Research Project: Evaluation and Maintenance of Flavor, Nutritional and Other Quality Attributes of Fresh and Fresh-Cut Produce

Location: Food Quality Laboratory

2020 Annual Report

Objectives
1. Evaluate the effects of pre-harvest production and post-harvest processing and storage treatments on fruit flavor, maintenance and/or enhancement in diverse accessions and breeding lines of Capsicum pepper, Malus sieversii apple, and blueberries; determine underlying molecular mechanisms controlling flavor quality. 2. Evaluate technologies to maintain the quality and marketability of fresh and fresh-cut produce through integrated microbiological and physiological approaches and innovations in post harvest handling, sanitation, and modified atmosphere packaging technology.

Approach
Mature produce from 30 to 240 accessions of non-cultivated and/or cultivated accessions of Capsicum peppers, Malus sieversii apple, and rabbiteye blueberry will be evaluated for flavor and nutritional quality-related substances using established gas chromatography, liquid chromatography and sensory methods and for shelf stability and overall marketability. Additional species to be studied include lettuce, tomatoes, strawberries, raspberries, and assorted microgreen.

Progress Report
A new project was recently certified following OSQR review for NP306 entitled "Reducing Postharvest Loss and Improving Fresh Produce Marketability and Nutritive Values through Technological Innovations and Process Optimizations," project number 8042-43440-006-00D. This is a long-term bridge project which is ending. Our previous studies have developed (patent-pending) a pathogen detection technology using a paper sensor array enabled by artificial intelligence (AI). In this study, we further expanded this technology to detect and quantify ripening stages and quality changes of fruits and vegetables based on their signature mixtures of volatile organic compounds (VOCs). The prototype sensor array was prepared by coating a paper substrate with a total of 23 chemical dyes. Several indicators (alcohols and aldehydes) exhibited a significant color change within 5 minutes of exposure to VOCs of fresh strawberries and fresh-cut iceberg lettuce. Other indicators (amines and thiols) showed distinct reactions at different stages of storage. Results demonstrate the potential for a non-destructive method for detecting ripeness, off odor, and other food quality attributes that can be used to facilitate real-time decision making in food inventory management and food consumption. On-going efforts are being made for improved storage stability and reactivity of sensors using novel matrices such as nanoclay. Tissue injuries sustained during fresh-cut processing stimulate enzymatic browning, and result in significant loss in quality and shelf life of fresh-cut lettuce. Through a collaborative effort by scientists in Beltsville, Maryland, and Salinas California, we comprehensively evaluated the browning potential of lettuce cultivars and the underlying chemical and molecular biological mechanisms. We reported that pedigree groups related to King Henry and Tall Guzmaine exhibited high degree of browning, while those related to Parris Island Cos had low browning potential. An untargeted metabolomic analysis showed a clear separation between fresh and browning tissues in both groups. Four positive indicators (e.g., caffeoylquinic acid) and three negative indicators (e.g., lactucopicrin-15-oxalate) were identified as predictive of lettuce browning potential. We also revealed that the phenylpropanoid biosynthesis genes were significantly up-regulated in the high browning cultivar as compared to the low browning cultivar, and the expression levels of auxin signal transduction pathway related genes in the low browning cultivar were higher than those in the high browning cultivar. These findings provide useful information and potential functional markers for lettuce breeders and industry to select low tissue browning cultivars and improve the quality and shelf life fresh-cut lettuce. NASA identified microgreens as potential food sources for astronauts on future long-duration missions and needs support from FQL. In collaboration with the University of Massachusetts, scientists at the USDA-ARS conceptualized, designed, and 3D printed a seed spacing and irrigation system (see invention disclosure for details). In addition, we also made significant progress in evaluating growth substrates and other materials to secure seeds while allowing for proper seed germination and root and shoot growth. In this regard, we have tested pre-seeded microgreen growth mats with peelable or penetrable top layers, hydrogels, as well as biodegradable growth mats. We also engaged a seed coating company to collaboratively develop a microgreen seed mat using a seed coating normally used to “pelletize” seeds, that may include additional chemicals to support seed germination and growth. We also developed novel ideas and insights for a soilless microgreen cultivation system comprising an interpenetrating hydrogel as a medium, a flexible nanofiber mat as spacer, a thin hydrogel film as coating, and an electro-spun mat with tunable degradability as a covering layer to be tested in the lab later. Information was shared with NASA via monthly informal microgreens meetings. Work on microgreens as functional foods has also involved collaboration with other units in USDA-ARS. In support of the Beltsville Human Nutrition Research Center (BHNRC) work, we joined an 8-week feeding study to evaluate the efficacy of microgreens to mitigate the effects of a high fat diet in rats. The results of the analyses are still inconclusive. Consequently, this year the experimentation will continue with 12-week feeding period tests. In optimizing growing conditions of microgreens we also studied relevant quality and safety parameters. Red cabbage microgreens grown from seed and inoculated with 8 log cfu/ ml of Non-STEC E. coli cocktail were grown under either white or 450 nm blue light with 16-hour photo period. Plants under blue light showed an accentuated red color and slightly lower levels of E. coli. Since blue light is a useful wavelength range for vegetative growth, and may favor anthocyanin production in red cabbage microgreens, it shows promise as part of a holistic approach to ensure quality and safety of microgreens. This will be followed up this year. Produce washing in flume systems is widely practiced in the fresh-cut produce industry for preparing ready-to-eat fresh-cut vegetable products. Optimization of flume wash system configuration is critical for improving washing efficacy. In collaboration with researchers at Rutgers University, we 3D-printed produce models with well-defined dimensions and surface physicochemical properties. We also inoculated the surfaces of plastic balls whose density could be customized so that they were neutrally buoyant, and flume-washed them under various conditions before recovering bacteria to measure sanitization efficacy. Generally, greater distance between two simulated produce pieces (less blocking of flow to one ball by another in the same flume) was associated with greater detachment of bacteria. Our data allowed the collaborator to further study the washing process using computer-generated models and assess the effects of product flow characteristics (i.e., production throughput, water usage etc.) on sanitizing efficacy. The current industry standard practice for washing fresh-cut produce is to wash in two sequential flumes. A major challenge bound to this procedure arises from the abundant organic exudates released from cut products, which deplete sanitizer rapidly and thus compromise their microbiological safety. We made significant progress in evaluating the potential application of our patented in-flight washer (IFW) in removing this organic matter before the product is introduced to the first flume, thereby improving sanitizer control and thus food safety and quality after packaging and during storage. Two IFW prototypes have been developed with configurable dimensions, water/air flow rates, directions, and distributions. Numerous trials have been conducted with various configurations, products (e.g., shredded lettuce, diced tomato, and diced cabbage) and product throughput levels (representative of 25-150% of typical levels for commercial processing) to test the organic removal capacity of IFW. Using turbidity and total dissolved solids (TDS) as indicators, we found that the second prototype with larger diameter and multiple water/air layers was able to remove 50-60% of organic materials released from shredded lettuce at a throughput of 3,600 lb./hr and water flow rate of 10 gal/min. These results demonstrated the potential of our invention in removing organic materials from fresh-cut produce, whose benefit in promoting microbiological safety and product quality during storage will be investigated in our next phase of study. Plasma-activated water (PAW) is a promising broad-spectrum antimicrobial and green alternative to conventional chemical treatment methods. However, little information is available currently regarding the sanitizing effectiveness of PAW in produce processing. In this study, we collaborated with researchers at Rutgers University to investigate the inactivation efficacy of PAW sanitization methods on fresh lettuce leaves contaminated with human pathogens. Chopped romaine lettuce pieces were treated by spraying or dipping in PAW, and survival of aerobic bacteria was analyzed. The results indicated that spray treatment with PAW reduced the inoculated population by over 0.5 log CFU/g, which was better than other methods. This result suggested the potential of PAW as a processing aid to improve food safety. Follow-up studies will be focused on the scale-up application of PAW on fresh and fresh-cut produce through our patented in-flight washer and commercial cutter with customer-designed nozzles installed. We examined the influence between quality digital images of fresh produce and sensory attributes perceived in-situ by sensory panelists, to reveal information that could help boost online fresh-produce sales. Digital images of fresh strawberries, cherry tomatoes, grapes, and blueberries were acquired using a high-resolution digital camera and features of the produce were analyzed using the ImagePro Plus software program. Consumer panels evaluated quality traits including appearance, texture, flavor, overall quality, and purchasing decision. The panels consisted of 32-40 members who evaluated strawberries, cherry tomatoes, grapes, and blueberries. The Pearson correlation coefficient (r) between assessment results of the image and the actual produce was high for appearance (0.87), moderate for overall quality (0.62), and low for texture (0.48). The study further demonstrated that the satisfaction gap between an image and the actual produce would be variable and dependent on specific characteristics of each commodity. Taking into consideration current COVID-19 situation, our work may be relevant for the fresh-produce online suppliers.

Accomplishments

Review Publications
Bolten, S., Gu, G., Luo, Y., Van Haute, S., Zhou, B., Millner, P.D., Micallef, S.A., Nou, X. 2019. Salmonella inactivation and cross-contamination on cherry and grape tomatoes during washing in simulated commercial wash water. Food Microbiology. 87:103359. https://doi.org/10.1016/j.fm.2019.103359.
Gu, G., Bolten, S., Mowery, J., Mowery, J.D., Luo, Y., Nou, X. 2020. Susceptibility of foodborne pathogens to sanitizers in produce rinse water and potential induction of viable but non-culturable state. Food Control. 112:107138.
De Frias, A., Luo, Y., Zhou, B., Zhang, B., Ingram, D., Vorst, K., Brecht, J., Stommel, J.R. 2019. Effects of door opening pattern of an enclosed refrigerated display case on product temperature and energy consumption. Food Control. https://doi.org/10.1016/j.foodcont.2019.107044.
Lewers, K.S., Newell, M., Luo, Y., Park, E. 2020. Consumer preference and physiochemical analyses of fresh strawberries from ten cultivars grown in Maryland. International Journal of Fruit Science. https://doi.org/10.1080/15538362.2020.1768617.
Li, J., Teng, Z., Weng, S., Srinivasan, P., Zhou, B., Turner, E.R., Luo, Y. 2019. Dynamic changes in the physicochemical properties of fresh-cut produce wash water as impacted by commodity type and processing conditions. PLoS One. https://doi.org/10.1371/journal.pone.0222174.
Vorst, K., Brown, W., Steinmaus, S., Brecht, J.K., Xie, Y., Luo, Y., Bornhorst, E.R., Zhou, B., Shaw, A., Monge-Brenes, A. 2020. Temperature profiling of open- and closed-doored produce cases in retail grocery stores. Food Control. https://doi.org/10.1016/j.foodcont.2020.107158.
Luo, Y., Bornhorst, E., Teng, Z., Zhou, B., Park, E., Turner, E.R., Simko, I. 2019. Identification of romaine lettuce (Lactuca sativa var. longifolia) varieties with reduced browning discoloration for fresh-cut processing. Postharvest Biology and Technology. 156:110931. https://doi.org/10.1016/j.postharvbio.2019.110931.
Turner, E.R., Buchanan, R., Luo, Y. 2020. Microgreen production, nutrition, safety, and shelf life: A review. Journal of Food Science. 85(4):870-882. https://doi.org/10.1111/1750-3841.15049.
Van Haute, Sam, Luo, Y., Bolten, S., Gu, G., Nou, X., Millner, P.D. 2020. Survival of Salmonella enterica and shifts in microbial community as impacted by tomato wash water particulate size and chlorine treatment. Food Control. 90:103470. https://doi.org/10.1016/j.fm.2020.103470.
Yang, M., Cousineau, A., Liu, X., Sun, D., Li, S., Gu, T., Luo, S., Luo, Y., Xu, M., Zhang, B. 2020. Direct metatranscriptome RNA-seq and multiplex RT-PCR amplicon sequencing on Nanopore MinION - promising strategies for multiplex identification of viable pathogens in food. Frontiers in Microbiology. https://doi.org/10.3389/fmicb.2020.00514.
Gu, G., Ottesen, A., Bolten, S., Luo, Y., Rideout, S., Nou, X. 2019. Microbiome convergence following sanitizer treatment and identification of sanitizer resistant species from spinach and lettuce rinse water. International Journal of Food Microbiology. 318:0168. https://doi.org/10.1016/j.ijfoodmicro.2019.108458.