اثر دما و غلظت در برش نوسانی پویا و ثابت خواص رئولوژیکی مخلوط پنیر فراوری شده کچاپ

ABSTRACT Steady and dynamic oscillatory shear rheological properties of ketchup–processed cheese mixtures: Effect of temperature and concentration

Ketchup, one of the most important tomato products, is produced basically from either cold or hot extracted tomatoes; or directly from concentrates, purees and tomato paste (Sahin and Ozdemir, 2004). Although ketchup is known worldwide; information 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 (Bistany 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 lowfat processed cheeses have entered the marketplace (Drake and Swanson, 1995).

Milk proteins are commonly used as food ingredients in the formulation 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 functional 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 increase 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 proteins 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 biopolymer 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 viscosity at key processing points, as well as helping to maintain consistent 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 dairybased foods including custards (Doublier and Durand, 2008), yoghurts (Keogh and O’Kennedy, 1998) and processed cheeses (Mounsey and Oriordan, 2008a). From a scientific perspective, processed 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 importance and extensive investigation of starch and milk proteins separately (Noisuwan et al., 2009). Based on the fact, ketchup–processed cheese (K–PC) mixture could be an example of such interactions as the former includes starch and the later is a source of casein (milk protein) and fat.

Viscosity, usually considered as an important physical characteristic of fluid or semifluid foods is related to the quality of products. Therefore, the flow behavior information may be useful in quality control, energy input calculations, process design and selection of process equipment, such as heat exchangers and pumps. In addition, the rheological properties of fluid foods affect the sensory quality and consequently consumer preference. However, these properties are widely influenced by chemical composition, pH value, processing conditions and also by some added ingredients (Bowland and Foegeding, 2001; Marchesseau and Cuq, 1995; Piska and Šttina, 2004). The viscoelastic properties of macromolecular dispersions can be determined by using dynamic oscillatory shear rheological tests (Rao, 1999). In a dynamic test, sinusoidal strain cycle is utilized for the determination of the storage modulus (G0), loss modulus (G00), complex modulus (G*) and complex viscosity (g*). Previous studies indicate the presence of a correlation between steady shear and dynamic shear properties of macromolecular dispersions (Chun and Yoo, 2004). The Cox–Merz superposition rule which relates the complex dynamic shear viscosity (g*) as a function of angular frequency (x) to the steady shear flow viscosity (g) as a function of shear rate (c_) has been experimentally confirmed for several macromolecular dispersions (Chun and Yoo, 2004), on the other hand, Da Silva and Rao (1992) reported that Cox–Merz rule was not valid for macromolecular dispersions with either hyperentanglements or aggregates.

No attempt has been made so far to study rheological properties of K–PC mixtures as a function of temperature and concentration. In particular, no information is available on the rheological properties of K–PC mixtures under both steady and dynamic shear. Therefore, this study was undertaken to determine the rheological behaviors of ketchup–processed cheese mixtures in steady and dynamic shear as affected by different temperatures (10–50 C), concentration ranges (0–30%).