Modified atmospheres (MA) have been shown to delay senescence of some leafy greens. This study explored the effect of modified atmospheres with elevated CO2 on baby kale and arugula shelf life and sensory quality. The ready to eat baby kale was stored at 5 °C in six different atmospheres: 1) 5% O2 + 5% CO2, 2) 5% O2 + 10% CO2, 3) 5% O2 +15% CO2, 4) 10% O2 + 10% CO2, 5) 10% O2 + 15% CO2 and 6) 20% O2 + 0.03% CO2 (Air), or in Air at 0 °C. Chlorophyll degradation was significantly retarded in every MA treatment compared to the kale stored in Air at 5 °C, which led to a significantly better visual quality scores. However, when high CO2 (15%) was combined with low O2 (5%) storage at 5 °C, significantly faster ascorbic acid degradation and ammonia accumulation occurred, resulting in off-flavor which was apparent after 25 days. However, storing baby kale in 10% O2 combined with 10% or 15% CO2 maintained visual quality without developing off odor or off flavor for 25 days at 5 °C. Use of modified atmosphere packaging can extend the shelf life of baby White Russian kale when it is stored and marketed at 5°C. Different result however was found in arugula. The arugula was stored at 5℃ in 5, 7, 10, or 0.03 % CO2 in combination with 15, 13, 10, or 21 % O2, respectively. Our results demonstrated that holding arugula in elevated CO2 combined with low O2 atmospheres increased its postharvest shelf life. After 13 days of storage at 5℃, the quality of arugula held in air was below the limit of marketability due to degradation. The visual quality of arugula stored in 15% CO2 and 5% O2 remained above the marketability limit until day 19 (6 days longer). High CO2 and low O2 also better maintained ascorbic acid and the aromatic volatiles, including 1-penten-3-ol and 2-hexanol that are important for the peppery and bitter flavor of fresh arugula.

AbstractCalifornia walnut growers and processors sell product all across the globe, and like with any raw food, safety assurance is critically important. The walnut industry of California utilizes either a chemical method of pasteurization or heat pasteurization to provide an effective kill step on any surface bacteria that might be present on the walnuts. In this study I only focus on two heat pasteurization methods: continuous and batch. The impact of the heat pasteurization methods on walnut storage quality is unknown. My thesis research analyzed the impact each pasteurization method has on walnut quality during storage, and the effect low oxygen storage may have in mitigating potential negative impacts caused by each heat pasteurization method. Quality was analyzed in the lab by measuring lipid degradation compound accumulation (peroxide value, free-fatty acid content), kernel color change, volatile compound accumulation (hexanal), and vitamin E degradation (tocopherol). Additionally, a trained sensory panel created a lexicon of descriptors in order to analyze the walnuts for changes in aroma, taste, and texture. Results showed that continuous walnut pasteurization accelerated lipid degradation, degraded vitamin E content, and increased negative sensory characteristics such as rancidity. Low oxygen storage of continuous pasteurized walnuts significantly reduced the rate of lipid and tocopherol degradation, and resulted in a lower sensory scores for rancidity. However, low oxygen storage of continuous pasteurized walnuts had significantly lower nut quality in comparison to either unpasteurized or batch pasteurized walnuts. Batch pasteurized walnuts had reduced lipid degradation and an improved sensory profile compared to unpasteurized walnuts, while vitamin E content was unaffected. Low oxygen storage of batch pasteurized walnuts further reduced lipid degradation, and caused an even greater improvement on sensory characteristics especially compared to continuous pasteurized walnuts. The best method of pasteurization and storage was found to be batch pasteurization stored in low oxygen conditions.

Raspberry (Rubus ideaeus L.) fruit are high value but have a short shelf life due to fungal decay, leakiness, off-flavor, and loss of firmness. High CO2 in combination with low O2 can increase raspberry storage time and quality. In fall 2020 and 2021, we examined raspberry shelf life and sensory quality periodically during storage at 5℃ for two weeks in four different CO2 atmospheres; 15 kPa CO2, 6 kPa O2 (15 kPa); 8 kPa CO2, 13 kPa O2 (8 kPa); 5 kPa CO2, 16 kPa O2 (5 kPa); or 0.03 kPa CO2, 21 kPa O2 (0.03 kPa) to identify an ideal postharvest atmosphere that will increase raspberry shelf life without degrading the sensory quality. Raspberry visual attributes, color, glossiness, decay incidence and leakiness deteriorated over time in all atmospheres, but high CO2 atmospheres reduced fruit discoloration and decay, and slowed the rate of deterioration and leakiness. After 5 days, the quality of air stored raspberries was significantly degraded, while raspberries stored in elevated CO2 maintained firmness with bright red color up to 10 days. In 2021, a trained sensory panel conducted a descriptive sensory evaluation of the raspberry fruit periodically during storage and found that raspberries stored in 8 or 15 kPa atmospheres had significantly lower off-flavor and higher tartness scores. Raspberries stored in 8 kPa atmosphere scored highest in raspberry flavor with substantial juiciness and sweetness scores. The fermentative volatiles; acetaldehyde and ethanol were higher in raspberries stored in 15 kPa atmosphere, while most other raspberry volatiles decreased with increasing CO2 concentration, including flavor-related volatiles α-ionone, α-terpineol, limonene and linalool. While raspberries stored in 15 kPa atmosphere had longer shelf life, the decrease in flavor-related volatiles was most pronounced in this fruit. However, fruit stored in 8 kPa atmosphere had better sensory quality and 10 days of shelf life and was the optimal atmosphere for raspberry in our study. Raspberries should not be stored in air beyond 5 days without modified or controlled atmospheres.

Background

Results

Conclusions

Basil, like most aromatic herbs, is typically marketed as a dried product, although demand for fresh basil has grown dramatically over the years due to its distinct sensory properties. However, unlike other culinary herbs, fresh basil is particularly sensitive to chilling temperatures which limits its transport and storage. Understanding the basis of chilling sensitivity is essential in designing storage regimes for quality maintenance after harvest. Since basil is a culinary herb valued for its unique aroma and flavor, preserving its sensory quality postharvest is also of utmost importance. I explored the biochemical basis of differential chilling sensitivity among genotypes and found that ‘Lemon’ and ‘Genovese’ were more sensitive to chilling than ‘Purple’, ‘Thai’, and ‘Holy’ basil. I also observed that different genotypes utilized various mechanisms to cope with postharvest chilling stress, as revealed by the dynamics between antioxidant enzymes and metabolites. Sensitive genotypes either had low activity of antioxidant enzymes such as catalase (CAT) and ascorbate peroxidase (APX), and/or the inability to recycle antioxidant metabolites such as ascorbic acid (AsA), which can result in the accumulation of reactive oxygen species (ROS) that damage cells and exacerbate chilling injury symptoms.

I also investigated whether controlled atmosphere storage can mitigate the impact of chilling temperatures on basil quality, particularly on the content of volatile compounds responsible for aroma and flavor. Storage at 5% CO2 alleviated the symptoms of chilling injury for up to 3 days in ‘Genovese’, but not in ‘Lemon’ basil. I identified potential volatile markers of chilling stress in both ‘Genovese’ and ‘Lemon’ which could play a significant role in early diagnosis of said stress. I also determined that the impact of atmosphere modulation on chilling injury is cultivar- or species-dependent, and its potential is hampered by relative sensitivity to CO2 injury. While both chilling temperatures and controlled atmospheres altered key volatile compounds implicated in basil aroma and flavor, we determined that temperature had a bigger influence on the changes in volatile profile.

The loss of volatile compounds in basil must be linked with sensory data to shed light on the effects on aroma and flavor as it relates to human perception. To determine impacts on sensory quality, I assessed changes in key flavor volatile compounds of ‘Genovese’ basil following storage at low temperature and controlled atmospheres and related these to descriptive sensory data based on perceived aroma and flavor. I found associations between major volatile compounds and basil sensory attributes involving aroma, mouthfeel, taste, and flavor-by-mouth. Storage at the chilling temperature of 5°C, regardless of atmosphere, resulted in a reduction in volatile constituents which impacted the perceived aroma and flavor, confirming the need for optimum handling and storage temperatures in order to preserve fresh basil external and internal qualities.

Blossom-end rot is a physiological disorder which causes significant losses in tomatoes, peppers, watermelons, and eggplants. Though blossom-end rot has been associated with calcium deficiency for decades, abiotic and oxidative stress are also significant factors in blossom-end rot development. The cause of cell death during symptom development is not well understood. This dissertation investigates the biological and chemical processes associated with cell death during blossom-end rot development. Due to confounding factors at the whole plant and fruit level, direct treatment and observation of the relevant tissue during blossom-end rot development is difficult. To improve experimentation, chapter 2 demonstrates and validates a new method for studying blossom-end rot using tomato pericarp discs. Chapter 3 uses greenhouse experiments and pericarp disc experiments to investigate blossom-end rot induced by excess calcium application. Chapter 4 uses the pericarp disc system to characterize the importance and timing of calcium and antioxidant related events during blossom-end rot symptom development. Chapter 5 investigates lignification during blossom-end rot as a result of accumulated reactive oxygen species and increased peroxidase activity in whole fruit affected by blossom-end rot. Overall, this research has provided the first causative evidence that calcium, ascorbic acid, and glutathione are protective against BER symptom development. Furthermore, this research shows that calcium and the ascorbic acid-glutathione antioxidant system are directly connected and both reactive oxygen species generation and calcium signaling are critical steps in blossom-end rot induction. Based on these results, the most efficient gains with regards to reduction in commercial blossom-end rot incidence would likely be made through optimizing calcium translocation specifically to the blossom-end pericarp and enhancing the ascorbic acid-glutathione antioxidant system.