Acid dips, edible coatings and films are alternative treatments to the use of chemicals to extend the storage life of litchi and rambutan fruits. The use of microorganisms to diminish the incidence of phytopathogens by competition and production of antagonistic compounds is another alternative. These postharvest treatments are well documented, but few reports combining microorganisms and coatings are found in the scientific literature. Therefore, it is important to study interactions between biological control agents and coatings, and their effects on postharvest quality of fruits. The aim of the present work was to study the effect of Lb. plantarum as an in situ biological producer of lactic acid and its interaction with chitosan coatings as well as their effect on postharvest life of rambutan and litchi fruits. The effect of chitosan on the growth of three species of lactic acid bacteria was determined. Lb. acidophilus and Lb. spp. grew to a lesser extent in presence of chitosan, whereas Lb. plantarum displayed greater resistance to the biopolymer under the conditions evaluated, hence it was selected for further use in this study. Two methodologies were used in order to determine the form of application of the selected microorganism in the fruit coatings. In the first one Lb. plantarum was dispersed in the chitosan solution and subsequently the film was formed by casting. The lactic acid bacteria did not show viability at chitosan concentrations of 5, 10, 20 and 30 g/L, but was able to survive at a concentration of 2 g/L. In the second methodology, the microorganism was incorporated by dipping chitosan films (up to 20 g/L), previously formed, in cellular suspensions. Bacteria were able to survive up to 10 days on chitosan film surfaces in relative humidities (RH) up to 92 %. The analyses of water vapor permeability (WVP) showed values for chitosan films of 2.1 gmm/kPadm2 at 0 to 75 % partial pressures. In order to improve chitosan WVP, a chemical crosslinking between the biopolymer and glutaraldehyde (0,02 % (w/v) was made. Our results showed a significant reduction in permeability of the modified chitosan (α < 0.05). The addition of fructose as prebiotic improved Lb. plantarum viability. The results showed that glutaraldehyde and fructose addition helped to maintain Lb. plantarum viabilities during longer periods than unmodified chitosan. An experiment with rambutan (Nephelium lappaceum) was conducted in order to evaluate the effect of Lb. plantarum and chitosan on the fruit quality retention. Ripe fruits coated with chitosan and bacteria were stored at 10 ° and 25 °C for 15 and 10 days, respectively. The limiting factors for rambutan conservation were related to pericarp discoloration, weight loss and fruit senescence. The viability analyses showed that Lb. plantarum combined with chitosan did not present significant changes (α < 0.05) in log CFU values during the storage period at 10 °C and 25 °C. Lb. plantarum alone and combined with chitosan, significantly diminished weight and color losses (a* and L*) with respect to control fruits. Total soluble solids (TSS) and pH of juice extracted from fruits stored at 10 °C did not show significant differences with respect to the initial values, but a significant difference was found at 25 °C possibly due to the more advanced senescence process of fruits stored at the higher temperature. In contrast to the results observed in the in vivo experiment, the viability of lactic acid bacteria on chitosan coatings was reduced 71.8 % after 10 days of storage at 25 °C and 75% HR. Lactic acid determinations showed that the lactic bacteria were metabolically active. The effect of Lb plantarum alone and in combination with unmodified chitosan and chitosan modified with glutaraldehyde on the preservation of color and quality of Litchi chinensis sonn was also investigated. The viability observed in fruits treated with Lb. plantarum showed higher log CFU values in comparison with control fruits, ranging from 5.3 to 4.5 log CFU/g fruit (α < 0.05) at 21 days of storage at 10 ºC and 75% RH. The application of bacteria on fruit pericarp significantly reduced color loss (α < 0.05), maintaining color values (L* and C*) above those of the control fruits at 21 days of storage (L* = 29.36 + 1.36 and C* = 25.77 + 1,32). The HPLC analyses to evaluate anthocyanins of the rind showed that cianidyn-3-rutinoside, the more abundant anthocyanin of litchi pericarp, had a smaller reduction than that registered in control fruits. These data were correlated with total anthocyanins determined by spectrophotometry (R2 = 0.9334). The analyses of phenols showed that Lb. plantarum diminished the oxidation of these compounds, effect that could be related to the observed reduction in the loss of anthocyanins. As a result, the application of Lb. plantarum significantly diminished browning and preserved the red color of litchi pericarp. Therefore, both rambutan and litchi in vivo experiments demonstrated that Lb. plantarum remained viable on fruit surfaces alone or in combination with chitosan. The development of microorganisms was evident by microbiological counts (log CFU values) and the lactic acid production. These effects explained the color preservation of rambutan and litchi fruits. Additionally, Lb. plantaum was able to maintain anthocyanins and phenols on the litchi pericarp.
Los tratamientos de Lb. plantarum solo o combinado con quitosano disminuyeron significativamente la pérdida de peso y color (a* y L*) con respecto a los frutos control. Los sólidos solubles totales (SST) y el pH del jugo de los frutos almacenados a 10 °C, no mostraron diferencias significativas con respecto a los análisis iniciales, pero si presentaron cambios significativos en el almacenamiento a 25 °C, lo que se atribuyó a la sobremaduración y senescencia de los frutos. En contraste a los resultados observados en el experimento in vivo, la viabilidad de las bacterias ácido lácticas en los recubrimientos de quitosano se redujo en un 71.8 % después de 10 días de almacenamiento a 25 °C y 75 % HR. Las determinaciones de ácido láctico el el pericarpio de los frutos mostraron que las bacterias lácticas se encontraban metabólicamente activas a 10 ºC. Para complementar el estudio se investigó el efecto de Lb. plantarum y quitosano, nativo y modificado con glutaraldehido, en la preservación del color y la calidad del Litchi chinensis sonn. La viabilidad obtenida en frutos tratados con Lb. plantarum mostró niveles más altos en comparación con los frutos control, de 5.3 a 4.5 log UFC/g fruto (α < 0.05) durante 21 días de almacenamiento a 10º C y 75 % HR. La aplicación de las bacterias lácticas en el pericarpio del fruto redujo significativamente la pérdida del color (α < 0.05), manteniendo los parámetros de color (L* y C*) en valores mayores a los de los frutos control durante los 21 días de almacenamiento (L*=29.36 + 1.36 y C*=25.77 + 1.32). Los análisis realizados para evaluar la concentración de antocianinas por HPLC, mostraron que la cianidina-3- rutinosido, la antocianina más abundante en el pericarpio de litchi, tuvo una disminución menor a la del fruto control. Estos resultados tuvieron una buena correlación con la concentración total de antocianinas determinada por espectrofotometría (R2 =0.9334). Por otra parte, los análisis de fenoles totales mostraron que Lb. plantarum disminuyó su oxidación, efecto que podría estar relacionado con la reducción de la pérdida de antocianinas. Como resultado de lo anterior, la aplicación de Lb. plantarum disminuyó significativamente el oscurecimiento y preservo el color rojo del pericarpio de litchi. Por lo tanto, en ambos experimentos in vivo en rambután y litchi se demostró que Lb. plantarum permaneció viable en la superficie de los frutos, solo ó en combinación con quitosano. El desarrollo del microorganismo fue evidente por las cuentas microbiológicas (log UFC) y por la producción de ácido láctico. Estos efectos explican la conservación del color en frutos de rambután y litchi. Además, Lb. plantaum fue capaz de mantener la concentración de antocianinas y fenoles en el pericarpio del fruto de litchi.
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