Location: Food Science and Market Quality and Handling Research Unit
2022 Annual Report
Objectives
1. Development of controlled, low-salt vegetable fermentations free of added preservatives using biofunctional lactic acid bacteria starter cultures to improve commercial product quality and reduce spoilage and food waste.
2. Identify beneficial chemical constituents of vegetables that facilitate the development of novel, clean-label, health-promoting fermented and acidified products that retain consumer-preferred appearance, textures, and flavor during processing, storage and distribution.
2a. Determine the effects of processing conditions on flavor characteristics and health-promoting metabolites in pickled vegetables.
2b. Determine the role of specific cell wall components in perceived sensory quality and susceptibility to softening of pickled cucumber and red bell peppers.
3. Determine the physical and chemical characteristics of sweetpotato genotypes to optimize commercial food processing methods and enable commercially viable, novel, value-added products that meet consumer preferences.
3a. Determine the effects of sweetpotato polymer structures and the influence of molecular mobilities on fried sweetpotato textural properties and fat absorption.
3b. Explore perceived sweetpotato sweetness and the impact of precursors in raw sweetpotato on the sugar and volatile compound composition of processed products.
Approach
Today’s consumers are interested in fermentation as a healthful food processing technology. Current industrial fermentations generate chloride waste and often use preservatives. To be successful, the ongoing development of low salt, clean-label commercial fermentation technology will require a better understanding of the indigenous microbiota and genetic diversity. Microbiomics approaches and starter cultures will be used to control Gram-negative bacteria, spoilage lactic acid bacteria, and other microbes causing quality defects in laboratory and small scale (bag-in-box) fermentations. Concomitant research on the texture, flavor and nutritional content of fermented and acidified vegetables is needed to assure product quality and consumer acceptability. A trained descriptive sensory analysis panel will create a standardized language (lexicon) to determine product quality attributes of fresh and processed vegetables. Mass spectrometry will be used to analyze the retention and production of health-promoting compounds, and establish connections between chemical composition, fermentation or processing technology, and quality. Food processing research will also include determining the chemical and physical properties of sweetpotato genotypes to identify characteristics that result in improved product quality for in-demand, novel, nutrient-rich processed products. Planned research contributes to the NP306 Action Plan 2020-2024, Component 1: Foods, problem statements 1.A, 1.B, and 1.C. Products from this research include: genotypically and phenotypically defined starter cultures for vegetable fermentations; new knowledge of health promoting small molecules, and flavor compounds of fermented and acidified vegetables along with a standardized sensory language for pickled vegetables; and knowledge of the chemical composition of novel sweetpotato varieties to enable commercial development of processed products.
Progress Report
Significant progress was made toward all three objectives by redirecting the scientific approaches to adapt to COVID restrictions; making use of other important research mechanism for on-site work; and leveraging key collaborations with university partners.
Objective 1: Enabling production of high-quality pickles with beneficial bacteria. The preservation of cucumber, cabbage, pepper and garlic by fermentation or acidification contributes more than 2 billion dollars to the United States economy. Like fresh vegetables, the production of pickles is a growing economic sector which registered a 3.5% increase in 2019. Processors in Michigan, Wisconsin, California, North Carolina and other states face challenges in preventing defects in pickle processing such as the formation of internal holes, loss of tissue firmness (softening), and foul-smelling spoilage. These defects lead to losses as high as 40% of production. Microbes naturally present on cucumbers are largely responsible for these defects. Beneficial bacteria added to pickling tanks at the start of fermentation may prevent spoilage microbes from growing. Several lactic acid producing bacteria that are naturally present in cucumber, lead the fermentation needed to make pickles and can confer health benefits for consumers. ARS scientists in Raleigh, North Carolina, studied the natural genetic diversity of a particular species of such bacteria known as Lactiplantibacillus (L.) pentosus, which prevails in cucumber fermentation. Key features that can distinguish 24 L. pentosus isolates from other closely related lactic acid bacteria were identified as well as unique characteristics of individual strains of L. pentosus. The team discovered that L. pentosus encode for sugar, amino acid and nucleotide transport systems that are absent in closely related lactic acid bacteria. Additionally, it was found that a subgroup of the L. pentosus isolates encode for genes that can provide a competitive advantage at the start of a cucumber fermentation, whereas others are equipped to better compete towards the end of the fermentation. In a parallel effort, the team of scientists in North Carolina studied the genetic diversity of two other genera of beneficial bacteria naturally found in commercial cucumber fermentations known as Levilactobacillus and Pediococcus. They found one genetically unique Levilactobacillus brevis that is adapted to pickling and identified four species of Pediococcus that naturally colonize pickles. Researchers are using an advanced phenotyping platform to identify the metabolic functions naturally expressed by the genetically unique lactic acid bacteria in the collection. These data will be used to select specific bacteria that can be added to vegetable fermentations to outcompete microbes associated with pickle defects. Retention of pickle quality during processing reduces food and economic losses, assures availability of quality foods for a growing population, and enhances the efficiency and resilience of the agricultural chain.
Objective 2: Electrical, thermal and sensory characterization of peppers to guide microwave process design. Pepper fruits (Capsicum annuum) such as bell peppers, banana peppers, and jalapenos are economically important commodities worldwide due to their unique sensory profiles and rich antioxidant properties. Preserved peppers are widely used in the United States food service industry and in households as ingredients for pizzas, sandwiches, salads, and pastas. However, quality deterioration during storage limits shelf-life and availability for consumption. Microwave processing of foods has demonstrated the ability to achieve improved quality through an efficient and environmentally sustainable process as compared to conventional thermal processing. ARS scientists collaborated with North Carolina State University to determine the dielectric and thermal properties that influence the heating characteristics of acidified red bell peppers when exposed to microwaves. Dielectric properties were modeled as a function of salt and acid composition to understand the role of ingredients in heating behavior. As the salt content increased, the loss tangent was found to increase, while acetic acid did not play a significant role. Dielectric properties could be predicted from conductivity measurements with at least 80% accuracy. The low loss tangent of fresh red bell peppers (~ 0.2) and transmitted power (< 50%) were identified as aspects to be addressed by equipment design to enhance heating uniformity and energy efficiency of the process. The sensory quality of pepper products is a key driver of the successful implementation of new preservation processes, but a standardized tool for evaluating product quality did not exist. We met this need with the development of a comprehensive, standardized vocabulary for describing and quantifying the quality traits of fresh and processed peppers. Commercially available fresh, pickled, and roasted peppers were used to establish definitions, methods of evaluation, and reference materials for each unique sensory attribute; and a scale ranging from 0 to 15 was used by trained descriptive sensory analysis panelists for scoring attribute intensities. A lexicon with 46 clearly defined terms was established and includes 14 aroma, 19 flavor, 9 texture, and 4 chemesthetic attributes that enabled characterization and differentiation of a wide range of pepper products. This sensory lexicon can be used by breeders and processors to guide the development of new varieties and innovative processing technologies for improved pepper products.
Objective 3: Role of starch structures in fried sweetpotato textures. Sweetpotato French fry (SPFF) textures have been associated with the dry matter content of the raw sweetpotatoes, especially in relation to starch content, but composition alone cannot fully predict the perceived textures. ARS scientists in Raleigh, North Carolina, studied the impact of the sweetpotato starch molecular structures, cooking properties, and granule sizes on fry textures from 16 diverse sweetpotato genotypes. These properties varied among the sweetpotato varieties, demonstrating the complexity of sweetpotato starch. While accounting for sweetpotato composition, it was found that starch properties explained an additional 5.9 to 45.4% of the variation in SPFF hardness, fracturability, crispness, smoothness, moistness, and oiliness. The percentage of A-type starch, which gelatinizes at higher temperatures than B- and C-type starches, was a significant factor in all these SPFF texture models. The model for predicting SPFF hardness was significantly affected by the percentage and gelatinization temperature of A-type starch, ratio of long amylopectin branches, and starch granule size. These starch properties helped explain variations in SPFF textures that could not be fully predicted by starch content alone. Therefore, starch structural, thermal, and granule size differences should be considered when developing new sweetpotato varieties and products. Additional sweetpotato chips and French fries were prepared from 20 sweetpotato genotypes with widely varying compositions and cooked textures and stored frozen for future analysis by a trained sensory panel and determination of polymer structures and molecular mobilities. Frying trials were conducted in collaboration with breeders at North Carolina State University to assist with selection of suitable genotypes for subordinate projects that support the United States sweetpotato industry.
Accomplishments
1. Improving the safety and quality of refrigerated pickles. Refrigerated pickles do not undergo thermal processing, which can leave them vulnerable to microbial contamination. Previous research has shown that pathogenic bacteria, such as Escherichia coli O157:H7 could survive for more than 25% of the shelf life of refrigerated pickles. ARS researchers in Raleigh, North Carolina, showed that a brief blanching of whole, raw cucumbers for 90 seconds in a 176°F water bath significantly reduced the native microbiota and is predicted to deliver a 5-log reduction of pathogenic E. coli on or within 1 mm of the surface of the cucumber. The proposed process can be implemented while maintaining the fresh-like qualities of refrigerated pickles during the typical shelf life and has the added benefit of retaining a fresher appearance for an extended shelf-life. This study illustrates that adding a brief blanching step in refrigerated pickle processing can reduce indigenous microbiota and target pathogens without negatively impacting quality attributes. This blanching process could assist pickled vegetable manufacturers in providing additional safeguards for consumers, while maintaining a high-quality product. As the Food and Drug Administration moves towards risk-based food safety initiatives, the data generated from our study may also be useful for a future risk assessment of refrigerated pickles. This research was disseminated in invited oral presentations at stakeholder conferences and a publication in the Journal of Food Science, resulting in several pickled vegetable manufacturers expressing interest in the study results.
Review Publications
Fideler Moore, J., Duvivier, R., Johanningsmeier, S.D. 2022. Changes in the free amino acid profile of pickling cucumber during lactic acid fermentation. Journal of Food Science. 87:599-611. https://doi.org/10.1111/1750-3841.15990.
Perez Diaz, I.M., Page, C.A. 2021. Whole-genome sequencing and annotation of selected Lactobacillales isolated from commercial cucumber fermentation. Microbiology Resource Announcements. 10(43):e00625-21. https://doi.org/10.1128/MRA.00625-21.
LaFountain, L.J., Johanningsmeier, S.D., Breidt, F., Stoforos, G.N., Price, R.E. 2022. Effects of a brief blanching process on quality, safety, and shelf life of refrigerated cucumber pickles. Journal of Food Science. 87(4):1475-1488. https://doi.org/10.1111/1750-3841.16112.
Little, C., Cruz-Martínez, V., St. Fort, D.P., Pagán-Medina, C., Page, C.A., Perez-Perez, Y., Taveirne, M., Lee, A., Arroyo González, N., Santiago Ortiz, C., Perez Diaz, I.M. 2022. Vegetable fermentations brined with low salt for reclaiming food waste. Journal of Food Science. 87(5):2121-2132. https://doi.org/10.1111/1750-3841.16084.
Rothwell, M.A., Zhai, Y., Pagan Medina, C., Perez Diaz, I.M. 2022. Growth of gamma-proteobacteria in cucumber fermentation is prevented by lactobacilli and the cover brine ingredients. Microbiology Spectrum. 10(3):e01031-21. https://doi.org/10.1128/spectrum.01031-21.
Page, C.A., Carter-Ogden, R., Lee, A.M., Perez Diaz, I.M. 2022. Genome sequences for Levilactobacillus brevis autochthonous to commercial cucumber fermentations. Microbiology Resource Announcements. 11(5):e00029-22. https://doi.org/10.1128/mra.00029-22.
Nakitto, M., Johanningsmeier, S.D., Moyo, M., Buguad, C., De Kock, H., Khakhasa, E., Forestier-Chiron, N., Dahdouh, L., Ricci, J., Mestres, C., Muzhingi, T. 2022. Sensory guided selection criteria for breeding consumer-preferred sweetpotatos in Uganda. Food Quality and Preference. 101:104628. https://doi.org/10.1016/j.foodqual.2022.104628.
