during the storage.

Graphical abstract

Physiochemical Responses of the Kernel Quality, Total Phenols and Antioxidant Enzymes of Walnut in Different Forms to the Low-Temperature Storage

Fresh walnut is obtaining high attention due to its pleasant taste and health benefits. This study aimed to evaluate the influence of storage temperatures (0 °C and -20 °C) on the kernel quality, total phenols, and antioxidant enzyme activities of walnuts in three forms (fresh kernels, walnuts with green husk, and walnuts with shell). For a short storage within 3 months at 0 °C, the results revealed that walnuts with green husk provided a better walnut kernel quality resulting from its lower acid value and peroxide value, together with a higher total phenol content and total antioxidant activity, compared with other forms of walnuts. In comparison, frozen storage at -20 °C for a long duration (up to 10 months), found that walnuts with shell showed advantages in improving the kernel quality (fatty acid content, total phenols, and total antioxidant activity) and antioxidant enzyme (peroxidase, catalase, and superoxide dismutase) activities in the kernels, leading to an acceptable range of acid value and peroxide value, compared with other forms of walnuts. Thus, frozen storage at -20 °C showed a potential application in maintaining the walnut kernel quality, especially the walnuts with shell.

An Efficient Virus-Induced Gene Silencing System for Functional Genomics Research in Walnut (Juglans regia L.) Fruits

The Persian walnut (Juglans regia L.) is a leading source of woody oil in warm temperate regions and has high nutritional and medicinal values. It also provides both tree nuts and woody products. Nevertheless, incomplete characterization of the walnut genetic system limits the walnut gene function analysis. This study used the tobacco rattle virus (TRV) vector to construct an infectious pTRV-JrPDS recombinant clone. A co-culture inoculation method utilizing Agrobacterium was screened out from four inoculation methods and optimized to set up an efficient virus-induced gene silencing (VIGS) system for J. regia fruit. The optimized VIGS-TRV system induced complete photobleaching phenotype on the walnut fruits of four cultivars, and the JrPDS transcript levels decreased by up to 88% at 8 days post-inoculation (dpi). While those of browning-related J. regia polyphenol oxidase (PPO) genes JrPPO1 and JrPPO2 decreased by 67 and 80% at 8 dpi, respectively, accompanied by a significant reduction in fruit browning phenotype. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis screening and Western Blot showed that the PPO protein levels were significantly reduced. Moreover, a model of TRV-mediated VIGS system for inoculating J. regia fruit with efficient silence efficiency via co-culture was developed. These results indicate that the VIGS-TRV system is an efficient tool for rapid gene function analysis in J. regia fruits.

Influence of Gum Arabic Enriched with GABA Coating on Oxidative Damage of Walnut Kernels

Because of the higher content of unsaturated fatty acids (UNSFA) and phenolics, walnut kernels are vulnerable to oxidative rancidity and browning due to unfavorable postharvest handling procedures. This study investigates the impact of gum arabic coating enriched with γ-aminobutyric acid (GABA) on oxidative rancidity and browning of kernels during storage at 20 °C. The results showed that the walnut kernels coated with gum arabic (5%) enriched with GABA (0.1 mM) exhibited lower oxidative rancidity and browning, manifested by lower peroxide value and malondialdehyde accumulation along with higher whiteness index. Moreover, kernels had higher UNSFA/SFA ratio as a response to lower lipoxygenase activity and H₂O₂ accumulation. The reduced oxidative browning in coated kernels was accompanied with lower polyphenol oxidase and higher phenylalanine ammonia-lyase activity leading to higher accumulation of phenolics and increased DPPH^• scavenging capacity. Based on our findings, gum arabic coating (5%) enriched with GABA (0.1 mM) may have a commercial potential for maintaining nutritional quality of walnut kernels.

Effects of Passive- and Active-Modified Atmosphere Packaging on Physio-Chemical and Quality Attributes of Fresh In-Hull Pistachios (Pistacia vera L. cv. Badami)

The effects of passive- and active-modified atmosphere packaging (passive- and active-MAP) were investigated on the physio-chemical and quality attributes of fresh in-hull pistachios stored at 4 ± 1 °C and 90 ± 5% R.H. Fresh pistachios were packaged under each of the following gas combinations: active-MAP1 (AMA1) (5% O₂ + 5% CO₂), AMA2 (5% O₂ + 25% CO₂), AMA3 (5% O₂ + 45% CO₂), AMA4 (2.5% O₂ + 5% CO₂), AMA5 (2.5% O₂ + 25% CO₂), and AMA6 (2.5% O₂ + 45% CO₂), all balanced with N₂, as well as passive-MAP (PMA) with ambient air (21% O₂ + 0.03% CO₂ + 78% N₂). Changes in quality parameters were evaluated after 0, 15, 30 and 45 days of storage. Results demonstrated that AMA6 and PMA had significantly lower (7.96 Log CFU g^-1) and higher (9.81 Log CFU g^-1) aerobic mesophilic bacteria counts than the other treatments. However, the AMA6 treatment decreased, kernel chlorophyll and carotenoid content, hull antioxidant capacity, and anthocyanin content. The PMA treatment produced a significant weight loss, 0.18%, relative to the other treatments. The active-MAP treatments were more effective than the passive-MAP in decreasing weight loss, microbial counts, kernel total chlorophyll (Kernel TCL), and kernel carotenoid content (Kernel CAC). The postharvest quality of fresh in-hull pistachios was maintained best by the AMA3 (5% O₂ + 45% CO₂ + 50% N₂) treatment.

Extending storage potential of de-hulled fresh pistachios in passive-modified atmosphere

BACKGROUND

The effects of passive modified-atmosphere packaging (passive-MAP) on the postharvest quality of de-hulled fresh pistachios (Pistacia vera L. cv. Kerman) stored at cold temperature (0 ± 0.5 °C) and 90 ± 1% relative humidity was investigated with fruits under ambient air condition as the control treatment. The fruit quality parameters measured included kernel firmness, color values (L^* , a^* , b^* , h°, and C^* ), weight loss, fungal decay and marketability, ethylene production, respiration rate, and sensory characteristics at 0, 30, 60, and 105 days of storage. The carbon dioxide (CO₂ ) and oxygen (O₂ ) concentrations in the package headspace were monitored during storage.

RESULTS

A modified atmosphere of 0.95-3.35% O₂ and 23.17-29.82% CO₂ was achieved in the passive-MAP treatment. Fruit respiration rates increased significantly relative to controls throughout storage (P ≤ 0.01). However, storage had no significant effect on ethylene production rates. Additionally, passive-MAP-stored fruits maintained firmness, shell lightness, kernel color, and sensory quality with minimum weight loss and fungal decay, compared with the control.

CONCLUSION

These results demonstrate storage life of fresh pistachios in passive-MAP can be extended up to 105 days, compared with 30 days in ambient conditions. © 2019 Society of Chemical Industry.