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Improvement of Methods for Processing Cucumbers, Sweetpotatoes, Peppers and Other Vegetables to Assure Quality and Safety and Reduce Waste

McFeeters, Roger
North Carolina State University
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  1. Determine the microorganisms and biochemical pathways that result in loss of lactic acid and spoilage of fermented cucumbers and then develop approaches for doing commercial cucumber fermentations without the use of sodium chloride which will greatly reduce salt waste from tank yard operations while assuring the microbiological stability of fermented cucumbers.
  2. Develop new approaches for preservation of acidified vegetables that increase consumer acceptance including acidification with reduced sour taste impact, use of alternative natural preservative compounds to prevent microbial growth, evaluation of pasteurizable plastic containers, and application of microwave heating for thermal processing.
  3. Investigate a range of pH, different organic acids, and temperature conditions that will ensure at least a 5-log reduction in acid tolerant pathogens in order to broaden the range of acidified vegetable products that can be safely manufactured without a thermal process.
  4. Determine mechanisms by which acids and acid preservatives disrupt the metabolism of E. coli O157:H7 cells that results in cell death in order to find alternative approaches that increase the rate of die off of acid tolerant pathogens in acidified foods.
  5. Determine the conditions of acid concentration, pH, temperature, competing microflora and fermentation time that will assure die off of E. coli O157:H7 and other acid tolerant pathogens in commercial cucumber fermentations.
  6. Increase the utilization of sweetpotatoes by increasing the time after harvest they can be made into high quality fried products and develop a new vortex dehydration technology to convert sweetpotatoes into functional flours.
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Non-Technical Summary: The acidified and fermented vegetable industry must address issues of: (1) excessive chloride waste from high salt fermentations, (2) high energy consumption from the use of 50 year old steam pasteurization technology, and (3) static or declining consumption of traditional product lines. For sweetpotatoes to make a greater contribution to the U.S. diet, they must be converted into forms that maintain or increase nutrient levels and that can be conveniently used by food processors in a variety of food products. To reduce chloride waste, methods to do commercial cucumber fermentations without use of sodium chloride will be developed. Reduction of energy consumption will be addressed by using microwave heating to more efficiently deliver heat to products and by developing practical means to pre-heat product and brine prior to filling containers. More convenient packaging, alternatives to traditional preservatives, acidification of nutrient rich vegetables to reduce sour taste intensity, and procedures to deliver probiotic bacteria will be developed to provide new approaches to add value to fermented and acidified vegetable products. Sweetpotato farmers and processors need new processing approaches that will result in increased production and consumption of this highly nutritious vegetable. A new vortex dehydration technology will be evaluated to determine if it can be used to produce high quality dehydrated sweetpotato flours from orange and purple flesh sweetpotatoes which can serve as functional food ingredients. There will be continued coordination with sweetpotato breeding programs to develop cultivars better adapted to year round production of sweetpotato fries and chips. A critical part of developing new processing methods or modifying current methods is to ensure the safety of the process. Refrigerated, fermented, and bulk acidified pickled vegetables cannot be thermally processed. While products produced by these preservation methods have a history of safe production, definitive data are lacking to show that the acid tolerant pathogens Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella enterica will die off over the range of conditions under which commercial processing and fermentations occur. Acid resistant bacterial pathogens have been shown to survive in acidified vegetable products that are not heat processed, particularly refrigerated products. This project will determine organic acid formulations (for a variety of commonly used and others generally regarded as safe acids), holding times and temperatures that assure a 5-log reduction of these pathogens in non-heat processed acidified vegetables. The effects of organic acids and food preservatives on the metabolism of E. coli O157:H7 will be determined to understand the mechanisms by which acids and acid preservatives kill acid tolerant pathogens. The knowledge gained will result in recommendations for safe manufacturing processes for acid and acidified vegetables, and potentially other food products containing organic acids including fruit beverages, salsas, dressings, and others.

Approach: The following steps will be taken to understand and control spoilage of cucumber fermentations. (1) Carry out experiments to confirm that bacteria (Lactobacillus buchneri; Pediococcus ethanolidurans) and yeasts (Issatchenkia occidentalis; Pichia manshurica) that have been repeatedly isolated and identified from anaerobic and aerobic (air purged) spoilage fermentations are in fact organisms that cause the spoilage. If necessary, additional identification of microorganisms from spoiled fermentations will be done. (2) Determine the metabolite changes that occur during spoilage processes using HPLC to measure changes in the major normal and spoilage metabolites including lactic acid, acetic acid, propionic acid, and butyric acid and formation of trimethy silyl derivatives followed by two dimensional gas chromatography-time of flight mass spectrometry (GCxGC-TOFMS) to determine changes in a broad range of non-volatile metabolites including amino acids, sugars, organic acids, alcohols, and amines. (3) Determine the range of conditions relevant to the cucumber fermentation process where organisms responsible for spoilage can grow and conditions where growth is inhibited. This will include their ability to be inhibited by potassium sorbate, sodium benzoate, and AITC at concentrations that do not interfere with a normal lactic acid fermentation. Spoilage will be produced experimentally in no salt and reduced salt fermentations of cucumbers from different growing areas by excess air purging or adjusting brine pH to ?3.8 in anaerobic conditions. These conditions have previously been used to produce spoilage by inoculation of filter sterilized brine from normal fermentations with the total population of microorganisms from fermentations that have spoiled. Results obtained from investigations of the microbiology and biochemistry of commercial spoilage of fermented cucumbers will be incorporated into development of methods to ferment cucumber in brines that contain calcium chloride to maintain cucumber firmness, but no sodium chloride. Experiments will be done with fermentations in replicated 55 gallon drums in a commercial tank yard. Control fermentations will be done with standard 6% NaCl brine containing 20 mM calcium chloride to compare with experimental fermentations with no NaCl in the cover brine. We plan to determine the minimum inoculation of L. plantarum culture required to initiate a rapid fermentation that will result in production of at least 80 mM lactic acid. Potassium sorbate (?0.1%), Na benzoate (?0.1%), and allyl isothiocyanate (?200 ppm) will be added at the beginning of fermentation to limit growth of yeasts that appear to begin the spoilage process by degrading lactic acid. Once we are confident that normal fermentations with sufficient lactic acid production will occur and that microbiological spoilage and cucumber bloating can be prevented in 55 gallon fermentation barrels, salt free fermentations will be carried out in 8000 to 10,000 gallon commercial scale tanks.

Funding Source
Nat'l. Inst. of Food and Agriculture
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Escherichia coli
Grains, Beans, Legumes