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, dutch bucket for tomatoes 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, 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.
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. Oxygen levels less than 2 kPa can result in fermentation of raspberries . 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.A descriptive sensory analysis was performed with 12 panelists who were trained ahead of the sensory evaluations to align their sensory perception. There were four one-hour training sessions over two weeks. During the training, panelists were provided with references for each attribute to compare against the training samples. The sensory evaluations took place in the UC Davis Department of Plant Sciences Sensory Lab, equipped with five separate evaluation booths with individual computers with sensory analysis software . Samples were prepared the morning of evaluation and stored at 5℃. The samples were brought to room temperature before being tasted by the panel. One sample included 3-4 raspberries and was provided to the panelists in sealed sensory tasting cups. Each sample was blinded with random 3-digit codes generated by the software .Panelists tasted three replications of raspberries at harvest , and again for each treatment after 5, 10, blueberry grow pot and 13-days in atmosphere storage and evaluated their taste, texture, and flavor. The panelists were instructed to cleanse their palates with crackers and water in between samples. On day 10, there was only one replication of the air treatment and two replications each of 15 kPa and 5 kPa atmospheres appropriate for sensory evaluation due to decay growth. On day 13, there were no samples of the air treatment, 5 kPa treatment had two replications, 8 kPa treatment had thee replications and 15 kPa treatment had two replications.
The panelists measured the intensity of sensory attributes of the raspberry samples and marked their score for each given attribute on a 10 cm straight line anchored with less and more using sensory evaluation software . This software transmuted the markings for each attribute into a numerical value ranging from 1 to 10 units, where 1 was less and 10 was more intensity. The tasted attributes were sweetness, acidity/tartness, firmness , juiciness, raspberry flavor, and off-flavor. The tasting lexicons were decided and agreed upon during the training.Data were analyzed using R statistical program . A total of 4 treatments and 3 replications across the four evaluation dates were analyzed for instrumental and sensory qualities of the raspberries. Data were assessed through ANOVA followed by Fishers Least Significance Difference test to reveal significant differences among treatments and evaluation times. A correlation analysis was also conducted to investigate the relationship between volatile compounds and sensory attributes. The sensory data was analyzed using principal component analysis using R and R Studio software and PCA plots are presented for 5- and 10-day evaluations. The sensory data on day 13 was insufficient for analysis due to decay.Raspberry fruit mouthfeel firmness, raspberry flavor and TSS decreased over time and juiciness and off-flavor increased over time . Firmness scores, both hand and mouthfeel, were significantly higher in raspberries stored in 15 kPa atmosphere followed by fruit held in 8 kPa atmosphere. The trend was opposite for juiciness and sweetness, where raspberries held in 0.03 kPa or 5 kPa atmosphere had the highest juiciness scores, and fruit held in 15 kPa atmosphere had a lower sweetness score than fruit held in air atmosphere. Tartness score was higher in fruit held in 8 kPa or 15 kPa atmosphere than fruit held in 0.03 kPa atmosphere. Raspberry flavor was significantly higher in fruit held in 8 kPa atmosphere than in 5 kPa atmosphere . After five days of storage, the PCA biplot showed that raspberry firmness was strongly associated with the 15 kPa atmosphere treatment, and less so with the 8 kPa atmosphere treatment . Fermentative volatiles: acetaldehyde, ethyl acetate, and ethanol were also clustered with the 15 kPa atmosphere treatment, and less so with the 8 kPa atmosphere treatment. Raspberry flavor was most closely associated with sweetness and tetradecane at the bottom of the biplot. No treatments were closely associated. Tartness, TA, TSS, heptanol, α-terpineol and limonene were associated with each other and the 5 kPa atmosphere treatment at the top of the bi-plot. Sweetness, off-flavor, and juiciness were clustered with each other and the 0.03 kPa atmosphere treatment. Most of the aromatic volatiles were associated closely with the 0.03 kPa and 5 kPa atmosphere treatments, and on the opposite side of the bi-plot from firmness and 8 kPa and 15 kPa atmosphere treatments. After ten days, firmness remained associated with treatments with high CO2 concentrations . Raspberry flavor, TSS, TA, and tartness were associated with each other and the 8 kPa atmosphere treatment, and raspberry flavor shifted to the top of the bi-plot. Juiciness, sweetness, tetradecane and heptanone were clustered on the top left with the 5 kPa atmosphere treatment. At the bottom of the bi-plot, ethanol and ethyl acetate were associated with the 15 kPa and 0.03 kPa atmosphere treatments, respectively, and acetaldehyde was in between 15 kPa and 0.03 kPa atmosphere. Aromatic volatiles maintained their association with lower CO2 atmosphere treatments, and were also associated with off-flavor as at ten days .Across evaluation days, total soluble solids and juiciness were positively correlated . Sweetness was negatively correlated with hand firmness and tartness, and mouthfeel firmness and juiciness were negatively correlated. Acetaldehyde and ethanol were negatively correlated and tetradecane was positively correlated with juiciness and TSS . 2-Heptanol was positively correlated with juiciness. Hexanal, hexanoic acid, α-ionone, linalool, and α-terpineol were negatively correlated with hand firmness, and all but hexanal were positively correlated with sweetness. Limonene was the only volatile significantly correlated with off-flavor , and α-terpineol was the only volatile correlated with tartness .Raspberry firmness remained stable or decreased more slowly with increasing CO2 concentration in storage, with the highest firmness in 15 kPa CO2. In agreement with our results, Haffner et al. , found that an atmosphere of 15% CO2, 10% O2 maintained the firmness of five raspberry cultivars stored for seven days at 1℃.Strawberries exposed to high CO2 exhibited changes in apoplastic pH, which may have induced cell to cell adhesion by precipitation of soluble pectin .