Controlled atmospheres or modified atmospheres are created by reducing O2 and/or elevating CO2 concentrations and have the general effect of slowing senescence and extending shelf life . The fundamental difference between CA storage and MA packaging systems is that in the CA storage system, gas levels are rigidly maintained, whereas in the MAP system, the gas mixture is flushed into the package once, if at all, and concentrations vary over time with product respiration and package permeability . Active MAP is performed by removing some amount of air from the package and replacing it with the desired gas combination . High CO2 concentrations have a general inhibitory effect on microorganism growth and development. CA composed of high CO2 and low O2 was found to be fungistatic in controlling Botrytis alli, Rhizopus nigricans, and Penicillium expansum . Nine red raspberry genotypes were tested in CA storage at 1℃ and decay was strongly suppressed across all the genotypes . Raspberries exposed to CO2 levels of 20% or higher exhibited delayed gray mold decay and extended shelf life . High CO2 concentration also slows further ripening and softening in berries. Applying CA, even for a short time of 0.5 to 3 days, increased strawberry shelf life by 3 days, as well as reduced the endogenous ethylene production and ultimately maintained lighter and brighter colored and firmer fruit . In addition, drainage planter pot lowering the O2 concentration in the storage atmosphere can be beneficial in extending the shelf life of fresh produce .
In mangoes, respiration rate decreased about 20-25% in low O2 atmosphere compared to air . Storing fruit with higher CO2 atmospheres; however, can result in off-flavor development, perhaps due to the initiation of anaerobic respiration and production of fermentative volatiles. Also, oxygen levels less than 2 kPa may cause fermentation of raspberry fruit . MA packaged strawberries developed off-flavor which the authors suggested might be linked to a specific cultivar’s susceptibility to accumulate ethyl acetate . The raspberry cultivar, Qualicum, produced more ethyl acetate in modified atmosphere packaging compared to “Meeker” and “Chilliwack” .Raspberry is a high value fruit but their shelf life is impacted by high perishability. In 2020, the United States produced 111,000 tons of raspberries, at a value of US$469 million and California alone contributed fresh raspberries worth of US$395 million . Raspberry’s delicate morphology coupled with high respiration and transpiration rates make them vulnerable to rapid deterioration after harvest. The typical shelf life of raspberries ranges from 3 to five days . Decay, leakiness, loss of firmness, darkening of the red color, and off-flavors are common limiting factors contributing to short storage life of raspberries . It is well established that cooling is by far the best technology for increasing the shelf-life of horticultural produce. Low temperatures slow pathogen growth and reduce the rate of deterioration of freshly harvested commodities, thus extending shelf life and the marketing period .
The recommended temperature for raspberry storage is 0-1℃ , but it is challenging to maintain this recommended temperature during transportation and marketing. Although low storage temperatures can slow the development of Botrytis cinerea infections, they don’t provide adequate control when pathogen inoculum loads are high . 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 theoptimum 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, 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. 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. Headspace gas samples were analyzed for CO2 and ethylene concentrations. 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 tissue paper was evaluated for the juice marks resulting from berry leaking and ranked based on their intensity;where 1 = none, 2 = very slight, 3 = slight, 4 = moderate, and 5 = severe. In 2021, an improved method was used. 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, plant pot with drainage 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 severity of infection on each fruit was scored using a scale of 1 to 5, where 1 = none; 2 = very slight, 1-3 decayed drupelets; 3 = slight, 4-6 decayed drupelets; 4 = moderate, 7-9 decayed drupelets; and 5 = severe, >9 decayed drupelets . 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. Ten raspberries were randomly selected from one clamshell per treatment and replication to evaluate color using a chromameter with the CIELAB color space. Coloration of the external surface of the raspberries was measured and expressed as L*, C*, and h color coordinates, indicating lightness, chroma and hue angle, respectively. Only one side, close to the apex of the fruit was measured. The same fruit were evaluated for glossiness. Glossiness refers to the light reflection intensity of the fruit. The fruit were visually inspected and subjectively scored from 1 to 3, where 1 = dull, 2 = moderately glossy, and 3 = glossy . The same ten berries were used to evaluate the degree of discoloration based on the number of discolored drupelets, and scored on a 1 to 5 scale where 1 = none, 2 = very slight, 1-3 discolored drupelets; 3 = slight, 4-7 discolored drupelets; 4 = moderate, 8-11 discolored drupelets; and 5 = severe, >11 discolored drupelets . Fruit firmness was also assessed subjectively using the same raspberries that were used to evaluate glossiness and discoloration. Each raspberry was pressed slightly with the thumb and middle fingers. Based on the palpability, the berries were scored from 1 to 5, where, 1 = very firm, rebounds from compression, high resistance; 2 = firm, partial rebound; 3 = soft, partial rebound; 4 = very soft, partial rebound and 5 = no resistance.
Ten randomly selected raspberries were juiced together using a hand juicer and cheesecloth, yielding 10-15 ml of juice. 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.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 major factor 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, 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 .