840—JOURNAL OF FOOD SCIENCE—Volume 61, No. 4, 1996 Flow Properties, Firmness and Stability of Double Cream Cheese Containing Whey Protein Concentrate C. SANCHEZ, J.-L. BEAUREGARD, J.-J. BIMBENET, and J. HARDY ABSTRACT As estimated on-line, the viscosity after cooling of double cream cheese curd containing heat-denatured WPC (DCC + ) increased from 1.4 Pa.s to 1.7 Pa.s when cooled to the range of 45°C to 24°C, and then decreased from 1.7 Pa.s to 1.0 Pa.s when cooled from 24°C to 15°C. The viscosity of DCC - (without heat-denatured WPC) increased from 1.5 Pa.s to 2.2 Pa.s at temperature shift from 40°C to 15.5°C. The firmness of stored DCC + and DCC - , respectively, decreased from 15.1N to 6.5N when cooled to temperatures from 45°C to 15°C, and from 17.9N to 9.9N when cooled from 40°C to 15.5°C, as recorded by cone penetrometry. The structure of DCC + cooled to 15°C collapsed after penetrometry, and DCC + cooled to 20°C destabilized during shearing in coaxial cylinder rheometer. A new phase in DCC + based on milk fat globules liberated by cluster disruption may be the cause of the structural and textural instability. Key Words: double cream cheese, rheology, whey protein concentrate INTRODUCTION THE INCORPORATION of whey proteins in cheese was primarily considered to reduce the cost of wastewater treatment and en- hance cheese yield (Jensen and Stapelfeldt, 1993). The func- tional properties of whey protein concentrates (WPC) and isolates (WPI) have led to a noteworthy increase in their indus- trial use as food ingredients. The tailoring of WPC and WPI quality requires a good knowledge of the impact of these whey proteins on the quality of the supplemented foods. This is im- portant for foods undergoing treatments after addition of whey proteins such as mixing, heating and homogenization. Physical treatments may considerably modify the functional properties of whey proteins by altering their structure, causing changes in the structural and textural features of foods. There are basically two methods to incorporate whey proteins in cheese: (a) heating the cheese milk to denature the whey proteins that complex with the caseins and are incorporated into the curd, (b) concentrating the whey by ultrafiltration (UF) in a first step, proteins being heat-denatured and aggregated before or after UF concentration, and in a second step adding the whey protein aggregates to the cheese milk. Detrimental effects on cheesemaking and on the quality of cheeses were observed with both methods (among others, Abrahamsen, 1979; Brown and Ersntrom, 1982; Marshall, 1986; Lelie `vre and Lawrence, 1988; Zoon, 1993). Heated milks showed poor rennetability because of the impaired aggregation properties of the rennet-converted casein micelles partly covered with denatured whey proteins. The attraction between protein aggregates diminished further as more of the calcium phosphate of the milk precipitated during heating (Lawrence, 1993). The poor aggregation between pro- teins resulted in weak curds and decreased syneresis, therefore producing softer cheeses than those obtained from nonheated milks. Higher moisture of cheeses made from milk supple- Authors Sanchez and Hardy are with Laboratoire de Physico- chimie et Ge ´ nie Alimentaires, ENSAIA, 2 avenue de la Fore ˆ t-de- Haye, 54500 Vandœuvre-le ` s-Nancy, France. Author Beauregard is with Centre de Recherches BEL, 7 bd. de l’Industrie, BP 77, 41102 Vendo ˆ me cedex, France. Author Bimbenet is with Laboratoire de Ge ´ nie Industriel Alimentaire, ENSIA, 1 avenue des Olympiades, 91305 Massy, France. Address inquiries to Dr. C. Sanchez. mented with heat-denatured whey proteins were also observed. In comparison with those made from milk without added pro- teins, they resulted in softer and greasier cheeses (van den Berg, 1979; Zoon, 1993). According to van den Berg (1979), such textural defects were caused by large whey protein aggregates that mechanically hindered shrinkage of the casein network and thus decreased syneresis. The ability of porous whey protein aggregates to hold water was also involved in the high moisture content of cheeses (de Wit and de Boer, 1975; Zoon, 1993). In order to avoid the cheese textural defects associated with the incorporation of whey protein aggregates to the starting milk, it was suggested to add the aggregates directly to the curd. Korolczuk and Mahaut (1991) found that a heat-denatured WPC (dWPC) incorporated by mixing to a microfiltered acid-coagu- lated milk improved the firmness of fresh cheeses. The increase in firmness was hypothesized to be due to the structuring prop- erties of whey protein aggregates since addition of soluble whey proteins showed no effect on cheese firmness. Banks and Muir (1985) previously added dWPC to a Cheddar-type curd and ob- served that aggregates interacted with the cheddaring process. Aggregates became entrapped between the curd layers, inhibit- ing fusion and destroying the usual ‘chicken-breast’ structure of normal Cheddar curd. The most successful efforts in incorporating dWPC to cheeses to increase yield, substitute fat and act as texturing agents have been with soft cheeses. However the impact of dWPC added to soft curds needs to be better understood in order to improve the control of cheese manufacturing. Our study was undertaken to solve a stability problem af- fecting double cream cheese when dWPC was added to the curd. The instability appeared when a spoonful of cheese removed at 5°C resulted in rapid collapse of the cheese near the sampling area with textural modifications. The cheese became very soft, ‘shiny’ and ‘sticky’ within 15 min following sampling. Since the double cream cheese curd underwent mixing, heat treatment, homogenization and cooling, we hypothesized that the added dWPC could be, at least indirectly, responsible for the cheese instability. Specific objectives of this study were to determine the impact of dWPC added to the curd on flow properties, firm- ness and stability of double cream cheese, and relate the results to changes in structure of the cheese. MATERIALS & METHODS Double cream cheese manufacturing Milk of standardized fat content was pasteurized, homogenized and cooled to an incubation temperature of 20°C. The mixture was then inoculated with a starter culture and incubated under gentle stirring. The drained curd was obtained by centrifugation of the mixture. After 24 hr storage at 5  2°C, the curd was heated to 70°C with 1% (w/w) salt under vigorous mixing (500 rpm), then heated further to 85°C in a scraped-surface heat exchanger (SSHE), homogenized (1st stage: 20 MPa; 2nd stage: 5 MPa; T: 80°C), cooled to 15–45°C in another SSHE, then packed in containers (1.2–1.3 kg) and stored at 5°C for 7 to 9 days before rheological tests. DCC + was obtained from the same process, with addition of 1% (w/w) dWPC to the curd before mixing. The dWPC was obtained from ‘La Socie ´te ´ des Fromageries BEL’ (Vendo ˆme, France) and contained 5.5% moisture, 65.5% total protein (N  6.38), 3.1% fat, 13.1% lactose, 6.3% ash and 0.9% Ca + + .