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Ciencia e investigación agraria

versión On-line ISSN 0718-1620

Cienc. Inv. Agr. v.37 n.2 Santiago ago. 2010

http://dx.doi.org/10.4067/S0718-16202010000200006 

Cien. Inv. Agr . 37(2):55-60. 2010
www.rcia.uc.cl

RESEARCH PAPER

 

Production of crystallized fruit from watermelon rind

Elaboración de productos cristalizados a partir de cascaras de sandía

 

Mauro Gontero, Adriano Brandelli, and Caciano Zapata Noreña

Instituto de Ciencia y Tecnología de Alimentos, Universidade Federal do Rio Grande do Sul. Av. Bento Gonçalves 9500, 91591-970 Porto Alegre, Brasil.

Dirección para correspondencia


Abstract. The aim of this work was to produce crystallized fruit from watermelon rind. The following procedure was developed: the outer peel was removed; the material was sliced into 7 mm cubes, blanched for, 0, 5 and 10 minutes, and then treated with 10% sodium chloride solution. This product was treated with solutions of sucrose (30 to 72 °Brix), and dried in a hot air dryer at different temperatures (40, 60 and 80 °C). Products were then analyzed by a sensory panel. The experimental design used was randomized blocks and the results were analyzed by the Tukey's test. The best acceptance of the sensory panel was for the product obtained by 5 minutes blanching followed by drying at 60 °C, whose intensity values for appearance, flavor and gummosis were between 6 and 7.

Key words: osmosis, watermelon, dehydration, agricultural waste.


Resumen

El presente trabajo tuvo como objetivo el aprovechamiento de las cascaras de sandía en la elaboración de productos cristalizados. Para esto fue eliminada la cutícula verde externa de la cascara, cortadas en forma de cubo de aproximadamente 7 mm de lado, escaldado con agua en su temperatura de ebullición utilizando 0, 5 ó 10 minutos y dejados inmersos en una solución de cloruro de sodio. Posteriormente fueron sometidas a concentraciones crecientes de soluciones de sacarosa (30 a 72 °Brix) y deshidratadas en un secador a diferentes temperaturas (40, 60 y 80 °C). Los diferentes tratamientos fueron evaluados por un panel sensorial. El diseño experimental empleado fue el de bloques completos aleatorios y los resultados del análisis sensorial fueron evaluados por la prueba de comparación de medias de Tukey. Fue constatado que el tratamiento con mayor aceptabilidad, por parte del panel sensorial, fue la que había sido sometida a un escaldado de 5 minutos y deshidratado a 60 °C, cuyos valores de intensidad para la apariencia, sabor y gomosidad estuvieron entre 6 y 7.

Palabras clave: Osmosis, sandía, deshidratación, residuo agroindustrial.


Introduction

Commercialization of fruits and derivatives are facing a growing demand due to improvements made in transportation systems, distribution. development of new products and processing methods. Fruits from non tropical zones are characterized by a wide edible portion and moderate amounts of residues. On the other hand. the processing of tropical and subtropical fruits like bananas, watermelons and mangos, generates a wide amountof by-products (Askar, 1998; Schieber et al, 2001). Therefore, it is necessary the development of efficient, economic, as well as environmentally correct technologies, in order to take advantage of the residues generated by these products through the whole production and commercialization chain.

A high percentage of food is currently lost due to the lack or misuse of conservation technologies and post-harvest storage, as well as damages caused during handling and transportation and rejection of products by the markel, because they do nol fulfil qualily patlerns. In order lo face this challenge, the process of osmotic dehydration is being used because it is a method allowing increasing the storage life of these products. The principie of conservation consists on reducing the water activity, which allows diminishing the speed of the chemical, biochemical and microbiological reactions responsible of food decay (Berbari et al, 1992). Rastogi et al. (1997) define osmotic dehydration as a process of pardal removal of free water of some food. like fruits and vegetables, when they are imersed in a hypertonic solution.

In recent years, these products have been appreciated by consumers, because they preserve their nutritive properties and are minimally processed (Torreggiani and Bertolo, 2001). Park et al. (2003) mention that this method has advantages in quality (color, flavor, texture) and for diminishing the costs in energy, distribution, packaging and storage.

Among the different products that may be obtained by osmotic processing are crystallized fruits, where there is a simultaneous water elimination and inclusion of sugars (Berbari et al, 1992). Subsequently, the product is subject to hot air drying (Southgate, 1992). Soler et al. (1995) classified fruits dehydrated osmotically by sugars like crystallized sugars, when they are not covered by a layer of sugar and glazed crystals, even when they were covered by a layer of an oversaturated sugar solution.

Alvarez et al. (1995) studied the effect of blanching and glucose concentration on the speed of air drying at 55°C in strawberries, observing that the coefficients of water diffusion were strongly affectedby blanching. Nieto et al. (2001) evaluated the previous effect of blanching and osmotic dehydration by glucose, made at atmospheric pressure and vacuum, on an air drying process at 60°C, in mango. They found that the values of water diffusion decreased and, there was a higher addition of glucose and modifications in cell starch and mucilaginous substances as a consequence of blanching. Sacchetti et al. (2001) report on sodium chloride that this salt is used due to the possibility of increasing the speed of the process of osmotic dehydration in apples, using sacarose solutions, without affecting their sensorial characteristics.

The objective of this work was the elaboration of crystallized products from watermelon rinds, evaluating the effect of blanching and the tem-perature of drying air on the sensorial characteristics of the final product.

Materials and methods

The watermelons (Citrullus vulgaris Schrad) were purchased in the Central Market of Porto Alegre, Rio Grande do Sul, Brazil. The initial moisture of the watermelon was 95,76%. After being washed and dried, the watermelons were stored at a temperature of 20±2 °C.

The watermelons were subject to the procedure of selection, wash and rind separation from the edible part and seeds shown in Figure 1. Then, the external green cuticle was eliminated from the rind (around 4 mm). The rind was diced in cubes of approximately 7 mm each side, and blanched inboiling water during 0, 5 or 10 minutes. After that, the samples were introduced in a NaCl solution at 10% during 5 hours. Then, the rind cubes were immersed in a sugar solution of 30 °Brix, measured with a table refractometer, remaining in that solution during 24 hours. Subsequently, they were placed in a sugar solution of 40 °Brix, and left to stand for another 24 hours. This procedure was repeated successively (50, 60, 70 °Brix) until the cubes were introduced in a solution of 72 °Brix.


In order to enable the fruit preservation, the solution of 60 °Brix was maintained at pH 4 (measured with pH-meter), for which citric acid was used.

After the process of sugars impregnation, the fruit syrup was retrieved and washed with distilled water and then drained, to eliminate the excess of solution from the surface. Subsequently, the samples were dried in hot air dryer at temperatures of 40, 60 and 80 °C, during 4 hours. Then, the surface of those products was smeared with syrup and covered with a layer of crystallized sugar and left in a oven at 60 °C for an hour. Finally, the cold product was packaged in high density polyethylene bags.

Sensorial analysis

The tests of flavor were made in individual cabins, illuminated with white fluorescent light. The samples were served in white porcelain dishes, coded with random 3-figure-numbers, along with a glass with water at room temperature.

The judges evaluated the samples according to the attributes (descriptive terms), established by the method of discriminating terms list: appearance (samples shine), flavor (associated to the sweet flavor of sacarose), chewiness (how easily the product is ground by the teeth and ready to consume) and gumminess (if the product is attached on the teeth during disintegration).

Eight previously selected evaluators, from 20 to 45 years old, analyzed the intensity perceived in each attribute with the help of a non structured horizontal scale. The evaluation card contained a 9.0 cm scale, between limits of totally like and totally dislike.

Statistical clesign

The experimental planning for the sensorial analysis was made with a completely randomized block design. The treatments evaluated are shown in Table 1. The Tukey comparison of means test was used in order to verify the existence of significant differences between the treatments (with a level of significance of 0.05). For the statistical analysis, the statistical software, SAS was used, version 9.1.3 (SAS Institute).


Results and discussion

The results of the means of the sensorial attributes from each sample, compared by the Tukey's test are in Table 2, with a p ≤ 0.05.


Some significant differences were observed in this work between the results from the comparison of the treatments as a consequence of the statistical tests (p ≤ 0.05). In regard to the ap-pearance, the results indicated that the blanched sample during 5 minutes and dried at 60 °C (E), which presented a higher significant intensity, in regard to the lower intensity of the treatment made with 10 minutes of blanching and drying at 40 °C (C).

In regard to flavor, the treatments A, B and D were equal to each other, but significantly different (p ≤ 0.05) from C and E, which were the samples obtaining the highest intensities (blanched during 10 minutes and dried at 40 °C and blanched of 5 minutes and dried at 60 °C, respectively), which were not different among them. In regard to the samples resulting with lower score in flavor, these were the treatments A and B, both without blanching.

When the chewiness was evaluated, the tests showing the highest intensity were subject to blanching for 10 minutes and drying at 40 °C (C), and the sample with 5 minutes of blanching and drying at 60 °C, without differences among them (p ≤ 0.05). Onthe contrary, the tests which were less intense on chewiness were the samples that were not blanched or subject to 80 °C.

In regard to gumminess, the tests concluded that the treatments A, B and D were statistically different from the treatments C (blanched for 10 minutes and dried at 40 °C) and E (blanching during 5 minutes and drying at 60 °C), with these two last showed a higher intensity. and were significantly equal to each other. On the contrary, the treatments with lower intensity were not subject to blanching or drying temperature of 80 °C (samples A, B and D).

In regard to the last attributes studied (chewiness and gumminess); it may be observed that in the treatments C and E, the panellists associated the easiness how the samples are ground and the absence of adherence in the teeth. Souza et al. (2007) mentioned that texture is one of the most appreciated sensorial qualities by the fruit consumers.

It may be stated from the results obtained that the treatment with higher acceptance by the panellists was the treatment subject to blanching for 5 minutes and drying at 60 °C (E). Blanching, in addition to avoiding the enzymatic darkness (fixing the color and texture), softens the cell tissue, increases the permeability of the cell walls, the dehydration speed and promotes the removal of the mucilaginous substances. Nieto et al. (2001), also state that, due to blanching, there is a diminished diffusivity, attributed to the shrinking tissue, the air expulsión and the alterations of the cell content (starch gelatinization and denaturalization of the mucilaginous substances) and these phenomena prevail on the damage of the cell wall structure and the rupture of the cell membrane that might ease the water transpon.

The salt could also have contributed in the sensorial characteristics. Studying the effect of banana soaked in sodium chloride and sacarose solutions, Ehabe et al. (2006) found that these solutes exert a significant effect on the moisture concentration, sugar and the color index in the final product. The same authors mention that the bananas improved their appearance sensorially, when they were soaked in sodium chloride solutions. Sacchetti et al. (2001) found that the addition of salt in the process of osmotic dehydration in apples increases the loss of water in the product, and that the salt and sugar concentrations have a synergic effect on the impregnation of the soluble solids. Lerici et al. (1985) indicate that the addition of 1% of sodium chloride in the apples osmo-dehydration may help to diminish the sweetness caused by the addition of sugar in the fruit.

The sample C (blanched 10 minutes and dried at 40 °C) had a similar acceptance similar to the sample E, in most of the attributes, except in the appearance. The sensorial profile is shown the Figure 2, where it is also observed that these treatments are also the most accepted by the panellists, supporting the results obtained by the Tukey's test. Morita et al. (2005) obtained a good acceptance inthe crystallized products of melón, in natura, obtained by the slow process of sugars impregnation and further drying at 50 °C.


Conclusion

The treatment more accepted by the sensorial panel due to the highest intensity of the attributes evaluated, was the watermelon rind subject to blanching for 5 minutes and drying at 60 °C during 4 hours. On the contrary, the treatments less accepted by the tasters were not subject to blanching or suffered drying temperatures of 80 °C.

Acknowledgment

Mauro M. Gontero was a interchange student of the ESCALA Program of Asociación de Universidades del Grupo Montevideo.

 

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Received June 26, 2009. Accepted July 31, 2009.

Corresponding author: czapatan@ufrgs.br

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