665 Research Article Received: 24 August 2008 Revised: 1 November 2008 Accepted: 1 December 2008 Published online in Wiley Interscience: 15 January 2009 (www.interscience.wiley.com) DOI 10.1002/jsfa.3498 Effect of ultrasound-assisted osmotic dehydration on cell structure of sapotas Sueli Rodrigues, a Maria CF Gomes, b Maria I Gall ˜ ao b and Fabiano AN Fernandes c * Abstract BACKGROUND: Drying is a traditional way of fruit preservation. Because of the high energy costs associated with air-drying, osmotic dehydration is often applied as a pretreatment to reduce air-drying time. Ultrasound is an emerging technology with several applications in food processing. The effect of ultrasound on fruit tissue depends on the tissue structure and composition, and ultrasound might be beneficial to improve air-drying efficiency, with consequent reduction in process costs. In this study the effect of ultrasound and ultrasound-assisted osmotic dehydration on sapota tissue structure was evaluated. RESULTS: Ultrasound induced cell disruption and breakdown of cells with high phenolic content (dense cells) and also induced elongation of parenchyma cells. Ultrasound application combined with high osmotic gradient enhanced water loss and solid gain because of the formation of microscopic channels. Ultrasound-assisted osmotic dehydration induced gradual distortion of the shape of cells, cell breakdown and formation of microscopic channels. Micrographs of the fruit tissue showed that ultrasound preferentially affected dense cells. CONCLUSION: Ultrasonic pretreatment was able to preserve the tissue structure of the fruit when distilled water was used as liquid medium. The application of ultrasound-assisted osmotic dehydration resulted in severe changes in the tissue structure of the fruit, with consequent increase in the effective water diffusivity, because of the formation of microscopic channels and cell rupture. c  2009 Society of Chemical Industry Keywords: sapota; image analysis; ultrasound; osmotic dehydration; drying INTRODUCTION Sapota (Achras sapota L.) is native to Central and South America, specifically from the Yucatan Peninsula of Mexico to Costa Rica, where the largest population of native trees still exists. Sapotas have become widespread throughout tropical regions of the world, including Central and South America, the West Indies, India and Florida in the USA. Sapota is grown on a commercial basis in India, The Philippines, Sri Lanka, Malaysia, Mexico, Venezuela, Guatemala and other Central and South American countries such as Brazil. The fruit can be nearly round, oblate, oval or ellipsoidal and varies from 5 to 10 cm in width. When immature, it is hard, gummy and very astringent. Although smooth-skinned, it is coated with sandy brown scurf until fully ripe. The flesh ranges in colour from yellowish to light or dark brown, may be coarse and somewhat grainy and becomes soft and very juicy, with a sweet flavour resembling that of pear. Some fruits are seedless, but regularly there may be 3 – 12 seeds which are easily removed. 1,2 Osmotic dehydration is the commonest pretreatment used before air-drying. The technique consists in immersing the fruit in a hypertonic solution to remove part of the water from the fruit. The driving force for water removal is the difference in osmotic pressure between the fruit and the hypertonic solution. The complex cellular structure of the fruit acts as a semi-permeable membrane creating extra resistance to diffusion of water within the fruit. 3 Application of osmotic dehydration changes the fruit texture, 4–7 especially because of pectin dissolution and breakage of tissue cell. This effect has been demonstrated for strawberries after long exposure of the fruit to the osmotic solution. 8 Ultrasonic waves can cause a rapid series of alternate com- pressions and expansions, in a similar way to a sponge when it is squeezed and released repeatedly (sponge effect). The forces involved in this mechanism can create microscopic channels that may ease moisture removal. In addition, ultrasound produces cavitation, which may be helpful to remove strongly attached moisture. The sponge effect caused by ultrasound application may be responsible for the creation of microscopic channels in porous materials such as fruits. 9–11 These microscopic channels were first reported by Fernandes et al. 12 in micrographs of melon tissue after ultrasound application. ∗ Correspondence to: Fabiano AN Fernandes, Departamento de Engenharia Qu´ ımica, Universidade Federal do Ceara, Campus do Pici, Bloco 709, 60455-760 Fortaleza, CE, Brazil. E-mail: fabiano@ufc.br a Departamento de Tecnologia dos Alimentos, Universidade Federal do Ceara, Campus do Pici, Bloco 858, 60021-970 Fortaleza, CE, Brazil b Departamento de Biologia, Universidade Federal do Ceara, Campus do Pici, Bloco 906, 60451-490 Fortaleza, CE, Brazil c Departamento de Engenharia Qu´ ımica, Universidade Federal do Ceara, Campus do Pici, Bloco 709, 60455-760 Fortaleza, CE, Brazil J Sci Food Agric 2009; 89: 665–670 www.soci.org c  2009 Society of Chemical Industry