1038 JOURNAL OF FOOD SCIENCE—Volume 64, No. 6, 1999 © 1999 Institute of Food Technologists Food Engineer ing and Physical Pr oper ties Ultrasonic Velocity in Cheddar Cheese as Affected by Temperature A. Mulet, J. Benedito, J. Bon, and C. Rossello ABSTRACT The ultrasonic velocity in Cheddar cheese is temperature dependent. This relationship can be used to make correc- tions when determining ultrasonic texture or to determine mean temperatures in cooling/heating processes. At 0 < T < 35 °C ultrasonic velocity was 1590 to 1696 m/s, at 0 and 35 °C, respectively. Differential Scanning Calorimetry thermograms linked the temperature dependence of ultrasonic velocity to fat melting. Three parts are distinguished in the curve as a consequence of the fat melting and the appearance of free oil. The most reliable temperature interval to carry out ultra- sonic measurements in Cheddar cheese is identified as 0 to 17 °C. Key Words: Cheddar cheese, DSC, fat melting, ultrasonic velocity INTRODUCTION ULTRASONIC TECHNIQUES HAVE BEEN USED IN MEDICINE (Wells, 1977), metal testing (Papadakis, 1976), and recently in the food industry. Ultrasonics provide a non-destructive, rapid, auto- mated, and low cost technique for quality evaluation (Povey and McClements, 1988). Ultrasonic techniques have been used in the beef industry to quantitatively determine carcass value and predict heritable mus- cling and quality attributes (Miles et al., 1990; Whittaker et al., 1992; Cross and Belk, 1994). Velocity, attenuation, and frequency spectrum composition are the commonly measured acoustical pa- rameters (Povey and McClements, 1988; Povey, 1989, 1998). The frequency spectrum composition has been used to detect hollow hearts in potatoes (Cheng and Haugh, 1994) and intramuscular fat (Whittaker et al., 1992). Ultrasound velocity measurement has been carried out to determine meat quality (Cross and Belk, 1994); to estimate the solid/liquid ratio in fats, oils, and adipose tissue (Miles et al., 1985); to measure the quality of fruits (Mizrach et al., 1991); or to estimate cod fillet moisture (Ghaedian et al., 1997). Structural defects may be determined in Parmesan cheese (Or- landini and Annibaldi, 1983), cut-time in cheese making (Gu- nasekaran and Ay, 1996), rheological properties of cheese (Lee et al., 1992), and cheese maturity (Benedito et al., 1998), all using variations in ultrasound technique. The ultrasonic velocity through cheese increased with increasing firmness during maturation (Benedito et al., 1998), and ultrasound velocity has been found re- lated to bulk modulus or shear modulus. These properties are tem- perature dependent (Povey and McClements, 1988). Therefore, temperature must be known to relate velocity to physical proper- ties. The influence of temperature on velocity was studied as relat- ed to oils, adipose tissue (Miles et al., 1985), and cod fillets (Ghae- dian et al., 1997). Velocity becomes more temperature dependent when phase changes occur. Differential Scanning Calorimetry (DSC) studies showed that dried cheese underwent a phase transition from about –30 to 38 °C, primarily due to changes in fat crystallinity (Tunick, 1994). Cooling of Cheddar cheese blocks is the primary means of con- trolling microbial activity to promote homofermentative metabo- lism (Fryer, 1982). Cooling rate is a very important factor affecting flavor development during aging (Miah et al., 1974; Grazier et al., 1993). A non-invasive method for monitoring internal temperature would be advantageous. Ultrasonic temperature determinations are performed by mea- suring ultrasonic velocity through a material at different tempera- tures and establishing a temperature-velocity relationship (Lyn- nworth, 1992). The study of the temperature-velocity relationship has been used to determine the composition of food products, such as fish (Ghaedian et al., 1998). Our objective was to quantify the relationship between ultra- sonic velocity and sample temperature in Cheddar cheese. This re- lationship was tested by using an unsteady heating experiment to determine the accuracy of the procedure. MATERIALS & METHODS Raw material Cheddar cheese (Kerrygold, Irish Dairy Board, Dublin, Ireland) purchased from a local supermarket was used. The cheese was kept refrigerated at 1 °C in a sealed plastic bag to avoid water loss, and all tests were performed within 7 d of purchase. Proximate analysis of cheese Protein was determined by a Kjeldahl method (Method 991.22. AOAC. 1996), fat by solvent extraction (Method 933.05. AOAC. 1996), ash by overnight heating at 550 °C (Method. 935.42. AOAC. 1995), and moisture by oven drying (Method. 24003. AOAC, 1984). DSC Thermal transitions of Cheddar cheese were determined by DSC (DSC5200C0, Seiko Instruments, Torrance, Calif., U.S.A.). The instrument was calibrated with indium at 5 °C/min from 25 to 250 °C. Cheese (30 mg) was introduced into an aluminum crucible (Seiko Instruments 5), dried at 130 °C in a forced-draft oven for 20 min (Tunick, 1994), and analyzed by DSC. Thermograms of oven dried and lyophilized samples were coincident. Water remov- al avoided interference with fat melting in the DSC curve. An empty crucible was used as a reference. Preliminary steps carried out before obtaining the DSC curves were as reported by Tunick (1994), the samples being tempered at 60 °C for 5 min, cooled to –50 °C at 5 °C/min and held at –50 °C for 5 min. A DSC curve was then obtained by heating the sample to 50 °C at 5 °C/min. Ultrasonic measurements The experimental setup for velocity measurements consisted of two narrow-band ultrasonic transducers (1 MHz, 0.75 crystal di- ameter, A314S-SU Model, Panametrics, Waltham, Mass., U.S.A.), a pulser-receiver (Toneburst Computer Controlled, Model JOURNAL OF FOOD SCIENCE FOOD ENGINEERING AND PHYSICAL PROPERTIES Authors Mulet, Benedito, and Bon are affiliated with the Dept. of Food Technol- ogy, Polytechnic University of Valencia, Cno de Vera s/n, 46071 Valencia, Spain. Author Rossello is with the Dept. of chemical Engineering, Univ. of Illes Balaers, Ctra, Valldemossa km 7.5, 07071 Palma de Mallorca, Spain. Address inquiries to Dr. Antonio Mulet (E-mail: amulet@tal.upv.es).