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Journal of Food Engineering 51 (2002) 249±254

Time-dependent viscosity of stirred yogurt. Part I: couette ¯ow
H.J. O'Donnell, F. Butler
Department of Agricultural and Food Engineering, University College Dublin, Earlsfort Terrace, Dublin 2, Ireland Received 8 September 2000; accepted 19 March 2001

Abstract Viscosity data at 5°C are presented forcommercial yogurt for a shear rate range 5±700 sÀ1 . Data were obtained using a conventional rotational rheometer. When subjected to a range of constant shear rates, yogurt viscosity demonstrated time-dependent and shear-dependent behaviour. The equilibrium structural parameter employed in the characterisation of the time-dependent nature of the yogurt was found to vary over the shear rate rangeinvestigated. At both the initial and equilibrium conditions, the shear rate/shear _ stress data were ®tted to an Ostwald power law model of the form s ˆ K cn with good correlation (average r2 ˆ 0:97). Experimental shear stress/time data at constant shear rate were modelled using a structural parameter approach and using the Weltmann model. The experimental shear stress data was best described by theWeltmann model. Ó 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Structural parameter; Yogurt; Time-dependent viscosity

1. Introduction Yogurt is produced by a fermentation process during which a weak protein gel develops due to a decrease in the pH of the milk. The pH of the milk is decreased due to the conversion of lactose to lactic acid by the fermentation culture bacteria. Inliquid milk, casein micelles are present as individual units. As the pH approaches pH 5.0, the casein micelles are partially destabilised and become linked to each other in the form of aggregates and chains which form part of a threedimensional protein matrix in which the liquid phase of the milk is immobilised. This gel structure contributes substantially to the overall texture and organolepticproperties of yogurt and gives rise to shear and timedependent viscosity. A large number of studies have been performed to characterise the viscosity of yogurt (Steventon, Parkinson, Fryer, & Bottomley, 1990; Ramaswamy & Basak, 1991a,b; Rohm, 1992; Benezech and Maingonnat, 1993; Skriver, Roemer, & Qvist, 1993; De Lorenzi, Pricl, & Torriano, 1995; Chan Man Fong, Turcotte, & De Kee, 1996). A number ofauthors have characterised the time-dependent viscosity of food products (Tiu & Boger, 1974; De Kee, Code, & Turcotte, 1983; Ramaswamy & Basak,

1991b; Benezech & Maingonnat, 1993; Alonso, Larrode, & Zapico, 1995; Chan Man Fong et al., 1996). Tiu and Boger (1974) employed a structural approach developed by Cheng and Evans (1965) which included a structural parameter k …0 T k T 1† which is anindex of the relative structural integrity of the sample. The assumption that the value of k at equilibrium, ke , is constant and independent of shear rate is central to that model. Another approach, applied by Ramaswamy and Basak (1991b), employed a model developed by Weltmann (1943) which described time-dependent viscosity at constant shear rate in terms of a logarithmic time model. Other modelshave also been reported which describe the time dependency of yogurt (De Kee et al., 1983; Chan Man Fong et al., 1996). In this study, the time-dependent viscosity of yogurt when subjected to a range of constant shear rates at 5°C was investigated. The objective of this work was to investigate the suitability of a structural model approach (Tiu & Boger, 1974) and the Weltmann model (Weltmann,1943) to characterise the time-dependent behaviour of yogurt. The aim was to use the viscosity data to model the tube ¯ow of yogurt. This work is described in the second paper.

2. Methods and materials
Corresponding author. Tel.: +353-1-716-7473; fax: +353-1-4752119. E-mail address: (F. Butler).

Batches of stirred natural yogurt in 500 g retail containers were purchased...
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