Di Capua, Marika (2015) Lactic Acid Bacteria as protective and functional cultures for the enhancement of chestnut (Castanea sativa Mill.) processing chain. [Tesi di dottorato]

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Abstract

Chestnuts are very perishable fruits, whose quality may be compromised during postharvest handling. Damage can be caused both by insects and fungi. Water curing, a commonly used postharvest method, is based on soaking fruits in water typically for about one week. Factors that affect effectiveness of water curing have only been explained partially. A decrease in pH, likely imputable to a light fermentation caused by lactic acid bacteria, may inhibit the growth of moulds. In this study a Lactobacillus pentosus strain was selected for its ability to inhibit fungi, and used as a starter culture during water curing. As second goal, a reduction of the environmental impact of the process was evaluated by using water that had been re-cycled from a previous curing treatment. Experiments were performed on pilot as well as on farm scale. In all trials, microbial dynamics were evaluated by means of a polyphasic approach including conventional and molecular-based analyses. According to results, the employment of an adjunct culture appears as a very promising opportunity. Even if no reduction in the duration of the process was achieved, waters exhibited a minor microbial complexity and fruits did not lose the natural lustre after the process. The second part of the project focused on the development of novel foods chestnut-based in order to seasonally adjust the offer and use the surplus of undersized production, providing, at the same time, a response to the growing demand for healthy and environmentally friendly products. Broken dried chestnuts were employed to prepare purees to be fermented with six different strains of Lactobacillus rhamnosus and Lactobacillus casei. The fermented purees were characterized by a technological and sensorial point of view, while the employed strains were tested for their probiotic potential. Conventional in vitro tests have indicated the six lactobacilli strains as promising probiotic candidates; moreover, being the strains able to grow and to survive in chestnut puree at a population level higher than 8 Log CFU mL-1 along 40 days of storage at 4°C, the bases for the production of a new food, lactose-free and with reduced fat content, have been laid. Subsequently, the effect of indigestible chestnut fiber and of chestnut extract on the viability of selected lactic acid bacteria strains was evaluated. Twelve strains were selected, on the basis of tolerance to low pH values and bile salts, and submitted to exposition to simulated gastric or bile juice in presence of chestnut extract with or without immobilization in chestnut fiber. The presence of chestnut extract proved to play a significant role on the gastric tolerance improvement of lactobacilli. The recorded protective effect could not be simply related to the starch or reducing sugars content. RP-HPLC demonstrated that in the chestnut flour, there are one or more hydrophobic peptides or oligopeptides, which specifically offer a marked resistance to simulated gastric juice, albeit present at low concentration. These beneficial effects proved to be dependent by the cultivar used to produce the flour. Moreover, were evaluated the suitability of chestnut extract as carrier for spray drying of two probiotic Lactobacillus rhamnosus strains and to develop a probiotic food chestnut based. The optimal settings for the spray-drying processes were defined and the loads of undamaged cells in the dried powders were quantified. Spray-dried cultures were incorporated into an anhydrous basis for chestnut mousse developed ad hoc. In this form, viable cells remained stable over 108 CFU g-1 during a 3 months long storage at 15°C. Sensorial analysis did not highlighted significant differences (p < 0.05) in preference between probiotic-supplemented and control mousses. Results suggest that chestnut mousse, a food product naturally rich in antioxidant compounds, may represent an excellent carrier for probiotics delivering. Finally, chestnut flour was even evaluated for the production of bakery goods gluten-free. Microbial dynamics in the chestnut-based sourdoughs were evaluated by PCR-DGGE. The impact of fermentation on volatile organic compounds formation during sourdoughs maturation was evaluated by means of gas chromatography coupled to mass spectrometry (GC/MS). Members of the Lactobacillus plantarum group and Pediococcus pentosaceous dominated the sourdough ecosystems. Nevertheless, RAPD-PCR allowed recording a relevant genotypic biodiversity among strains coming from gluten-free flour combinations. A total of 59 volatile compounds were identified, mainly alcohols, esters, acids, aldehydes and ketones. Principal component analysis of samples at the beginning and at the end of ripening offered a good separation of the samples and highlighted the effect of fermentation on the sensorial profile.

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