Steady and dynamic oscillatory shear rheological properties of ketchup–processed cheese mixtures: Effect of temperature and concentration Mustafa Tahsin Yılmaz a,⇑ , Safa Karaman b , Hasan Cankurt a , Ahmed Kayacier b , Osman Sagdic b a Food Technology Department, Safiye Cikrikcioglu Vocational College, Erciyes University, 38039 Kayseri, Turkey b Food Engineering Department, Engineering Faculty, Erciyes University, 38039 Kayseri, Turkey article info Article history: Received 23 June 2010 Received in revised form 12 October 2010 Accepted 19 October 2010 Available online 25 October 2010 Keywords: Ketchup Processed cheese Dynamic rheology Temperature Concentration abstract The steady and dynamic shear properties of ketchup–processed cheese (K–PC) mixtures were investi- gated at different temperatures (10–50 °C) and PC concentrations (0–30%). The K–PC mixtures showed a shear-thinning behavior with low magnitudes of yield stress. The consistency coefficient (K) and appar- ent viscosity (g 50 ) decreased with increase in temperature and concentration. The mixtures followed the Arrhenius temperature relationship, indicating that the magnitudes of activation energies (E a ) were in the range of 8.83–17.16 kJ mol 1 . Storage (G 0 ), loss (G 00 ) and complex (G*) modulus increased with increase in frequency while complex viscosity (g*) decreased. The K–PC mixtures at concentrations of 0–15% exhib- ited weak gel-like behavior. Increase in the PC concentration resulted in a decrease in G*, G 0 , G 00 and g* up to the 15% of PC concentration, showing a plateau value between 0% and 30% concentrations. Cox–Merz rule was not applicable to K–PC mixtures. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Ketchup, one of the most important tomato products, is pro- duced basically from either cold or hot extracted tomatoes; or di- rectly from concentrates, purees and tomato paste (Sahin and Ozdemir, 2004). Although ketchup is known worldwide; informa- tion on this product is limited in the literature (Sharoba et al., 2005). Ketchup is time-independent, semi-solid non-Newtonian fluid having a definite yield stress (Sharoba et al., 2005). Based on dynamic shear data, it was also reported that the Cox–Merz rule was not applicable for commercial tomato ketchup samples (Bista- ny and Kokini, 1983), exhibiting properties similar to weak gels (Rao and Cooley, 1992). Processed cheese is a dairy product obtained by mixing natural cheeses with emulsifying salts and water under the influence of heat and agitation. After homogenization of molten blend, the product is packed and cooled. In recent years, reduced- or low- fat processed cheeses have entered the marketplace (Drake and Swanson, 1995). Milk proteins are commonly used as food ingredients in the for- mulation of various food products due to their unique functional characteristics like emulsifying, gelling, thickening, foaming and water binding (Kinsella and Whitehead, 1989). In addition to func- tional properties, milk proteins are also utilized because of their high nutritive value and GRAS status (Bryant and McClements, 1998; Harper, 2000; Hudson et al., 2000). They are used at optimal levels to improve texture, flavor and color of foods as well as to in- crease the amount of proteins with high biological value (Kilara, 1994). Because of the aforementioned reasons, ketchup could be supplemented or enriched with the sources containing dairy pro- teins like processed cheese to obtain a nutritionally enriched product. Starch is commonly used as thickening agent (Sidhu et al., 1997; Srivastava, 1982). It is added to ketchup formulations because it has some functional properties such as formation of typical bio- polymer gel network (Mohammed et al., 1998) and alteration of rheological properties such as thixotropic and rheopectic flow properties (Sharoba et al., 2005). Therefore, starch is assumed to play multifunctional role in a condiment system, providing viscos- ity at key processing points, as well as helping to maintain consis- tent suspension (Alam et al., 2009). For these reasons, starch is added to tomato ketchup in industry to achieve good quality of the final product (Hoover and Ratnayake, 2002). Starch and milk proteins are present together in various dairy- based foods including custards (Doublier and Durand, 2008), yo- ghurts (Keogh and O’Kennedy, 1998) and processed cheeses (Mounsey and Oriordan, 2008a). From a scientific perspective, pro- cessed cheese is reported to provide an ideal model system to examine the interactions of the major food components – protein/ fat/polysaccharides (starch) and water (Mounsey and Oriordan, 2008b). Studies about the interactions between starch and dairy proteins are limited in the literature compared to those of mixtures of milk proteins and other biopolymers in spite of the industrial 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.10.016 ⇑ Corresponding author. Tel.: +90 352 437 4901/41523; fax: +90 352 437 1383. E-mail address: mtyilmaz@erciyes.edu.tr (M.T. Yılmaz). Journal of Food Engineering 103 (2011) 197–210 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng