Efecto de los tratamientos de gel de aloe, aplicados en pre- o post-recolección sobre la calidad de frutos de hueso y uva mesa

Abstract

Durante siglos, el Aloe vera se ha usado por sus propiedades medicinales y terapéuticas y en general las plantas de Aloe sp son catalogadas como medicinales por sus efectos antiinflamatorios, hipoglucémicos, protectores gástricos y aliviadores de quemaduras y heridas. Por otra parte, está bien constatado el papel del aloe inhibiendo la proliferación de bacterias, tanto Gram positivas como Gram negativas, así como mejorando enfermedades provocadas por diferentes especies de hongos. La comercialización en fresco de productos hortofrutícolas es una actividad económica muy significativa en España, como resultado del desarrollo de una agricultura moderna, que incorpora nuevas tecnologías, nuevos productos y variedades. La aparición de podredumbres ocasionadas por hongos es una de las principales causas de deterioro de los frutos durante su post-recolección. Para combatir las enfermedades existen diferentes fungicidas, si bien su uso se está eliminando o restringiendo, por lo que los agricultores y productores necesitan de herramientas eficaces capaces de controlar estas enfermedades. Con el fin de aportar soluciones a este problema, en esta Tesis se plantea realizar una serie de investigaciones, estructuradas en 5 capítulos, encaminadas a la evaluación del efecto de los tratamientos con geles de aloe sobre el control de podredumbres causadas por hongos filamentosos, así como su efecto sobre la calidad y fisiología de los frutos. De este modo, se pretende establecer si estos geles se podrían aplicar, bien en pre-cosecha o en post-recolección, en frutos con elevado interés comercial como son ciruela, cereza, melocotón, nectarina y uva de mesa, para reducir la incidencia de podredumbres y mantener su calidad durante un periodo de conservación más prolongado. En el primer capítulo, primero se estudia el efecto in-vitro de diferentes concentraciones de gel de Aloe vera sobre la inhibición del crecimiento del micelio de dos de los hongos más comunes responsables de las podredumbres de la fruta, como son Penicillium digitatum y Botrytis cinerea. A continuación se evalúa el efecto antifúngico en post-cosecha del gel de A. vera (con la adición o no de timol) sobre dos variedades de nectarinas infectadas con B. cinerea, P. digitatum y R.striata Haw., y Aloe vera L.) recolectadas en 3 estaciones del año. Además, también se evalúa la actividad antifúngica de las hojas de las 8 especies contra los hongos patógenos más comunes de la fruta (Botrytis cinerea, Penicillium digitatum, Penicillium expansum y Penicillium italicum). Los resultados muestran que existen diferencias significativas en las propiedades y la composición química del gel obtenido de las hojas según la especie de Aloe y del momento en el que se realice la recolección. La aparición de podredumbres sobre las hojas es diferente entre las distintas especies de Aloe, siendo las hojas de A. ferox, A. mitriformis y A. saponaria en las que menos crecieron los hongos inoculados artificialmente. El porcentaje de heridas infectadas por los diferentes hongos en las hojas de las diferentes especies de Aloe se correlacionó negativamente con la concentración de aloína de sus geles, lo que indica que la aloína puede ser uno de los componentes más importantes del gel de aloe más importante relacionado con la actividad antifúngica. Así pues, otras especies de Aloe, como A. ferox, A. mitriformis y A. saponaria, podrían usarse, junto con A. vera, para la fabricación de geles que podrían usarse comercialmente como tratamientos pre- o post-cosecha para reducir la carga microbiana, retrasar los procesos de maduración y mantener la calidad de la uva de mesa y los frutos de hueso.During centuries, Aloe vera plant has been used due to its medicinal and therapeutic properties and in general, Aloe sp. are categorised as medicinal plants by their effects as anti-inflammatory, hypoglycaemic, gastrointestinal protector and wound healing. On the other hand, it is well known the Aloe vera role on inhibiting bacterial proliferation, both gram positive and gram negative, as well as improving several diseases caused by fungi species. The commercialisation of horticultural fresh produce is an important economic activity in Spain, as a result of the development of a modern agriculture by incorporating new technologies, new products and cultivars. Occurrence of fungal diseases caused by different fungi species is one of the main causes of fruit deterioration during postharvest storage. There are different commercial fungicides to counteract these diseases, although their use are being controlled and restricted, and thus farmers and producers are needed of some efficient tools able for controlling the postharvest diseases. In the present PhD Thesis, structured in 5 chapters and with the aim to have available new solutions to this problem, several investigations have been carried out to evaluate the effect of treatments with aloe gels on controlling decay caused by mycelia fungi as well as their effects on fruit quality and physiology. Accordingly, the possible use of aloe gels, applied either at pre- or postharvest, on fruit commodities with great commercial interest such as plum, sweet cherry, peach, nectarine and table grape were studied. In the first chapter, the in vitro effect of several Aloe vera gel concentrations on inhibiting the mycelium growth of the two most common fungi responsible for fruit decay, such as Penicillium digitatum and Botrytis cinerea, will be firstly assayed. Secondly, the postharvest antifungal effect of Aloe vera gel (alone or with the addition of thymol) on two nectarine cultivars artificially infected with B. cinerea, P. digitatum and Rhizopus stolonifer will be analysed. Results from this chapter permit to conclude that Aloe vera gel was able to inhibit the in vitro and ex vivo growth of these common fungi during postharvest storage and also to delay some specific ripening processes of these stone fruits.In the second chapter, it is shown that application of Aloe vera gel as preharvest treatment on sweet cherries, peaches and nectarines reduced the microbial load of these fruits at time of harvesting without affecting the on-tree ripening process or quality attributes, and thus would reduce decay occurrence of stone fruits during postharvest storage without modifying the organoleptic quality. In the third chapter, pre-harvest treatments with Aloe vera gel (commercial or home-made) and their effects on quality attributes, microbial population and decay occurrence on table grape at time of harvesting and after postharvest storage were studied. Treatments were carried out on ‘Autumn Royal’ and ‘Crimson’ table grape cultivars grown under conventional or organic-farming, respectively. Results from these chapter demonstrated that Aloe vera gel could be used before harvesting as a treatment to inhibit microbial deterioration and to reduce decay occurrence during postharvest storage of table grape, with an additional on delaying the fruit ripening process. In the fourth chapter, the effect of post-harvest treatments with Aloe vera or Aloe arborescens gels on different parameters related to quality of ‘Santa Rosa’ plum and ‘Redheaven’ peach was assayed. It is shown that gels both aloe species could be applied as postharvest treatment to delay the ripening process, since a significant reduction in ethylene production rate, weight loss, colour changes and ripening index were obtained in treated fruit compared with untreated ones. In the fifth chapter, the leaves and gels of 8 different Aloe sp. harvested at 3 seasons along the year were characterised (Aloe arborescens Mill., Aloe aristata Haw, Aloe claviflora Strydenburg, Aloe ferox Mill., Aloe mitriformis Mill., Aloe saponaria Ait., Aloe striata Haw., and Aloe vera L.). Additionally, the antifungal activity of the leaves from the 8 Aloe sp. against the most common fungi attacking fruit commodities (B. cinerea, P. digitatum, P. expansum and P. italicum) was studied. Results showed that significant differences existed on gel properties and chemical composition depending on the Aloe sp. leaf used and the harvesting season. Leaf decay occurrence and severity was different depending on the aloe sp., the A. ferox, A. mitriformis and A. saponaria being the leaves with reduced fungal growth after artificial fungi inoculation. The percentage of damaged injuries of the different Aloe sp. was negatively correlated with aloin concentration in the gel, which would indicate that aloin could be the most important component of the Aloe gels related to antifungal activity. Thus, other Aloe sp. such as A. ferox, A. mitriformis and A. saponaria could be used, toghether with A. vera to made gel with potential application as pre- or post-harvest treatments to table grapes and stone fruits to decrease fruit decay, delay de ripening process and maintain fruit quality.