Effect of moisture content and water mobility on microwave expansion of imitation cheese J.M. Arimi, E. Duggan, M. O’Sullivan, J.G. Lyng, E.D. O’Riordan * UCD School of Agriculture, Food Science and Veterinary Medicine, UCD Dublin, Belfield, Dublin 4, Ireland article info Article history: Received 25 November 2008 Received in revised form 4 November 2009 Accepted 5 January 2010 Keywords: Imitation cheese Moisture content Water mobility Microwave expansion Crispy abstract The effects of moisture content (60–45%), and refrigerated storage (1–9 days) on the functional proper- ties, water mobility and microwave expansion of imitation cheese are investigated. Cheese hardness, elastic (G 0 ) and viscous moduli (G 00 ) increased with decreasing moisture content and decreased during storage. Flowability, loss tangent (tan d) and water mobility decreased with decreasing moisture content and increased during storage. Microwave expansion of imitation cheese decreased with decreasing mois- ture content and increased with prolonging storage time prior to microwaving. The changes in texture, rheology and flowability of cheese with decreasing moisture content, changes in water mobility and stor- age time reflected the level of cheese plasticisation by water and had strong linear correlations with microwave expansion. This work provides evidence that moisture content influences microwave expan- sion of imitation cheese by providing the driving force for expansion and plasticising the cheese matrix. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In modern society, consumers are ‘cash-rich’, ‘time-poor’ and well-informed on issues of diet and health. Therefore consumers are not only looking for snack foods that are convenient (i.e. quick to prepare or ready-to-go) but also that are healthy. This has helped make microwavable snack foods a popular option, as such products require minimal preparation time and are normally low in fat compared with fried snacks (Ernoult, Moraru, & Kokini, 2002). Most microwave expandable snack food substrates are starch- based, e.g. popcorn (Lin & Anantheswaran, 1988), rice and corn pel- lets (Chen & Yeh, 2000; Gimeno, Moraru, & Kokini, 2004). However, our group has developed an imitation cheese-type product, which is essentially a protein-based matrix containing resistant starch, and it expands when microwaved to form a crispy product (Arimi, Duggan, O’Riordan, O’Sullivan, & Lyng, 2008; Arimi, Duggan, O’Sullivan, Lyng, & O’Riordan, 2008; O’Riordan, O’Sullivan, & Lyng, 2008). Although starch is present within the matrix, it has been shown to remain intact (i.e. not gelatinise) during microwave heating, suggesting that the expansion observed is not that of a starch matrix (Arimi, Duggan, O’Sullivan et al., 2008). Moisture content within the cheese matrix is thought to be the key factor influencing expansion. Previously we have hypothesised that, during microwaving, moisture turns into superheated steam and provides the driving pressure for expansion while at the same time plasticising the cheese matrix by making it softer, more flexible and thus more able to expand (Arimi, Duggan, O’Riordan et al., 2008). In recently published work, the degree of expansion of high moisture imitation cheese during microwaving was shown to in- crease with increasing pre-expansion storage time. This phenome- non was related to an increase in water mobility in the unheated cheese during storage prior to microwaving (Arimi, Duggan, O’Sullivan et al., 2008). The increase in water mobility during stor- age was suggested to cause increased plasticisation of the cheese matrix which could be responsible for the observed changes in expansion. Additionally, it may be that this increase in the mobility of water renders it more easily evaporated, leading to increased pressure of steam within the product to drive the expansion (Arimi, Duggan, O’Sullivan et al., 2008). Should the changes in water mobility during storage result in increased plasticisation of the cheese matrix, then it could reasonably be expected to be re- flected in changes in the textural properties (e.g. hardness), melt- ability and rheological properties of cheese. Changes in these properties of imitation cheese during storage were reported by Montesinos-Herrero (2003) and McDarby (2007) and were associ- ated with the increased plasticisation of cheese by water. The objective of the present study is to provide experimental evidence that the moisture content contributes to the microwave expansion of imitation cheese by providing both the driving force (in the form of steam) and by plasticising the matrix, thus allowing it to expand. A further objective was to assess whether the time-dependent increase in expansion is due to increased plastici- sation of the matrix. In the previous studies, high moisture cheese 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.01.001 * Corresponding author. Tel.: +353 1716 7016; fax: +353 1716 1147. E-mail address: dolores.oriordan@ucd.ie (E.D. O’Riordan). Food Chemistry 121 (2010) 509–516 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem