Ultrasound application as pre-treatment for drying of fruits

Transcription

1 Ultrasound application as pre-treatment for drying of fruits Fabiano A. N. Fernandes a and Sueli Rodrigues b a Universidade Federal do Ceará, Departamento de Engenharia Química, Campus do Pici, Bloco 709, Fortaleza CE, BRAZIL. fabiano@ufc.br b Universidade Federal do Ceará, Departamento de Tecnologia de Alimentos, Campus do Pici, Bloco 858, Fortaleza CE, BRAZIL. sueli@ufc.br ABSTRACT Drying is one of the processes that present higher costs in food processing. Pre-treatments are often used to reduce drying time and consequently its cost. In the past years, our group has studied the use of ultrasonic pre-treatments to induce structural changes in the fruit tissue as to improve the effective water diffusivity during air-drying and consequently reduce air-drying time. This study has evaluated the effect of ultrasound pre-treatment on several fruits: banana, papaya, melon, strawberry, sapota, malay apples, star fruit and pineapples. The influence of ultrasound on water loss, sugar gain, effective water diffusivity, drying time and tissue structure modification was evaluated and analyzed. Ultrasound pre-treatment consists in immersing fruit pieces in water or in an osmotic solution and to subject the fruit and solution to ultrasonic waves (at frequencies ranging from 18 to 40 khz) for a period of time (usually less than 60 min). The ultrasonic waves produce a sponge effect which compresses and releases the fruit cells inducing in the appearance of microscopic channels within the tissue structure of the fruit. These microscopic channels are responsible for the increase in the effective water diffusivity in the fruit during air-drying processes. Our studies showed significant differences on the results for different fruits. Ultrasound showed to have higher influence on fruits with high water content (pineapples, melons, malay apples) and high content of fibers and phenolic cells (sapotas). Low influence was observed in very porous fruits (strawberries) and on dense fruits with strong cell attachment (papaya). These observations are in accordance to the modifications observed in the fruit tissue structure by means of photo-micrography, which show different structures and microscopic channels lengths and concentration. Overall, the ultrasound pre-treatment was able to reduce drying time by 20% and to reduce drying costs in up to 30%. Keywords: fruit; drying; ultrasound INTRODUCTION Drying is the most common method of food preservation. Air-drying is generally carried out at mild temperatures (between 40ºC to 70ºC) to reduce the degradation of the fruit. The process takes from 8 to 24 hours depending on the fruit that is being processed, on its initial moisture content, on the desired final moisture content and on the temperature used in the process. Temperature is maintained by direct heating of the air passing through the samples making the air-drying process heat intensive and expensive. A pretreatment can be employed to reduce air-drying time. One of pre-treatment that can be used to reduce air-drying time is ultrasound application. Power ultrasound can produce chemical, mechanical or physical changes on the processes or products where it is applied [1]. When low frequency power ultrasound is applied, ultrasonic waves travel through the solid medium causing a rapid series of alternative compressions and expansions, in a similar way to a sponge when it is squeezed and released repeatedly (sponge effect) [2]. The sonication also causes cavitation in a liquid medium, which consists in the formation of bubbles in the liquid that can explosively collapse and generate localized pressure and temperature increase [3-4]. The rate of cavitation or alternate compressions and expansions depend on the frequency of the ultrasonic waves. The mechanical and physical effects of acoustic waves can be used to enhance many processes where mass transfer takes place, drying included. The forces involved by the sponge effect caused by ultrasonic waves can create microscopic channels that may ease moisture removal. These microscopic channels can be used by

2 water molecules as a preferential pathway to diffuse toward the surface of the fruit increasing its effective water diffusivity. The ultrasonic waves also reduce the diffusion boundary layer and increase the convective mass transfer in the sample [2,5-6]. In addition, the cavitations produced by ultrasound are beneficial for the removal of moisture strongly attached. In this paper, an overview of the effects of ultrasound application in the dehydration of eight fruits (banana, genipap, malay apple, melon, papaya, pineapple, pinha and sapota) is presented. MATERIALS & METHODS The ultrasound pre-treatment procedure consists in the immersion of the fruit in water to which ultrasound is applied. The pre-treatment can be carried out at ambient temperature reducing the probability of food degradation [1]. Fruit samples were cut to obtain cubes, cylinders or triangular shaped slices, depending on the fruit that was studied. Moisture content was determined by heating in a drying oven at 105 o C for 48h, according to AOAC method. The initial soluble solids content of the fruit (ºBrix) was determined by refractometry. The samples were immersed in distilled water and were subjected to ultrasonic waves during a period of 10 to 45 minutes. The process was carried out placing the samples in an ultrasonic bath. Ultrasound should be applied for at least 10 minutes, because the effect of ultrasound showed to be insignificant at lower times. The changes caused by ultrasonic waves on the effective water diffusivity become slight after 20 minutes and after minutes the changes are insignificant [7-8]. The water to fruit ratio was maintained between 3:1 and 4:1 (weight basis). The pre-treatment was carried out under ambient temperature, because higher temperatures did not show to enhance the effects of ultrasound application [4]. The frequency of the ultrasonic waves was 25 khz and the intensity was 4000 W/m 2. After the pre-treatment, the fruit was dried in a forced circulation air-drying oven. Air temperature in the oven was set at 60ºC. Cross-flow air moved from side to side of the dryer at 0.5 m/s, flowing parallel to the width of the dryer shelves. RESULTS & DISCUSSION Each fruit presented a different behavior when ultrasound was applied. Melons and pineapples lost water to the liquid medium during the pre-treatment. Papayas, sapotas, genipaps and bananas gained water (Table 1). Table 1. Water loss and sugar gain of fruits subjected for 30 minutes to ultrasound pre-treatment. Fruit Banana (Musa spp.) Genipap (Genipa americana L.) Malay apple (Syzygium malaccense L.) Melon (Cucumis melo L.) Papaya (Carica papaya L.) Pineapple (Ananas comosus L.) Pinha (Annona squamosa L.) Sapota (Achras sapota L.) Strawberry (Fragaria vesca L.) Sugar Gain [%] Water Loss [%] The concentration gradient of soluble solids between the fruit and the liquid medium should result in the gain of water by the fruit during the pre-treatment. Most fruit presented such behavior, except for melons and pineapples. These latter fruits present high moisture content (90% for melons and 83% for pineapples) and were significantly affected by the sponge effect of ultrasonic waves. The gain of water was not significant and, except for genipaps, it was lower than 10%. Sugar gain was negative for all fruits indicating that the fruit lost soluble solids to the liquid medium. This result was expected because of the concentration gradient of soluble solids between the fruit and the liquid medium that favors the mass transfer of soluble solids from the fruit to the liquid medium.

3 The loss of soluble solids for each fruit was very different, ranging from 1.5% to 52.9%. Fruits with high initial moisture content lost more soluble solids to the liquid medium than fruits with low initial moisture content. Fruits with high initial moisture content may ease the diffusion of soluble solids toward the liquid medium. The exception was pinha that rapidly lost soluble solids to the liquid medium because of the low cohesion between the cells in its tissue structure. The solid loss observed for sapotas was lower than the solid loss observed for other fruits, which may be explained by the high starch content of sapotas (160 ± 50 g kg-1, dry basis) that create an extra resistance for solid loss. The ultrasonic treatment showed to be an interesting process to produce dried fruits with low sugar content, because during ultrasound pre-treatment the fruit lost sugar, whereas under osmotic dehydration the uptake of sugar is very high. The loss of soluble solids was also influence by the effects of ultrasound on the tissue structure of the fruit. Ultrasonic waves cause a rapid series of alternative compressions and expansions, in a similar way to a sponge when it is squeezed and released repeatedly (sponge effect). The sponge effect results in the creation of microscopic channels in the fruit tissue. The tissue structure of melons was very sensitive to ultrasound application and several microscopic channels appeared in the tissue structure [9]. These microscopic channels ease the diffusion of water and soluble solids in and out of the fruit. The number, length and width of the microscopic channels affect the mass transfer between the fruit and the liquid medium. Short microscopic channels were formed in papayas. Consequently, the overall mass transfer did not increase and only a small quantity of soluble solids (1.5%) was transferred to the liquid medium (Figure 1). Figure 1. Photomicrographs of pineapples (A) and papaya (B) cubes subjected to ultrasound pre-treatment. Arrows indicate the cell interspaces and microscopic channels. Magnification of 380x. Few microscopic channels were formed in pineapples and strawberries, but they were long, increasing the overall mass transfer. Consequently, the sugar loss in pineapples and strawberries were high (18.9% and 9.5% respectively) (Figures 1 and 2). The microscopic image analysis of fresh sapota presents two types of cells: parenchyma cells and dense cells (Figure 3). The parenchyma cells (white cells in Figure 3) comprise mostly round shaped cells. Only few parenchyma cells are slightly distorted. The tissue presents few small cell interspaces, mostly near the junction between three cells. Dense cells (black cells in Figure 3) are found near one another and contain high concentration of phenolic compounds. Long microscopic channels were formed by breakdown of dense cells. The breakdown of cells creates large regions where no cell membrane was observed. The breakdown of dense cells was more intense than when distilled water is used as liquid medium.

4 Figure 2. Photomicrographs of strawberry samples after 30 min of ultrasound pre-treatment: (A) micro-channel formation and (B) micro-channels region with disrupted cells (380X). Figure 3. Photomicrographs of sapota samples after 30 min of ultrasound pre-treatement: (A) fresh sapota and (B) microchannels region with disrupted dense cells (380X). Microscopic channels were formed in all fruits during ultrasound application. The time required to form long channels, which increases the mass transfer rate, depended on the fruit. Microscopic channels were formed by elongation of cells in melons, which was a fast process. In other fruits, like in pineapples, microscopic channels were formed by disruption of cells, which required more time to occur. In the latter case, an increase in sugar loss was observed after a certain time (20 minutes for pineapples) as shown in Figure 4. The effect of the ultrasonic pre-treatment was mainly observed during the air-drying stage where a significant increase in effective water diffusivity was observed. Effective water diffusivity was affected by ultrasound application in some fruits (Table 2). The ultrasonic pre-treatment had a positive effect on drying because the increase in the effective water diffusivity resulted in shorter air-drying time if compared to the fresh fruit with no pre-treatment.

5 Figure 4. Evolution of sugar loss during ultrasound application using distilled water as liquid medium. Table 2. Effective water diffusivity of fruits submitted to ultrasound pre-treatment. Fruit Banana (Musa spp.) Genipap (Genipa americana L.) Malay apple (Syzygium malaccense L.) Melon (Cucumis melo L.) Papaya (Carica papaya L.) Pineapple (Ananas comosus L.) Pinha (Annona squamosa L.) Sapota (Achras sapota L.) Strawberry (Fragaria vesca L.) Effective Diffusivity of the fresh fruit [m 2 /s] Effective Diffusivity of the fruit treated in ultrasound for 30 minutes [m 2 /s] The effective water diffusivity during the air-drying process increased after the bananas where pre-treated for 20 minutes under ultrasonic waves ( m 2.s -1 ). Pre-treating bananas using osmotic dehydration resulted in an effective water diffusivity of m 2.s -1. Both values were higher than the diffusivity obtained for the fresh fruit ( m 2.s -1 ) during air-drying [10]. The effective water diffusivity after application of ultrasound was 14.8% higher than the diffusivity of the fresh fruit and 7.3% higher than the diffusivity of the osmo-dehydrated bananas. The increase observed in the effective water diffusivity for bananas was lower than for melons. CONCLUSION Fruits pre-treated with ultrasonic waves have presented significant loss of sugars when the process was carried out using distilled water as the liquid medium. This process can be applied to produce dried fruits with low sugar content, which might be used in the production of foodstuffs with reduced calories. The application of the ultrasound pre-treatment increased the water diffusivity of the fruit, in most cases. This phenomenon may occur because of the formation of micro-channels during the application of ultrasound. The increase in the effective water diffusivity at the air-drying stage makes the use of ultrasound an interesting technique that can be used complementary to classical air-drying. ACKNOWLEDGEMENT The authors thank the financial support of the Brazilian funding institute CNPq.

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