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.
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.
ARS scientists at Raleigh, North Carolina made progress on all three objectives. The microbial, chemical, and physical factors that impact the preservation of fruits and vegetables in consumer-preferred forms were studied. The findings can be used to enhance production of nutritious foods. Objective 1: Enabling production of high-quality pickles with beneficial bacteria: The Economic Research Service estimates that pickling cucumbers use 10% of the land dedicated to farming in the USA. Several states nationwide produce these types of cucumbers that are well-suited for pickling. Pickling extends the period for the safe consumption of cucumbers, enhances the flavor profile, and increases its economic value. Several processing defects compromise the quality of pickles produced in the USA. Pickle processing defects include 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. ARS researchers studied the genetic diversity of three species of beneficial bacteria taken from commercial cucumber fermentations employing classical microbiological techniques and modern DNA analysis tools. ARS scientists found 22 genetically unique types among the three bacterial species that are generally useful in pickling and in restoring human gut health. The DNA genomic sequence of the 22 unique bacteria were generated, aligned and annotated this year and made available in the public domain in July 2021. Seven of the 22 unique bacteria outperformed in fermenting cucumber juice under varied conditions of temperature, salt, and acid contents. ARS scientists will evaluate the ability of these beneficial bacteria to outcompete microbes associated with pickle defects, and the DNA genomic sequences will be fully analyzed in the coming year to identify beneficial features for pickling. 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: Presence and persistence of bitter molecules during cucumber pickling: Bitterness is rare in pickling cucumber but results in consumer rejection of pickle products, leading to costly losses of entire production batches. Cucurbitacins are bitter molecules that can be found in cucumber and related fruits and vegetables, such as watermelon and zucchini. ARS scientists in Raleigh North,Carolina determined the distribution of cucurbitacins in cucumber and their stability during pickling. Bitter and non-bitter pickling cucumbers were grown under controlled greenhouse conditions, harvested, and preserved by acidification or fermentation. Cucurbitacins are bitter in trace amounts and authentic standards are difficult to obtain. To address these challenges, ARS scientists used high resolution mass spectrometry to detect and identify these bitter molecules in cucumber pickles. Regular pickling cucumber grown under controlled greenhouse conditions did not contain any known cucurbitacins. The bitter cucumber type contained significant amounts of cucurbitacin C (CuC). This molecule was present throughout the cucumber fruit but was most concentrated in the interior portion (seed cavity). Fermentation reduced the content of this bitter component by 90%. The quantitative distribution of CuC in bitter pickling cucumber and the effect of common pickling processes on reduction in CuC content were shown here for the first time. This research lays the foundation for further studies into fermentation as a de-bittering method and identified a natural source of CuC for future exploration of its function in foods and health. Enhancement of health-promoting amino acid content in fermented cucumber pickles: Consumer interest in pickled vegetables creates an opportunity to positively impact health through the development of vegetable products that are naturally enriched in bioactive compounds through fermentation. Gamma-aminobutyric acid (GABA) is a bioactive compound that may be present in both fresh and fermented vegetables. GABA is known to reduce blood pressure, improve decision making, reduce anxiety, and enhance the immune system. Previous research by ARS scientists at Raleigh, North Carolina showed that GABA is made during the natural, lactic acid fermentation of cucumber and is stable during further processing and storage. Reducing the salt used for fermentation enhanced the content of this bioactive compound in finished pickle products, but the amount of GABA in pickles is limited by the free glutamate present in fresh cucumbers. ARS scientists evaluated fermentation methods to further increase GABA in fermented cucumber pickles. A study of the genomes of lactic acid bacteria identified several strains of Lactobacillus gasseri that contain the genetic elements for glutaminase, an enzyme that could enhance GABA production during fermentation of the glutamine-rich cucumber. Although not native to vegetable fermentations, L. gasseri was able to grow in pure cucumber juice in the presence of both salt and acid. However, addition of this microbe to a starter culture for cucumber fermentation did not increase glutamate or GABA content. Microbiome analysis showed that this L. gasseri culture did not participate in the fermentation when added together with the primary starter culture, Lactobacillus pentosus. Further research is needed to test the conditions under which these or other microbes could be beneficial for GABA enhancement in fermented vegetables. On the other hand, direct addition of GABA precursors to cucumber fermentations increased GABA content 10-fold regardless of brine acidification or starter culture addition. Therefore, pickles that deliver clinically relevant levels of GABA in a typical United States serving size can be produced by addition of glutamate as an ingredient in reduced-salt cucumber fermentations. Objective 3: Effects of sweetpotato polymer structures and molecular mobilities on fried sweetpotato textural properties and fat absorption: Americans consume nearly twice the amount of sweetpotatoes from two decades ago. Part of this growing popularity is from value-added sweetpotato products, such as fries and chips. Both starch content and individual sweetpotato starch cooking properties impact French fry textures, but the specific polymer traits of different sweetpotato varieties is not well known. Understanding the nature of the polymers and their effect on fry and chip textures would help breeders and processors select sweetpotatoes for producing consumer-preferred products. ARS scientists in Raleigh, North Carolina used cell wall active enzymes to modify the polymer structures of sweetpotatoes to learn more about how polymer structures influence chip texture and fat contents. Sweetpotato chips treated with a pectinase or a commercial blend of cell wall degrading enzymes had significantly lower breaking forces than untreated chips. Other enzymes, including protease, hemicellulase, cellulase, and pectin methyl esterase, did not change the hardness of the chips. All enzymatic treatments increased sugar leaching from the sweetpotatoes, indicative of cell wall damage, but only the pectinase and enzyme blend with pectinases affected the mechanical properties of the chips. Therefore, it is proposed that sweetpotato chip hardness is largely influenced by the pectic material properties. The effects of enzyme treatments on sweetpotato chip fat contents and glass transition temperatures are ongoing. In addition, 22 sweetpotato cultivars from the North Carolina State University breeding program with varying textural attributes were processed into chips and fries, and the cell wall and starch architectures will be determined and associated with sensory textures and molecular mobilities measured by time domain nuclear magnetic resonance. Additional research was conducted by ARS scientists to assist the breeding program with processing and evaluation of new cultivars and measure sweetpotato thermal properties for selecting cultivars that are optimal for chip and fry processing. The findings will help uncover which sweetpotato components dictate fried product textures, evaluate whether sweetpotato friers could use an enzyme treatment to manipulate product texture and reduce fat contents, and enable the selection of sweetpotato cultivars best suited for fried products.
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