Phase structures impact the rheological properties of rennet-casein-based imitation cheese containing starch Aiqian Ye * , Sheelagh Hewitt Fonterra Research Centre, Private Bag 11 029, Palmerston North, New Zealand article info Article history: Received 19 October 2007 Accepted 9 May 2008 Keywords: Rennet casein Potato starch Imitation cheese Rheological property Microstructure Binary continuous phases abstract The dynamic rheological and microstructural properties of rennet-casein-based imitation cheeses containing various concentrations of potato starch were investigated using a stress-controlled rheometer and confocal laser scanning microscopy. The influence of added starch on the size of the oil droplets in the imitation cheeses was also examined. Imitation cheeses with 0–15% protein replaced by starch were processed in a Rapid Visco Analyser (RVA) at 90  C for 10 min at a shear rate of 800 rev/min and were then evaluated using oscillatory shear measurement and a temperature sweep (20–90  C). The storage modulus (G 0 ) of the rennet casein imitation cheeses increased abruptly at added starch concentrations >4%. In the temperature range 20–90  C, tan d of the imitation cheeses decreased with increasing starch concentration and was <1 at added starch concentrations >4%. A binary continuous phase consisting of a protein phase and a starch phase was observed in systems containing >4% starch, whereas the starch was dispersed in the protein matrix as small particles of irregular shapes at added starch concentrations 4%. As the dispersed phase, the size of the oil droplets increased with starch addition in the imitation cheeses. The marked increase in G 0 and the reduction in tan d may be attributed to the formation of a binary continuous separated phase structure in imitation cheeses containing added starch that is driven by thermodynamic incompatibility between rennet casein and starch. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Processed cheese/imitation cheese products are manufactured by blending various edible oils/fats, proteins, other ingredients and water into a smooth homogeneous blend with the aid of heat, mechanical shear and emulsifying salts. As processed cheeses/ imitation cheeses are used mainly as an ingredient in prepared foods such as pizzas, burgers and toasted sandwiches, their rheological properties, as related to firmness and meltability during heat treatment, are very important (Guinee, Caric, & Kalab, 2004). The influences of various ingredients and processing conditions on the rheological properties of processed cheeses/imitation cheeses have been widely studied using various determination methods, e.g. large strain and small strain rheological tests (Bowland & Foegeding 1999, 2001; Guinee et al., 2004; Lee, Klostermeyer, Schrader, & Buchheim, 1996; Shirashoji, Jaeggi, & Lucey, 2006). Starch has been added to imitation cheese as an ingredient for cost and health reasons (Guinee et al., 2004; Montesinos-Herrero, Cottell, O’Riordan, & O’Sullivan, 2006; Mounsey & O’Riordan 2001). However, the properties of imitation cheese have been shown to change when starch is incorporated into the protein and fat structures in the cheese (Guinee et al., 2004; Montesinos-Herrero et al., 2006; Mounsey & O’Riordan, 2001). Mounsey and O’Riordan (2001) reported adding 3% of different native starch types to rennet-casein-based imitation cheese. The addition of starch resulted in a change in the melt and texture properties. In addition, these workers recently reported that an increase in the firmness and brittleness and a decrease in the fluidity and flowability were observed in imitation cheese in the presence of a high level of pre-gelatinized maize starch (Mounsey & O’Riordan, 2008a, 2008b). These changes in the physical properties were attributed to a modification in the structure of the imitation cheese. The distributions of fat and protein in the matrix of imitation cheeses containing starch were different from those in the control samples containing no starch (Montesinos-Herrero et al., 2006; Mounsey & O’Riordan, 2001). The levels of changes in the properties and the structure of imitation cheese were dependent on the type of starch added. Guinee et al. (2004) concluded that the influence of added starch on the properties of imitation cheese was probably related to the degree of gelatinization of the starch during processing, storage-related retrogradation and gelation of the starch molecules, which related to the ratio of amylose to amylopectin in the starch. A starch gel would impede the flow of the cheese at high temperature. Mounsey and O’Riordan (2001) found that the addition of starch increased the degree of fat emulsification. * Corresponding author. Tel.: þ64 6 350 4649; fax: þ64 6 356 1476. E-mail address: Aiqian.Ye@fonterra.com (A. Ye). Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd 0268-005X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2008.05.004 Food Hydrocolloids 23 (2009) 867–873