Atmospheres enriched with CO2 can create fungistatic conditions, and therefore, inhibit the growth of fungi. Raspberries exposed to CO2 levels of 20 kPa or higher had delayed gray mold decay and extended shelf life . Nine red raspberry genotypes were stored in controlled atmospheres with 12.5 kPa CO2 and 7.5 kPa O2 for 50 days at 1℃, and decay development was strongly suppressed across all genotypes . Our objective was to determine the optimum atmosphere to extend raspberry shelf-life and maximize quality during transit or storage by assessing fruit response to a range of CO2 atmospheres.Freshly harvested raspberries were obtained immediately after harvest in fall 2020 and 2021. Berries were field packed into clamshells and precooled at a commercial facility in Watsonville, California. Cooled fruit were transported on the same day in an air-conditioned vehicle to the UC Davis postharvest laboratory within 3 hours. Raspberries were held at 5℃ overnight, and the next day, the baseline quality of a sample of fruit was analyzed before randomly assigning the remaining clamshells to different atmosphere treatments at 5℃. Fruit were removed from the atmosphere treatments after 6, 10 and 14 days in 2020 and 5, 10, dutch buckets and 13 days in 2021 and immediately evaluated to assess changes in the fruit’s physical quality over time in storage. The performance of fruit in each treatment atmosphere was evaluated from the perspective of raspberry shelf life and quality. Respiration rate and ethylene production were measured at 5℃ on each evaluation date. Fruit for the 0-day evaluation were cooled overnight before measurement.
After removing stored raspberries from the atmospheres, fruit were held at 5℃ in air for 18-20 hours to off-gas before being sealed inside a 10-liter container for 1 hour at 5℃ prior to headspace gas sample collection. Respiration and ethylene production rates were calculated and expressed as ml CO2/kg/hr and µl ethylene /kg/hr, respectively. One clamshell per treatment and replication was weighed before sealing in the plastic bags. Percent weight loss was calculated by deducting the measured final weight from the initial weight, dividing by the initial weight, and multiplying by 100. Leakiness was assessed subjectively on one clamshell per treatment and replication. In 2020, a single layer of paper towel was laid on a tray. The whole clamshell of raspberries was gently poured onto the tray, then the tray was shaken five times, back and forth; gently, but enough to move the berries. The raspberries from one clamshell were arranged on a white paper divided into 40 square blocks; an individual raspberry was placed horizontally on each block for leakiness evaluation. A similar paper was used to cover the raspberries and pressed very gently onto the fruit for 1 second. The top paper and the fruit were removed, and the papers’ printed square blocks were evaluated and scored for liquid stains resulting from berry leaking. The scores for each fruit were assigned based on the intensity, where 1 = none, 2 = very slight, 3 = slight, 4 = moderate and 5 = severe . The number of fruit with a score of 2 or higher were divided by the total number of fruit to determine the percentage of affected fruit. Leakiness severity was calculated by summing up the severity scores of fruit with a score of 2 or higher and dividing by the total number of leaky fruit.
Decay evaluation was done visually on the fruit from the same clamshell as leakiness. The number of fruit with a score of 2 or higher was divided by the total number of fruit and multiplied by 100 to determine the percentage of decayed fruit. Decay severity was calculated by summing up the severity scores of fruit with a score of 2 or higher and dividing by the total number of decayed fruit. The juice was used for measuring total soluble solids content with a tabletop automatic refractometer , and results were expressed as the percentage of TSS. Four grams of juice were diluted with 20 ml of dH20 and then titrated with an automatic titrator . Titratable acidity was expressed as percentage of citric acid , the dominant organic acid in raspberries.A second clamshell of raspberries from each treatment and replication was frozen with liquid N2, and immediately broken into drupelets with a mortar and pestle. Drupelets were mixed among fruit from each clamshell and stored in a –80 ℃ freezer until analyzed. These frozen raspberries were used for measuring total anthocyanin content . The TAC was measured using a microvolume UV-Vis spectrophotometer by adapting a method from Abdel-Aal & Hucl . Liquid N2 was added to the frozen raspberry drupelets and then immediately ground with a blender for 1 min and turned into a fine powder. An aliquot of raspberry powder was added to 10 mL of acidified ethanol solution and vortexed for 1 min. The solution was incubated for 30 min at 50℃ and then filtered through a 0.45-micron polytetrafluorethylene filter . The supernatant was collected and held in a –20℃ freezer until evaluated by spectrophotometry. Absorbance was measured at 530 and 700 nm on cyanidin 3-glucoside equivalence. The acidified ethanol solution was used as a blank.Firmness is an important indicator of quality in raspberry fruit, as well as many other fruit.
The decrease in raspberry firmness after harvest was inhibited or slowed by storage under increasing CO2 concentrations, and high CO2 stored fruit had significantly higher firmness than air stored raspberries. CO2 has other effects on fruit physiology, it influences ethylene biosynthesis by regulating 1-aminocyclopropane-1-carboxylic acid synthesis and oxidization. ACC synthase is inhibited by high CO2. ACC oxidase activity is stimulated by low levels of CO2 and inhibited by higher CO2 . The association of high CO2 atmospheres with the maintenance of raspberry fruit firmness was further supported by González et al. who found that raspberries stored in a continuous flow of CO2 for 14 days had higher firmness than berries exposed to CO2 for 3 days or an intermittent CO2 treatment. In strawberries, elevated CO2 has also been shown to enhance firmness . Strawberry fruit exposed to high CO2 atmospheres exhibited changes in apoplastic pH levels and in turn may have increased cell to cell adhesion by precipitation of soluble pectin . Solubilization of CO2 produces H+ and HCO3- that could influence pH . The increase in firmness following exposure to high CO2 atmospheres, as related to pectin polymerization, is mediated by calcium. In strawberry, modification of pectic polymers decreased the amount of water soluble pectins and increased the chelator soluble pectins , which is the majorfactor in firmness increase . However, in our study, we did not find any increases in raspberry firmness as a result of exposure to up to 15 kPa atmosphere for 14 days, although the rate of softening was reduced. Forney et al. found that CA did not maintain raspberry firmness during 2-3 days storage at 1℃, and resulted in fruit softening compared to air stored raspberries. The effect of the modified atmospheres in delaying further ripening, as evidenced by differences in other raspberry quality parameters such as color, may be one reason why the firmness was maintained. Bing cherries stored in low O2 maintained a higher percentage of green stems, brighter color and higher TA, indicating delayed ripening as compared to air stored cherries . However, O2 may not have had much effect in our experiment because the lowest O2 concentration we utilized was 6 kPa and the other O2 concentrations were ≥ 13 kPa. The 15 kPa atmosphere could be the one exception. Given the relatively low O2 content and the high CO2 content, the combination of 15 kPa CO2 and 6 kPa O2 may have had additional effects on fruit metabolism beyond the effects of the high CO2 alone, grow bucket strengthening the effect of the 15 kPa atmosphere on fruit quality. However, elevated CO2 atmospheres can delay ripening without the added effect of low O2. In our study we observed an increase in leakiness and a decrease in glossiness during storage. Leakiness is initiated in raspberries by physiological breakdown of the cells, a typical symptom of a plant tissues’ senescence . Physiological breakdown is evidenced by juice leakage and softness, and contributes to the fast deterioration of raspberry fruit quality . We observed a significant increase in leakiness over time after harvest; however, the rate of increase was slower with less leaky raspberries when stored in 15 kPa atmosphere. The effect of high CO2 in slowing further ripening and overripening likely contributed to the slower rate of leakiness development. When evaluating different raspberry cultivars, Harshman et al. did not detect a clear association between fruit firmness and PB resistance, indicating that initial fruit firmness is not related to PB incidence. Forney et al. reported that storage in 12.5 KPa CO2 and 7.5 KPa O2 was less effective in delaying PB than delaying decay. Perhaps, their fruit had already begun senescence prior to CA exposure. Visible decay on the fruit surface significantly reduces raspberry fruit quality. Decay incidence in our studies was reduced by storage under high CO2 concentrations, with the maximum effect achieved at 8 and 15 kPa atmosphere. In agreement with our study, Haffner et al. found significant inhibition of raspberry decay by using high CO2 atmospheres as compared to air stored fruit. High CO2 concentrations create a fungistatic effect that slows microbial activity of fungi as well as the metabolic activity of fruit. High CO2’s fungistatic effect is due to its solubility in the aqueous phase of the produce and fungi. CO2 in the intercellular environment lowers the pH, inhibiting enzyme-catalyzed processes and enzyme production, interacting with cell membranes, and affecting the physicochemical characteristics of proteins . Altered expression of proteins in both fungi and fruit tissues can therefore alter decay development . In addition, maintaining cellular integrity as a result of CO2’s firming effect may have also inhibited fungal activity. Petrasch et al. also, reported mycelium developed faster on softer strawberry fruit than on firmer fruit. In apple and pear CA storage, Von Schelhorn et al. determined that control of fungal development was a secondary impact, and the major prolongation of shelf life was due to delayed ripening of the fruit. While the atmospheres and time-frame of apple and pear storage are very different from those for raspberry, we also found some strong effects of atmosphere on raspberry senescence, apart from decay, which may have contributed to the fruit’s ability to resist decay. CA impacts on fruit physiology may promote decay resistance in addition to direct effects on fungal development. Modified atmospheres reduce respiration rates and delay fruit ripening , which is also in agreement with our findings. In addition, higher firmness can reduce fruit damage and stronger cell walls resist cell wall degrading enzymes produced by pathogens, hindering a microbe’s capacity to infect the fruit . Maintaining a bright red color is an important postharvest quality attribute for raspberries, as dark red color is associated with overripe fruit . High values of hue angle indicate more orange-red color and low values more blue-red color. Our results showed that raspberry fruit stored in 15 kPa atmosphere maintained a stable hue angle after five days, but the hue angle declined in raspberries stored in air or lower CO2 concentrations. In strawberries, holding fruit in 15 kPa CO2 and 5 kPa O2 decreased endogenous ethylene biosynthesis and resulted in a lighter, brighter hue and this finding is also aligning with our finding where high CO2 held raspberries had significantly lower ethylene production rate than air held raspberries. pH also plays a crucial role in raspberry fruit color. CO2 in the intercellular environment lowers the pH . Hydration of CO2 and the production of HCO3 − and H+ may reduce intracellular pH . In strawberry, reducing pH from 3.81 to 3.21 resulted in a 37 to 13 percent shift in flavylium form, and also increased the stability of fruit color more than any other factors . The red flavylium cation remains stable only in acidicconditions . In addition, elevated CO2 atmospheres during storage and/or transportation were found to maintain a lighter, brighter color in strawberry . Anthocyanins play a vital role in raspberry color expression.