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, pot with drainage holes 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 storageare 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. The visual appeal of raspberry fruit decreases with time after harvest, along with increased levels of certain anthocyanins .

In our study, total anthocyanins increased over time, except in raspberries stored in 15 kPa atmosphere; storing raspberries in 15 kPa atmosphere maintained the anthocyanin content close to the levels at harvest. In agreement with our finding, Gil et al. found that high CO2 concentrations inhibited the increase in anthocyanin content after harvest by affecting its biosynthesis, degradation or both. These results indicate the ability of high CO2 atmospheres to maintain raspberry fruit color tone, even after two weeks of storage. Anthocyanin content is also related to raspberry skin color. Palonene et al. found a significant correlation between anthocyanin concentration and color values, as the darkest raspberries had higher anthocyanin content. In our research, we also found higher anthocyanin content and low hue angle in raspberries stored in air or low CO2 atmospheres. Moore also stated that the hue angle or a*/b* could predict raspberry anthocyanin content. We observed an increase in raspberry discoloration after harvest, which has not been reported previously to our knowledge. Discoloration occurred when the raspberry drupelets changed color from red to light pink. In blackberries, a similar phenomenon, red drupelet reversion , occurs, a type of physiological disorder . Edgley et al. reported RDR was associated with a decrease in anthocyanin content and was primarily caused by mechanical damage during harvest which causes lost membrane integrity and a decrease in cellular structural integrity. There may also be some change in pH from membrane leakiness leading to colorchanges in the anthocyanins. Slight changes in pH significantly impact anthocyanins, as acidity of the solution impacts the ratio between different forms of the pigments . In our study, discoloration increased with time in storage, but was inhibited by high CO2; anthocyanin content was also maintained close to harvest levels with high CO2. Also, high CO2 atmospheres maintained fruit firmness and the integrity of the cell wall, and reduced senescence. It seems that these effects may be related to the decrease in discoloration development with high CO2 atmospheres. Overall, high CO2 atmospheres were effective in increasing raspberry shelf life and maintaining postharvest quality. Raspberries held in 15 kPa atmosphere maintained the highest firmness and glossiness, and the brightest red color, with the least leakiness and decay, followed by raspberries held in 8 kPa atmosphere. Total anthocyanin content increased over time after harvest in all raspberries, regardless of storage atmosphere, but the increase was greatly inhibited by high CO2 in a concentration dependent manner. Raspberry visual attributes deteriorated over time after harvest, large pot with drainage but the atmosphere influenced the rate of deterioration. High CO2 slowed ripening and created fungistatic conditions. Air stored raspberries rapidly lost shelf life and quality after five days at 5℃ and should not be stored longer without modified or controlled atmospheres. As little as 5 kPa atmosphere can contribute to maintaining raspberry quality for very short periods and 8 kPa atmosphere can maintain quality for up to 10 days be potential an alternative to 15 kPa atmosphere for storing below 10 days. It would be beneficial to investigate the effects of these atmospheres on the sensory quality of raspberry, to ensure that flavor quality is maintained.Raspberry fruit are appreciated for their distinctive aroma and flavor. Visual appearance, texture, flavor, and nutritional compounds are generally considered as fruit quality . Visual quality is indicated by color, absence of disease or decay, texture, and aroma, which altogether appeal to consumers as freshness. ‘Texture’ is a qualitative characteristic of fruit appreciated by the consumer, including firmness, juiciness, and crispness. Multiple irreversible physiological and biochemical changes occur during ripening that impact fruit quality . According to Ponder et al. the ratio of sugar and organic acid determines raspberry taste. De Ancos further reported that total soluble solids content ranges between 9 and 10 % and titratable acidity between 1.5 and 1.8% for good raspberry taste. Raspberry aroma is composed of volatile chemicals .

Raspberry volatiles are vital for olfactory sensory quality perception as well as mold resistance . It has been reported that raspberry has approximately 200 aromatic volatiles. . The main volatile compounds contributing to raspberry flavor are α and β-ionone, linalool, α and β- pinene, caryophyllene and citral . As a non-climacteric fruit, raspberry taste and flavor mostly develops while they are ripening on the plant. Kader suggested that berries should be picked when fully ripe to ensure good flavor quality. Some raspberry research has focused on three phases of ripeness: semi-ripe, ripe, and over-ripe and suggested that semi-ripe fruit may be more suitable for shipment and good sensory quality . Wang et al. evaluated raspberry fruit harvested at 5%, 20%, 50%, 80% and 100% ripe. They concluded that 50-80% ripe berries developed the same level of TSS, TA and sugars as 100 % ripe berries but, 5-20% ripe berries never attained those qualities.High CO2 atmospheres can be beneficial to extend the postharvest life of raspberry fruit, slowing further ripening and reducing decay development . However, high CO2 concentrations have the capacity to disrupt enzyme systems, including the lipoxygenase pathway which is involved in the formation of aromatic volatile compounds . In addition, use of high CO2 atmospheres can result in off-flavor development, which might be due to initiation of fermentative respiration. Earlier research by Li and Kader reported higher accumulation of ethanol in strawberries treated with low O2 and/or high CO2 than air stored berries. Ke et al. suggested that low O2 and high CO2 concentrations contribute to alcohol production. The objective of this research was to investigate the effects of a range of CO2 atmospheres during cold storage on raspberry fruit sensory quality.Freshly harvested raspberries were obtained immediately after harvest in Fall 2021. Berries were commercially 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 pilot plant within 3 hours. Raspberries were held at 5℃ overnight, and the next day, a fruit sample was analyzed for objective and sensory quality to determine the baseline quality. The remaining clamshells of fruit were randomly assigned to different atmosphere treatments at 5℃. Fruit were removed from the atmosphere treatments after 5, 10, and 13 days and immediately evaluated to assess changes in the fruit’s objective and sensory quality over time in storage.Raspberries in clamshells were stored at 5℃ for up to 13 days in one of four atmosphere treatments: 15 kPa CO2, 6 kPa O2 ; 8 kPa CO2, 13 kPa O2 ; 5 kPa CO2, 16 kPa O2 ; or 0.03 kPa CO2, 21 kPa O2 . These atmosphere treatments represent the mixture of O2 and CO2 concentrations that would be found in a modified atmosphere package targeting 15, 8 and 5 kPa CO2 as well as a package without modified atmosphere . The gas concentrations were measured during set up and periodically with a CO2/O2 gas analyzer . The four atmospheres were humidified by bubbling through water prior to fruit exposure in a continuous flow-through system at a rate of 100 mL/minute. Raspberry fruit remained in the clamshells during treatment. There were nine plastic bags, each holding six clamshells inside, for each atmosphere and replication. The bags were modified with an inlet and outlet port and connected to a separate flow board for each atmosphere. Instrumental and sensory fruit quality was evaluated at the start of the experiment and again on each evaluation date.Ten randomly selected raspberries from 1 clamshell per replication were juiced together using a hand juicer and cheesecloth, yielding about 10-15 ml of juice. The juice was used for measuring TSS with a tabletop automatic refractometer , and results were expressed as the percentage TSS. Four grams of juice were diluted with 20 ml of dH2O and then titrated with an automatic titrator . Titratable acidity was expressed as percentage citric acid , the dominant organic acid in raspberries .A clamshell of raspberries from each treatment and replication was frozen with liquid nitrogen, and immediately broken into drupelets with a mortar and pestle. Drupelets were mixed among the fruit from each clamshell.