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ARS Home » Southeast Area » Raleigh, North Carolina » Food Science Research » Research » Research Project #428731

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

2016 Annual Report

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.

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
Progress was made on all Objectives. Under Objective 1, Subobjective 1a: Microbial populations in newly developed alternative salt fermentations (calcium chloride brines instead of sodium chloride brines) were characterized. Samples from commercial cucumber fermentations brined with sodium chloride and calcium chloride were sequenced to determine predominant bacterial and yeast species using a high-throughput sequencing method. Results indicated that by the third day of fermentation the community of potentially pathogenic bacteria that can be naturally present on the cucumber fruit had mostly died off, and the lactic acid bacteria were the predominant cultures in sodium and calcium based brines. This was true for fermentations with and without a starter culture. The predominant species of lactic acid bacteria in fermentations studied was somewhat ambiguous, because of close sequence similarity between two very similar species, both of which may have genetic advantages (based on genome data analysis) that allowed the culture(s) to dominate in cucumber fermentations. These data fill critical knowledge gaps for newly developed low and alternative salt commercial scale fermentations. Under, subobjective 1b: Data from commercial scale cucumbers fermentation in winter months in northern climates, including freezing point depression due to brine salt type (sodium vs. calcium based fermentations), salt concentration, thermal conductivity, and other parameters was used to identify ways to optimize brining and reduce freezing damage. The thermal conductivity, diffusivity, resistivity, and volumetric specific heat of fermented cucumbers brined with calcium chloride as the primary salt were similar to those reported for cover brines with sodium chloride, indicating that freezing patterns will be alike in tanks containing cover brines with different formulations containing calcium chloride. However, freezing damage of fermented cucumber stocks brined without sodium chloride during winter is of concern for processors. The freezing point of a cover brine solution containing 2.2% calcium chloride was 30°F (-1°C) as compared to 14°F (-10°C) for a 6% sodium chloride cover brine solution. Supplementation of calcium chloride cover brines with glycerin or a higher concentration of calcium chloride is needed to match the freezing point of the sodium chloride cover brine solution and reduce tissue damage, although this may not feasible at the commercial scale. Three different external insulating systems for fermentation tanks were identified for further studies as an alternative to prevent freezing damage of fermented cucumbers brined with or without sodium chloride. This research will aid in the adaptation of alternative or reduced salt fermentation on commercial scale in cold climates. Under, subobjective 1c: Development of new alternative salt fermentation processes depends on the ability to predict the impact of those processes or new ingredients on shelf-life of finished products. Research was conducted to develop an accelerated shelf life model (ASLT) for rapid and accurate prediction of the shelf-life of pickle products. A trained sensory panel determined basic tastes and prominent flavor attributes of a variety of fermented cucumber dill pickle products. The most defining texture attributes include crunchiness, crispness, hardness, and fracturability. A temperature based ASLT was carried out on fermented cucumber pickles produced from cucumbers that were fermented in commercial, open-top tanks with both 6% sodium chloride or 1.1% calcium chloride brines, desalted, and processed into hamburger dill chips (HDC) with comparable finished product formulations. Hamburger dill chips produced from cucumbers fermented in calcium chloride brines had a lower initial firmness, but the rate of texture degradation for calcium chloride fermented HDC was significantly slower than that of traditional HDC at all storage temperatures. Texture degradation rates fit a first order kinetic model of time versus instrumentally measured mesocarp firmness. Sensory texture attributes of crispness and crunchiness were highly correlated to the instrumental firmness measured by puncture testing, enabling the selection of an appropriate cutoff value for mesocarp firmness to predict the end of shelf-life in the ASLT model. This research will enable pickle processors to predict the end of shelf-life based on instrumental texture measurements that relate to the sensory perception and consumer acceptability of texture changes in products during storage. Under, Objective 2, Subobjectives 2a and 2b: Ten lactic acid bacterial strains that have been previously characterized for commercial use in probiotic dairy products were evaluated for use in probiotic acidified cucumber products. An assay to measure acid resistance of these cultures to lactic and acetic acids was developed as a way to rapidly determine prospects of the cultures for long term survival in probiotic products. Significant differences in acid resistance and growth rates were found among the 10 cultures. Interestingly, we found that the faster growing cultures were, in general, less acid resistant. Selected cultures were tested for long term survival in a simulated acidified cucumber pickled vegetable product. We found that one of the most acid resistant strains identified could survive for up to 70 days under refrigeration conditions without significant loss of viability. Potential methods for the incorporation of probiotics in commercial pickle products have been designed and relevant variables identified. This research may contribute to the development of probiotic refrigerated pickle products. Under, subobjective 2c: Lactic acid fermentation of vegetables is an economical, traditional preservation process that is believed in many cultures to confer health benefits to consumers. As identified in other food systems, lactic acid bacteria responsible for fermentation may have enzymes that play an important, but unexamined, role in vegetable fermentation. Certain lactic acid bacteria produce bioactive peptides with associated benefits that include: weight control, antioxidant activity, immunomodulation, and decreased risk of cardiovascular disease and certain types of cancer. Research was initiated to develop methods for documenting the degree of proteolysis that occurs as a result of fermentation and characterize the peptide composition of raw and fermented cucumbers. Methods for extraction and quantification of protein and peptides from raw and fermented cucumbers were evaluated, and mass spectrometry tools were explored for peptide characterization. The high salt and low pH of fermented cucumbers create challenges with routine methods for total protein quantification. Further research will address these challenges to select appropriate methods for analysis. This research has the potential to help develop enhanced nutritional value of fermented vegetable products. Under Under Objective 3: The efficacy of acidification methods for the preservation of two varieties of tomatoes was evaluated. Cherry and Roma tomatoes were preserved with cover brines containing citric acid or hydrochloric acid and preservatives. Removal of tomato cores, pricking and dicing techniques were incorporated with the aim to achieve the efficient equilibration of the system. Pricking of the tomatoes resulted in the eventual separation of the exocarp from the flesh. Removal of tomato cores improved equilibration, but was slower than dicing. A combination of tomato dicing with acidification and added chemical and natural preservatives resulted in microbiologically stable products for 60 days, with retention of texture and color quality. A combination of garlic oil and hops as a natural preservative combination, resulted in the development of an incomplete fermentation. No significant differences were observed in the preservation patterns of the two tomato varieties. These results may enable the preservation and utilization of surplus vegetables that would otherwise be discarded. Under Objective 4: Studies were conducted to evaluate the effects of postharvest handling and long-term storage on chemical changes in sweetpotato (SP) roots in relation to the quality of fried chips. Fifteen SP genotypes with a wide range of reducing sugar content (0.46-17.51 mg/g fw) and flesh colors were evaluated for long-term storage and chip processing to reduce browning and formation of acrylamide, a known neurotoxin and potential carcinogen. Harvested roots were cured, stored at 15°C and samples were taken at 2 month intervals for chemical analysis and processing of fried chips. Curing SP roots prior to long-term storage as commonly practiced resulted in an increase in reducing sugars in 13 genotypes and darker chip color for 11 cultivars. However, curing did not affect acrylamide formation during frying for most cultivars, with the exception of two cultivars where acrylamide formation decreased after curing. Therefore, sugars were unlikely to be the limiting factor in acrylamide formation. This study indicates that SP genotypes and chemical changes during postharvest handling and storage should be considered when selecting the SP variety to be used in the production of chips with minimal browning and low acrylamide content. Additionally, over 200 SP genotypes with yellow and orange flesh color from the breeding programs were analyzed and evaluated for sugar profiles, asparagine, carotene and dry matter contents that influence health benefits and product quality. From this work, 15 promising cultivars were sent to two food companies for further evaluation on attributes for French fry processing.

1. Development and validation of a near-infrared spectroscopy method for the prediction of acrylamide content in potato French fries. Acrylamide, a known neurotoxin and potential carcinogen, can be formed in carbohydrate rich foods including french-fries. Efforts to reduce acrylamide content include breeding programs to select potato varieties that form less acrylamide during processing, and development of processing technologies to prevent acrylamide formation. A rapid method for accurately predicting the acrylamide content in potato french-fries was developed using near-infrared spectroscopy (NIRS). This NIRS method can be used to screen hundreds of samples without the need for organic solvents and expensive analytical instruments such as gas chromatography or liquid chromatography in conjunction with mass spectroscopy (GC-MS or LC-MS) required for the standard analysis of acrylamide in foods. The developed method therefore would provide substantial benefits to potato breeding programs and the food processing industry.


Review Publications
Adedipe, O.E., Johanningsmeier, S.D., Truong, V., Yencho, G. 2016. Development and validation of a near-infrared spectroscopy method for the prediction of acrylamide content in French-fried potato. Journal of Agricultural and Food Chemistry. 64:1850-1860. DOI: 10.1021/acs.jafc.5b04733.
Johanningsmeier, S.D., Harris, G., Klevorn, C. 2016. Metabolomic technologies for improving the quality of food: Practice and promise. Annual Review of Food Science & Technology. 7:413-438. DOI: 10.1146/annurev-food-022814-015721.
Medina-Pradas, E., Perez Diaz, I.M., Breidt, F., Hayes, J.S., Franco, W., Butz, N., Azcarate-Peril, A. 2016. Bacterial ecology of fermented cucumber rising pH spoilage as determined by non-culture based methods. Journal of Food Science. 80(1):M121-M129. doi: 10.1111/1750-3841.13158.
Perez Diaz, I.M., McFeeters, R.F., Moeller, L., Johanningsmeier, S.D., Hayes, J.S., Fornea, D., Gilbert, C., Custis, N., Beene, K., Bass, D. 2015. Commercial scale cucumber fermentations brined with calcium chloride instead of sodium chloride. Journal of Food Science. 80(12):M2827-M2836. doi: 10.1111/1750-3841.13107.
Reina, L., Perez Diaz, I.M., Breidt, F., Azcarate-Peril, M., Medina, E., Butz, N. 2015. Characterization of the microbial diversity in yacon spontaneous fermentation at 20 ºC. International Journal of Food Microbiology. 203:35-40. 10.1016/j.ijfoodmicro.2015.03.007.
Caldwell, J.M., Perez Diaz, I.M., Sandeep, K.P., Simunovic, J., Harris, K., Osborne, J., Hassan, H.M. 2015. Mitochondrial DNA fragmentation as a molecular tool to monitor thermal processing of plant-derived, low-acid foods and biomaterials. Journal of Food Science. 80(8):M1804-1814. doi: 10.1111/1750-3841.12937.
Caldwell, J.M., Perez Diaz, I.M., Simunovic, J., Harris, G., Hassan, H.M., Sandeep, K.P. 2015. Mitochondrial DNA fragmentation to monitor processing parameters in high acid, plant-derived foods. Journal of Food Science. 80(12):M2892-M2898. doi: 10.1111/1750-3841.13139.
Johanningsmeier, S.D., McFeeters, R.F. 2015. Metabolic footprinting of Lactobacillus buchneri strain LA1147 during anaerobic spoilage of fermented cucumbers. International Journal of Food Microbiology. 215:40-48. doi: 10.1016/j.ijfoodmicro.2015.08.004.