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Research Project: Improved Processes for the Preservation and Utilization of Vegetables, Including Cucumber, Sweetpotato, Cabbage, and Peppers to Produce Safe, High Quality Products with Reduced Energy Use and Waste

Location: Food Science Research

2018 Annual Report

Objectives
Objective 1. Enable the commercialization of reduced NaCl fermentation technology by identifying suitable starter cultures, optimizing processing parameters to prevent freezing of the fruits; and develop a shelf-life model that allows processors to rapidly and accurately predict end-use quality. Subobjective 1a: Develop starter cultures for cucumber fermentations brined with calcium chloride. Subobjective 1b: Identify parameters for long term storage of fermented cucumbers brined with calcium chloride so that freezing damage of the fruits can be prevented and removal of goods from tanks is facilitated during the winter season. Subobjective 1c: Develop an accelerated shelf-life model for fermented cucumber pickles. Objective 2. Determine the factors that maximize survival of probiotic bacteria and retention or production of healthful components in pickled vegetables to facilitate the commercialization of new cucumber pickling methods for delivery of live, probiotic, lactic acid bacteria to consumers. Subobjective 2a: Determine the acid resistance and growth characteristics of probiotic lactic acid bacteria that influence their survival in acidified and fermented cucumbers. Subobjective 2b: Identify suitable methods for the incorporation of probiotic lactic acid bacteria in refrigerated cucumber pickles. Subobjective 2c: Evaluate production and/or retention of bioactive peptides in fermented cucumbers. Objective 3. Enable new commercial methods for process-ready-fermentation or acidification to convert surplus vegetables at grocery stores and farmers' markets into value-added products with enhanced probiotic content. Objective 4. Integrate sweetpotato genotypes intended for commercial processing with their potential for increased levels of bioactive compounds and high quality food products.

Approach
Consumer interest in vegetable products with enhanced health benefits will drive future developments in the fermented and acidified vegetable industry. One major limitation for expansion is the generation of chloride waste and excess water use from traditional preservation technologies that rely on high sodium chloride (NaCl) concentrations. Cucumber fermentation in calcium chloride (CaCl2) brine was developed and transferred to commercial processors as an alternative to reduce chlorides in waste waters. Long-term adoption of this reduced NaCl preservation technology requires: a) defining conditions for bulk storage in outdoor tanks to prevent freezing damage in cold climates; b) understanding the microbial ecology of low salt vegetable fermentations so that appropriate starter cultures can be defined; and c) developing a shelf-life model to accurately predict end-use quality. The ability to deliver probiotic bacteria in pickles and to adapt acidification and fermentation technologies to preserve a wide range of surplus vegetables from farms and retail markets will provide new knowledge and technology for processors to engage the growing consumer market interested in healthy eating. For sweetpotatoes, consumer demands for readily available processed products have increased in recent years. Development of new cultivars that are bred for desirable processing characteristics, low acrylamide formation and high bioactive compound content, will benefit sweetpotato processors and farmers and boost the contribution of this nutritious vegetable in the U.S. diet. The proposed research will support developments that broaden the markets for sweetpotatoes, acidified and fermented vegetables and will reduce food and processing wastes.

Progress Report
Adding a malate decarboxylase negative starter culture to low salt cucumber fermentations for reducing gas production and bloater defect, resulted in a slow and incomplete sugar conversion. Combination of acidification and mdc- starter culture resulted in an incomplete fermentation with reduced bloater defect, as compared to non-inoculated and acidified fermentations. Cucumber fermentation cover brines containing calcium chloride were reformulated to contain calcium hydroxide, less than 4% sodium chloride and an acid to enhance buffer capacity enabling a complete fermentation and inhibit the gas producing wild microbiota. Use of the reformulated brine and mdc- starter culture in cucumber fermentations resulted in complete sugar conversion with final pH of 0.5 units higher than fermentations brined without the base, increased lactic acid production and reduced relative abundance of enteric bacteria (92%) and gas production. Although the combination of mdc-starter culture and reformulated brine enabled a complete fermentation with less carbon dioxide production, an insignificant reduction in bloater defect was observed. Cucumbers are commercially fermented and stored in bulk in outdoor open top fiberglass tanks. During winter, snow and ice that accumulates around and on top of the tanks affect heat transfer in an unpredictable manner. We evaluated the performance of inexpensive and resilient fermentation tank insulation and determined heat loss associated with storage and preservation of fermented cucumbers during winter. Three insulation configurations were tested in a collaborative effort with processors and engineers at North Carolina State University: conical top-cover, flat top-cover, and perimeter insulation. Changes in temperature during storage were recorded in different tank configurations. A mathematical model was developed to simulate temperature profiles and heat loss in an idealized fermentation/storage vessel. Comparisons of these insulated tank configurations suggested a significant difference in temperature between flat cover and uncovered tanks when exposed to temperature fluctuations during spring season in Pinconning, Michigan. In another study, glycerin (2-20%) as a food-grade antifreeze agent was used to reduce freezing point and improve quality of bulk stored fermented cucumbers brined with calcium chloride (1.1-20%). Cover brines supplemented with 14.5% vol. glycerin, 18% wt. calcium chloride, or 14% vol. glycerin and 5% wt. calcium chloride were needed to match the freezing point of 6% wt. NaCl cover brine solution typically used for cucumber fermentation. Results indicated that supplementation of CaCl2 and glycerin in fermentation cover brines located on top of the cover boards in fermentation tanks may aid to minimize the damage of fermented cucumbers induced by freezing and also facilitate the removal of fermented fruits from the vessels during winter. As described in our previous report, strain selection and process modifications may enable the delivery of enough probiotic lactic acid bacteria in one serving of refrigerated pickles to confer health benefits. Further study on probiotic pickles was conducted using two methods for incorporating cultures, and pickles were evaluated for consumer acceptability, color, texture, and composition of organic acids and sugars. The first product was prepared by direct addition of a commercially available, freeze-dried culture of probiotic Lactobacillus plantarum and a 24 h ambient temperature pre-conditioning step. The second product was prepared by adding the probiotic Lactobacillus plantarum culture in lower numbers and allowing it to ferment for 5 days prior to refrigerated storage. Both products were compared to a similar, standard-formulation refrigerated pickle product after 42 days of refrigerated storage. No differences were observed in color or texture, and the probiotic pickles had higher consumer liking than the control product that contained no probiotic culture. The higher liking scores appear to be related to an enhanced flavor profile in the probiotic pickles. This research demonstrates that high quality, refrigerated pickles containing probiotic cultures are acceptable to consumers and can be produced. Future research is needed to fully understand all of the factors that contribute to probiotic culture survival in pickled vegetable products. Progress was made in evaluation of surplus vegetables from the United States market for value-added fermented or acidified foods. Vegetables were grouped based on the sugar content data published in the USDA Food Composition Database. Four groups have been defined as low, low-medium, medium-high and high sugar content. Representative vegetables of each group were selected for experimentation based on pounds produced annually and the estimated market value as reported by the USDANASS. Fermentation cover brines were defined for each vegetable group considering the natural buffering capacity and acid production potential from the natural sugar content. Buffer capacity for representative vegetables for each group was indicated by the amount of lactic acid, acetic acid or a mixture of the two acids that was necessary to decrease the pH of each vegetable slurry to 3.3. Vegetable slurries and pieces were packed in glass jars to characterize acid equilibration rate. Consumer demands for fried products processed from orange-fleshed sweetpotatoes as carotene-rich foods have increased. Processing of high quality fried products is challenging due to variation in chemical constituents in raw sweetpotatoes that may affect product color and formation of acrylamide, a potential carcinogen. We evaluated the changes in sugar and free amino acid levels in sweetpotato genotypes during storage as related to fried chip browning and acrylamide content. Fifteen sweetpotato genotypes were cured for 7 days, subsequently stored up to 10 months at 58ºF as commercially practiced, and samples were taken at 2-month intervals. For raw roots, sugar and amino acid profiles were analyzed by high performance liquid chromatography and liquid chromatography mass spectrometry. Peeled roots were cut into 1.5 mm thick slices and fried at 157ºC in canola oil for 3 min. The fried chips were analyzed for Hunter color values, and acrylamide content by liquid chromatography mass spectrometry. Results showed that total reducing sugar levels ranged from 0.34-30 mg/g fresh weight and significantly increased (p = 0.05) in all genotypes after curing and until 4-6 months of storage, and then slightly decreased afterwards. The mean acrylamide content in chips processed from the 15 genotypes at all storage times ranged from 300 ng/g to over 9,000 ng/g. Analysis of variance revealed significant interactions between genotype and storage time for the levels of free amino acids, sugars, and acrylamide. The increases in reducing sugars were associated with darker chip color in 8 genotypes. Asparagine levels, rather than reducing sugar, correlated significantly with acrylamide formation (r=0.588, p = 0.05). Therefore, sweetpotato genotypes and chemical changes during postharvest storage should be considered in varietal development for processing of fries with less browning and low acrylamide content. Near-infrared spectroscopic model with high prediction accuracy has been developed for rapid quantification of beta-carotene, anthocyanins, total phenolics, asparagine and sugars in sweetpotato genotypes to facilitate the breeding effort. Quality of French fries and chips processed from 40 new breeding lines were evaluated and 12 promising genotypes were selected for further evaluation by food processing companies.

Accomplishments
1. Phenotypic and genotypic diversity of Lactobacillus buchneri. Characterization of spoilage organisms is a key step in the development of stabilization procedures for low salt vegetable fermentations. Lactobacillus buchneri, a common non-pathogenic bacteria, can degrade lactic acid and cause spoilage of fermented cucumbers, especially in vegetable fermentations with reduced salt concentrations. To better understand the ability of this organism to cause spoilage, ARS researchers at Raleigh, North Carolina, isolated the bacteria from spoiled, fermented cucumber and compared their differences in genetic make-up, growth characteristics, and ability to degrade lactic acid in laboratory media and fermented cucumbers. Several unique strains of L. buchneri were identified and characterized, demonstrating the diversity and adaptability of this organism from a single environmental niche. These microbial strains can be used as target spoilage organisms for developing low salt preservation techniques and further evaluated for industrial applications that rely on their unique metabolic capabilities.

2. Determining the presence of nitrate and nitrite in fermented and acidified vegetables. The influence of nitrate and nitrite in foods on human health has been controversial, with literature citing both positive and negative health effects. ARS researchers at Raleigh, North Carolina, measured the concentration of these compounds in a wide variety of acidified vegetables (made by adding vinegar or other acids to fresh fruits and vegetables), as well as some fermented foods currently available in the U.S. market. This was done in collaboration with a researcher from Jiangnan University, Wuxi, China, who was a visiting scientist at the Raleigh, North Carolina location. The naturally present antioxidants in foods were also of interest in the study of nitrite and nitrate levels in foods due to interactions between these compounds, so antioxidant levels were also measured. We found that nitrite was relatively rare in acidified vegetables, but was present in some of the fermented foods tested. Nitrate, on the other hand, was found to be present at varying levels in many acidified products. These results provide new information for evaluating nitrate and nitrite content in pickled fruit and vegetable products, and may be used to help assess the potential health consequences of these compounds in U.S. consumer diets.

U.S. DEPARTMENT OF AGRICULTURE

Food Science Research: Raleigh, NC