Effect of pulsed vacuum and calcium lactate on the structure of red guavas osmotically dehydrated G. S. Vieira a , L. M. Pereira a , S. M Carmello-Guerreiro b , M. D. Hubinger a* a Department of Food Engineering, Faculty of Food Engineering, State University of Campinas, Brazil b Department of Botany, Biology Institute, State University of Campinas, Brazil * (mhub@fea.unicamp.br) ABSTRACT The effect of osmotic dehydration with vacuum pulse application and calcium lactate addition on microscopic features and mechanical properties of guavas was studied. The osmotic process was carried out at atmospheric pressure or with vacuum pulse of 100 mbar for 20 min at the beginning of the process, followed by the osmotic process at atmospheric pressure for 3 hours. Calcium lactate was added to the sucrose solution (60°Brix and 32°C) at 0.6 and 1.2% (w/w). Texture properties (stress and strain at failure) and the structure of guavas observed by light microscopy were evaluated. Through the micrographs of guavas tissue, image analysis was used to quantify the cellular plasmolysis. Severe cellular damage caused by the osmotic dehydration process at atmospheric pressure and the structural maintenance of fruit tissue due to the vacuum pulse application at the beginning of osmotic process were observed. Calcium lactate addition on osmotic solution was also able to promote the structural maintenance of guavas osmotically dehydrated at atmospheric pressure, showing turgid cells with well-defined cellular contours and minimizing the cell plasmolysis. For guavas osmotically dehydrated with vacuum pulse application, the calcium salt promoted the strengthening of cell wall and caused an increase in guavas hardness and resistance to deformation. Keywords: Osmotic dehydration, vacuum pulse, calcium lactate, light microscopy, plasmolysis. INTRODUCTION Osmotic dehydration process is considered an important tool for tropical fruits conservation and development of new fruit products, producing fruits with an extended shelf-life, sensorial characteristics similar to fresh products and a high nutritive value [1-2]. However, structural changes, mainly in product texture, are frequently observed as a consequence of osmotic treatment, which can impair the consumer acceptance. During the osmotic dehydration process, the cells lose water, causing plasmolysis and loss of turgor pressure, shrinkage and deformation of cells (cell wall and plasma membrane). The detachment of the middle lamella is also an important cellular change, which can affect the mechanical behavior of plant tissues [3-4]. This process can be performed at atmospheric pressure (OD) or with vacuum pulse application (PVOD) for a small period at the beginning of the process. The solid-liquid interface area and the mass transfer between both phases increase with vacuum pulse application [5]. Moreover, a structural preservation of processed products is observed with the application of vacuum pulse on osmotic process [6]. The addition of calcium salts on osmotic solution can also be an alternative to avoid or reduce the structural changes. Calcium ions interact with pectin present in the cell wall of plant tissues, preserving the structural integrity of fruit and vegetable products [7-8]. This interaction calcium - pectin provides rigidity to the cell wall, improving the mechanical properties and resulting in an osmotically dehydrated product with better texture characteristics [2, 9]. Thus, the objective of this work was to evaluate the effect of osmotic dehydration with vacuum pulse application and calcium lactate addition on microscopic features and mechanical properties of guavas. MATERIALS & METHODS Materials Red guavas (Psidium guajava L.) of the “Chinese” cultivar, supplied by Torrezin Ind. e Com. Polpas e Frutas Congeladas (Valinhos, SP, Brazil), were used in the study. The fruits were selected in order to obtain uniform samples according to its shape, size (100.10 ± 3.65 mm in length, 85.49 ± 2.75 mm in diameter and pericarp thickness of 12.63 ± 1.55) and ripening grade (9°Brix and 70% of skin yellowness).