Location: Food Science and Market Quality and Handling Research Unit
2023 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, particularly towards Objectives 1 and 3. Objective 2 was stalled due to a critical vacancy, which was just filled in May 2023. New collaborations were initiated with scientists at the USDA-ARS Immunity and Disease Prevention Research Unit, the University of Arkansas, and Louisiana State University.
Under Objective 1 enabling production of high-quality pickles with functional live bacteria: The preservation of cucumber, cabbage, pepper and garlic by fermentation or acidification enables year-round access and consumption, uses 2.5 billion pound each year in North America and 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 during cucumber 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. Several lactic acid producing bacteria are naturally present in cucumber; lead the fermentation needed to make pickles; and could be leveraged to control spoilage bacteria. ARS scientists in Raleigh, North Carolina, discovered that the formation of internal holes in fermented cucumbers could be reduced by 85% when a particular type of plant derived-lactic acid producing bacterium, known as Lactococcus lactis, is added to fermentations. This finding could significantly reduce economic losses for pickle processors. Additionally, the team learned that several spoilage-associated lactic acid bacteria utilize a variety of acids, including the sorbic and benzoic acids commonly used as preservatives, and the lactic acid produced in fermentation. Because such spoilage associated lactic acid bacteria also tolerate up to 8% sodium chloride, their control in commercial cucumber fermentations necessitates innovative interventions. Researchers continue to evaluate the ability of pickle-adapted beneficial bacteria to outcompete microbes associated with processing defects to improve quality of products, availability of foods for a growing population, and profitability.
Healthy pickles for a healthy population: While 40% of healthy fresh produce is wasted in the United States, affecting invaluable resources and the environment, vulnerable populations experience malnutrition, dysfunctional gut microbiomes and poor health. An integral solution lies in the development of waste-reducing, extended shelf-life fermented vegetables that are produced with respect for the environment and also improve consumer health. ARS scientists in Raleigh, North Carolina, characterized a collection of vegetable-derived-lactic acid producing bacteria to optimize low-salt-natural-fermentation technology for producing pickle blends from surplus vegetables. ARS researchers found that a genetically distinct member of the Levilactobacillus brevis species, a type of lactic acid producing bacteria, only colonizes cucumber fermentation. Such pickle specific bacterium lacks the ability to utilize citric acid and degrades 1,2-propanediol, a compound relied on by pathogens in the human gut to cause infection. Such observation suggests that Levilactobacillus brevis from pickles could exclude pathogens from the human gut by competing for the common energy source. Researchers from the Food Science and Market Quality and Handling Research Unit in Raleigh, North Carolina, are partnering with the ARS Immunity and Disease Prevention Research Unit in Davis, California, to determine if fermented vegetables can confer health benefits to consumers. The upcycling of surplus vegetables into health-promoting pickles can ameliorate environmental impacts, enhance the resilience of the agricultural system and better support the nutritional needs of United States citizens.
Under Objective 2 pickled vegetable composition and quality. The perceived quality of foods come from flavor, texture, and other sensory characteristics, as well as its nutritional and health-promoting properties. However, most of the natural components of foods influencing these properties have yet to be characterized. We procured, installed and tested new instrumentation for comprehensive two-dimensional gas chromatography with high resolution time of flight mass spectrometry to identify compounds that drive flavor and enhance quality. This effort will be complemented by profiling of nutritional components to investigate the retention and production of health-promoting compounds that occurs during fermentation or acidification. Methods and data processing workflows were optimized and an outgoing agreement was formed with the University of Arkansas Metabolomics Core to enable research on the impact of fermentation or fresh pack processing on nutritional and flavor components of pickled vegetables.
In regards to Objective 3 new sweetpotato varieties are continuously being developed for enhanced agronomic characteristics (e.g., disease resistance, higher yields) and improved nutrition (e.g., provitamin A), but these new varieties must also meet consumer preferred culinary attributes, such as texture and sweetness. ARS scientists in Raleigh, North Carolina in collaboration with North Carolina State University and the International Potato Center investigated relationships between intrinsic and physical sweetpotato properties and the perceived textures and sweetness of baked sweetpotatoes. Fifteen diverse genotypes were grown on three separate farming plots, and the baked sweetpotatoes were evaluated by a trained sensory panel for flavor and texture profiles (e.g., sweetness, firmness, particle size, cohesiveness). Sweetpotato composition (dry matter, starch, sugars, cell wall materials), amylase activities, starch properties (thermal properties, granule sizes, crystal type ratios) and mechanical texture analysis were also measured. Mouthfeel-related textures were related to sugar and starch contents, starch granule type, and amylase activities, while chewing force textures were correlated to mechanical properties of baked sweetpotatoes. The sweetness of baked sweetpotato was impacted by more than just sugar content, which only accounted for 34% of the variation in sweetness among the genotypes. Maltose was a predominate sugar affecting sweetness, and its contents were highly correlated with starch contents along with a functional ß-amylase. Sweetness was also associated with the perceived particle size. Volatile organic compound profiles of these genotypes showed that there are hundreds of components contributing to the perception of characteristic flavors in sweetpotato. These findings advance Sub-objective 3b: “Explore perceived sweetpotato sweetness and the impact of precursors in raw sweetpotato on the sugar and volatile compound composition of processed products” by identifying the factors that affect baked sweetpotato textures, sweetness perception, and flavor.
Accomplishments
1. Sweetpotato cell wall polymers affect fried chip textures and fat contents. The United States sweetpotato industry has experienced a large growth over the past 2 decades along with increased demand for value-added sweetpotato products such as chips. The challenge with sweetpotato chip production is that common United States sweetpotato varieties were not bred for chipping, which leads to issues such as undesirably soft to hard textures and higher oil contents. ARS researchers in Raleigh, North Carolina investigated the impact of sweetpotato cell wall polymers on sweetpotato chip texture and oil uptake to identify important sweetpotato attributes that can be bred for or modified by postharvest treatments for desirable chip attributes. Slices treated with a pectin strengthening enzyme resulted in firmer chips, and chips treated with an enzyme that degrades proteins or an enzyme blend that weakens the cell wall structure were softer. Oil contents were also lower in chips treated with the enzyme blend. This demonstrated chip texture and oil contents are dependent on cell wall polymers and can be modified using post-harvest processing treatments, such as food-grade enzymes. This research was spotlighted in an article in the Institute of Food Technologists’ Food Technology trade magazine and was an invited presentation for an Institute for the Advancement of Food and Nutrition Sciences’ Innovation Showcase. The research will be foundational for the development of new varieties and post-harvest processes specifically for sweetpotato chips.
Review Publications
Perez Diaz, I.M., Medina, E., Page, C.A., Johanningsmeier, S.D., Daughtry, K.V., Moeller, L. 2022. Prevention of microbes-induced spoilage in sodium chloride-free cucumber fermentations employing preservatives. Journal of Food Science. 87(11):5054-5069. https://doi.org/10.1111/1750-3841.16345.
Allan, M.C., Read, Q.D., Johanningsmeier, S.D. 2023. Impact of sweetpotato starch structures, thermal properties, and granules sizes on sweetpotato fry textures. Food Hydrocolloids. 137:108377. https://doi.org/10.1016/j.foodhyd.2022.108377.
Allan, M.C., Johanningsmeier, S.D. 2022. Sweetpotato chip texture and fat content: Effects of enzymatic modification of cell wall polymers. Journal of Food Science. 87(9):3995-4008. https://doi.org/10.1111/1750-3841.16267.
Page, C.A., Perez Diaz, I.M. 2023. Whole-genome sequencing and annotation of Pediococcus ethanolidurans and Pediococcus pentosaceous isolates from commercial cucumber fermentation. Microbiology Resource Announcements. 12(5):e00050-23. https://doi.org/10.1128/mra.00050-23.
Page, C.A., Perez Diaz, I.M., Pan, E., Barrangou, R. 2023. Genome-wide comparative analysis of Lactiplantibacillus pentosus isolates autochthonous to cucumber fermentation reveals subclades of divergent ancestry. Foods. 12(13):2455. https://doi.org/10.3390/foods12132455.
Perez Diaz, I.M., Page, C.A., Mendez-Sandoval, L., Johanningsmeier, S.D. 2023. Levilactobacillus brevis autochthonous to cucumber fermentation is unable to utilize citric acid and encodes for a putative 1,2-propanediol utilization microcompartment. Frontiers in Microbiology. 14:1210190. https://doi.org/10.3389/fmicb.2023.1210190.
