LWT 38 (2005) 379–386 Use of dyed solutions to visualize different aspects of vacuum impregnation of Minas cheese L.C. Hofmeister, J.A.R. Souza, J.B. Laurindo à Universidade Federal de Santa Catarina, Departamento de Engenharia Quı´mica e Engenharia de Alimentos, Caixa Postal 476, 88040-900 Floriano ´polis, SC, Brazil Received 15 November 2003; accepted 3 May 2004 Abstract The process of salting Minas cheese (a Brazilian cheese) was studied using salt solutions dyed with methylene blue, in order to visualize different aspects of the salting. Vacuum impregnation experiments were carried out in a hermetic chamber, where cheese samples were immersed in the salt solutions, after the manufacturing pressing stage, under absolute pressure ranging from 16 to 21.3 kPa. Impregnation experiments were carried out under continuous and intermittent vacuum conditions. After 1, 2 or 3 h, the chamber was opened, the impregnated cheese was withdrawn and cut, and the sections were analysed with respect to salt concentration and photographed. The pictures indicated that solute impregnation increased with the vacuum intensity and with the impregnation time. Solute penetration decreased with the force applied during the pressing step and as the cheese aged. These results could be associated with the decrease in the average size and number of the interconnected pores. The experiments showed that intermittent vacuum salting promoted a greater cheese impregnation, which can be explained by the action of deformation–relaxa- tion mechanisms. Despite their qualitative nature, dyed solutions can be useful to study impregnation of porous foods. The experimental results for salt concentration showed that vacuum salting of Minas cheese is a simple and effective procedure for obtaining products with more homogeneous salt concentration. r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Cheese; Salting; Vacuum; Time; Pictures 1. Introduction The majority of foodstuffs are considered as porous media, where the porosity, the connectivity and the average dimension of the pores play an important role in heat and mass transfer during food processing. In the Unit Operations used by the food industry to promote solid–liquid contact, impregnation has an essential role in its efficiency (Andre´s, Fito, & Chiralt, 1995). The processes used to extract oils and other solutes from solids, the osmotic dehydration and the salting of cheeses are examples where the degree of impregnation is decisive. In the osmotic dehydration of foods two flows occur simultaneously, in counter-current: the output of water (carrying some solutes) from the solid and the input of the impregnation solution (Andre´s et al., 1995; Salva- tori, Andre´s, Chiralt, & Fito, 1999; Azoubel & Murr, 2004). Salting of cheeses and meats and the osmotic dehydration of fruits are processes controlled by diffusion, which require long contact times between the liquid and solid phases (Shi & Fito, 1994; Guamis et al., 1997). Vacuum impregnation has been used as an alternative to osmotic impregnation, with the aim of increasing the impregnation efficiency and saving time (Fito & Pastor, ARTICLE IN PRESS www.elsevier.com/locate/lwt 0023-6438/$30.00 r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2004.05.019 à Corresponding author. Federal University of Santa Catarina, Department of Chemical and Food Engineering, CP 476, Floriano- polis, SC88040-900, Brazil. Tel.: +55-48-331-9930; fax: +55-48-331- 9687. E-mail address: joao@enq.ufsc.br (J.B. Laurindo).