Aplicación de diferentes pretratamientos termofísicos y biológicos para la mejora de las propiedades antioxidantes de productos en polvo procedentes de tallo de brócoli

Abstract

[EN] Reducing food waste is one of the main objectives of the 2030 Agenda, due not only to the environmental problem represented by this organic waste, but also to the social, economic and health problem generated by the loss of food already produced and that has consumed resources, upset by the challenge of feeding a growing population. The fruit and vegetable sector is one of the sectors that contributes most to food waste. In response to the continuous search for measures to ensure sustainable development, the food industry, and in particular the fruit and vegetable industry, is faced with the challenge of implementing a circular economy system that allows the recovery of waste and its reincorporation into the food chain. One of the approaches that allows an integral use of waste is its transformation into powdered functional ingredients, through a simple process that includes the stages of shredding, dehydration and milling, obtaining stable powdered products rich in bioactive compounds, and contributing to the concept of healthier and more sustainable diets. Pretreatments can be incorporated into this valorization process to bring about structural or biochemical changes that increase the concentration of compounds of interest or improve their characteristics for subsequent drying. In previous studies, physical pretreatments such as the intensity of plant tissue disruption and freeze-thawing have been studied. This work is intended to deepen in the use of biological (fermentation) and thermo-physical (autoclaving, microwaves, ultrasound, pasteurization) pretreatments to know their effect on the residue and the characteristics of the powder obtained. Therefore, the aim of this final degree thesis was to study the impact of different pretreatments and dehydration techniques on the physicochemical and antioxidant properties of powdered products obtained from broccoli stems. For this purpose, a preliminary study was carried out in bench dryers, after which the temperature and drying time that resulted in stable powders with better antioxidant properties were defined. On the other hand, different levels of microwave, ultrasound and fermentation pretreatment variables were tested to select the conditions at which these pretreatments would be applied. As such, the impact of pretreatment on the physicochemical properties of the pretreated broccoli residue (moisture, water activity, total soluble solids), including antioxidant properties (total phenols, total flavonoids, and antioxidant capacity by ABTS and DPPH methods) was studied; as well as the impact on the plant tissue structure by low temperature scanning electron microscopy (Cryo-SEM). Once the pretreatment conditions were selected, the powdered products were obtained from broccoli residue subjected to the different thermophysical and biological pretreatments and subsequent freeze-drying or hot air drying in a pilot plant dryer. The impact of processing on the physicochemical properties (xw, aw and xss), including antioxidant properties (total phenols, total flavonoids, and antioxidant capacity by ABTS and DPPH free radical methods), of the powdered products obtained was then analyzed. Cell viability after fermentation and drying was also determined on the fermented residues. After the preliminary study, a higher amount of antioxidant compounds was found in the powders obtained by hot air drying compared to the freeze-dried ones, especially after drying at 60 °C for 10 h. When broccoli residues were subjected to the different pretreatments, an improvement in all antioxidant properties was observed compared to fresh stems. This increase was particularly pronounced in the residues subjected to ultrasound, which was supported by microscopic observations. When dehydrating the pretreated residues, a considerable increase in antioxidant properties was observed after air drying at 60 °C. Of all the pretreatments applied, sonication resulted in powders with better antioxidant properties than the non-pretreated powders, while the other pretreatments failed to improve the antioxidant properties of the powdered products. As for obtaining a powder with probiotic properties, only powders obtained by freeze-drying the fermented residues presented counts close to 107 CFU/g. Thus, it is recommended to continue investigating the hot air drying stage to ensure that the resulting powder retains its probiotic character. In conclusion, the present work has shown that the applied pretreatments can improve the antioxidant properties of broccoli residue, but this improvement does not translate into powdered products with better characteristics, based on the antioxidant properties tested. The present work is related with the SDOs 2, 3 and 12.

[ES] El consumo de brócoli ha experimentado un aumento significativo en los últimos años debido a su bajo contenido calórico y su alto valor nutricional. Este último se deriva de su abundancia en fibra, potasio, folatos, y vitaminas C y K1, así como de la presencia de glucosinolatos e indoles, los cuales se acumulan especialmente en la inflorescencia o parte comestible de la planta. El tallo, que representa el 38% del peso total de la planta, es habitualmente descartado por su textura más firme, sabor menos intenso y escaso atractivo visual, lo que representa un serio problema medioambiental. Su transformación mediante técnicas sencillas de trituración y deshidratación permite obtener en un ingrediente en polvo estable y altamente concentrado en nutrientes, que podría ser utilizado en la formulación de alimentos para mejorar su perfil nutricional, contribuyendo de esta manera a la economía circular y promoviendo el consumo dietas saludables y sostenibles. Sin embargo, es de esperar que la aplicación de otros tratamientos previos al secado del tallo de brócoli contribuya a la mejora de las propiedades funcionales de dichos polvos.

Es por esto que el presente estudio se plantea con el objetivo principal de evaluar el impacto que la fermentación con Lactobacillus plantarum (FERM), la pasteurización (PAST), el autoclavado (AUTO), la aplicación de ondas microondas (MW) o el tratamiento con ultrasonidos (US) antes de la deshidratación ejercen sobre las propiedades fisicoquímicas (humedad, actividad del agua y contenido en sólidos solubles totales) y antioxidantes (contenido en fenoles y flavonoides totales, capacidad antioxidante por los métodos de los radicales DPPH y ABTS y contenido en compuestos específicos mediante HPLC) de polvos procedentes de tallo de brócoli. La temperatura del aire en el secador convectivo se fijará en 50, 60 ó 70 °C según un estudio previo a escala de laboratorio, de manera que la supervivencia del lactobacilo empleado en la fermentación resulte máxima. Como alternativa al secado con aire caliente también se obtendrán polvos mediante liofilización.