The Effects of Temperature Cycling on Cheddar Cheese

Transcription

1 The Effects of Temperature Cycling on Cheddar Cheese By D. Reinemann J. Mitchell A. Hade J. Leggett 7 May, 1998 This work was supported by a grant from the Energy Center of Wisconsin. 1

2 Introduction The Effects of Temperature Cycling on Cheddar Cheese Thermal storage is a technique commonly used for shifting electrical demand. Thermal storage involves storing available heating or cooling energy in a storage medium during peak use hours and using this stored energy during peak hours to reduce or even eliminate electric loads during that period. This strategy can be used to reduce the cost of operating the refrigeration system in a warehouse. This project investigated the possible use of cheese as a thermal storage medium in refrigerated warehouses. Cheese is stored in refrigerated warehouses during the aging process that develops the taste and texture of the cheese. Cheese is typically aged for 60 days or more in warehouses containing several million pounds of cheese. Thermal modeling was performed by Zehr (1997) to estimate the technical and economic potential for thermal storage in a cheddar cheese warehouse. Zehr s study concluded that the potential demand and cost savings were substantial but the analysis did not explore the possible effects that temperature variation may have on the quality of the cheese. The experiment described in this report was designed to produce the most extreme temperature variations predicted for thermal storage to determine the effect of these on the aging process of cheddar cheese. In this report, the model used to determine the temperature response of cheese in a temperature-cycled warehouse is described first. Then the experiments conducted on stored cheese blocks are described. Finally, the results of an evaluation of the cheese exposed to temperature cycling are presented. Lumped Capacitance Model of Warehouse air Temperature A finite difference model was developed by Zehr to predict warehouse temperatures and estimate the electrical power consumption for various thermal storage scenarios. The model parameters were based on an actual cheddar cheese warehouse in Marshfield, Wisconsin. This model was used to specify the parameters used in the temperature cycling experiment described in this report. The lumped capacitance model simulates heat transfer between three masses: the new cheese brought into the warehouse; the cheese already stored in the warehouse; and the air inside the warehouse. In addition, the model accounts for cooling loads due to lighting, motors and other equipment, as well as infiltration of the outside air. The values for these loads were provided by Zehr. A conventional cheese warehouse is maintained at F for 24 hours per day. In the thermal storage scenario, the refrigeration system is operated to maintain air temperature at 30 F for twelve hours each day. During the other twelve on peak hours, the refrigeration system is shut off. This situation was modeled and the resulting air 2

3 temperature determined from the heat transfer model. During the off period, the temperature is in the range of F. Pallets of cheese enter the warehouse at a uniform temperature of F from the production line. In the model, the mass of the previously stored cheese was taken to be 2.8 million lb., the annual average mass of cheese in storage in the Zehr model. The heat transfer coefficients were provided by Zehr. Thermal conductivity of the cheese (both new and stored) was set at Btu/ (hr-ft2-f), and the effective convection coefficient between the cheese and air was set at Btu/ (hr-ft-f). (The effective convection coefficient accounts for the thermal resistance of the cheese when a model for average temperature is used.) The following management conditions were chosen as a worst-case scenario: 1. the outside ambient temperature was set at 92 F; 2. the cheese entered the warehouse at 97 F 3. an entire daily production (156,000 lb.) was inserted into the warehouse instantaneously, rather than on an incremental basis; and 4. The new cheese was placed in the warehouse at the beginning of the twelvehour period when the coolers were off. Simulations indicated that the temperature of the warehouse rises quickly from 30 F to approximately 46 F within two hours after the refrigeration system is shut off. Thereafter, the temperature rises slowly, and at the end of the twelve-hour period, the air temperature climbs to 48.3 F. (See Figure 1.). Based on the simulations, the temperature of the experimental chamber was thus held at 30 F for 12 hours and 48 F for the other 12 hours of the day. 3

4 Figure 1. Results of Warehouse Model. Warehouse Simulation Temperature (F) Bulk Cheese New Palette Warehouse Air Time (hrs) Finite Element Model of the Temperature Profiles in a Block of Cheese The warehouse model describes the average cheese temperature. A detailed finite element model was developed to estimate the temperature variations inside a 40-lb. block of cheddar cheese exposed to the experimental air temperature variations. Because of symmetry, only one quarter of the block of cheese needed to be considered. This portion of the block was then simulated as 16 nodes of equal size. Each node was assumed to be at a constant temperature. Zehr s value of thermal conductivity of cheese was used. The results of the simulation are given in Figure 2. The locations of the temperature nodes are described in Figure 4. A convection coefficient that included free convection and radiation relations was used to determine transfer between the block and the air. The value of this coefficient was 2.3 Btu/(hr-ft-F). The thermal resistance of the cardboard was assessed using a conductivity of 0.04 Btu/ (hr-ft2-f) and a thickness of 0.25 inches. The bottom of the box was assumed to be adiabatic. A one-day simulation using the values given earlier is shown in Figure 2. 4

5 Figure 2. Internal Cheese Temperature Finite Model Element Results. Simulation 50 Temperature (F) W arehouse Air Center of Block Corner of Block Time (hrs) The lower air temperature limit (30 F) was chosen to avoid the possibility of freezing the outside layers of cheese. The upper air temperature (48 F) was chosen to avoid the conventional accepted abuse temperature of stored cheese (50 F). The average cheese temperature (38 F) was thus somewhat below typical cheese storage temperature (40 45 F). Experimental Apparatus The effect of temperature variation on cheese quality was evaluated on two 40-lb. blocks of cheddar cheese. These blocks of cheese were obtained from a local cheese plant, transported in an insulated container to the UW and placed in the experimental chamber within 2 hours of production. The internal cheese temperature and air temperature in the experimental were monitored with a Campbell 21X Data logger which also controlled the air temperature in the chamber. The experimental chamber was a refrigerator fitted with 150-Watt strip heater and a fan was used to circulate air in the chamber. The blocks of cheese were placed one atop the other, which allowed for equal surface area for each the blocks and an adiabatic surface similar to the finite element model. (The bottom block rested on a rack with it s bottom surface exposed.) The experimental set-up is shown in Figure 3. In addition to the two blocks inside the refrigerator, another 40-lb. block was used as a control. This block was stored in a cooler maintained at a constant temperature of 45 F. The locations of the thermocouples for measuring the internal temperature of the cheese are shown on Figure 4. 5

6 Figure 3. Experimental Apparatus. Figure 4. Thermocouple Locations. Measurements were taken by the data logger every minute for each thermocouple; and, minimum, maximum, and average values were determined at the end of each hour. Data were obtained over a three-day period to assure that a steady response had been achieved. The three-day temperature plots were reduced to one-day plots by averaging the three points at identical locations in the cycle. The air temperature in the experimental chamber over a three-day period is shown in Figure 5. The cheese temperatures are shown in Figure 6. The results indicate that the experimental chamber was able to maintain the temperature to within two degrees of the setpoint. 6

7 The model predictions shown in Figure 2 are in agreement with the experimental results in Figure 6. The corner temperatures agree very well (within 5%) with the modeled corner temperatures, while the center displayed a somewhat slower response than predicted. This suggests that the chosen conductivity value for cheddar, Btu/(fthr-F), may be higher than that of the cheese blocks used. The close corner temperatures suggest that the refrigerator produced convection conditions close to those predicted for the warehouse conditions. Evaluation of Cycled Cheese The aging of cheddar cheese is an important factor in the taste and quality of the cheese. Aging is determined by bacterial growth on and inside the cheese, and is therefore strongly related to the temperature of the cheese. The optimum temperature for aging is considered by the industry to be 45 F for cheddar. The maximum allowable temperature is 48 F, and the minimum effective temperature is 40 F. Temperatures above 50 F are considered detrimental to the aging process and to affect the overall quality of the cheese. 7

8 Figure 5. Experiment Chamber Air Temperature. 55 Temperature (F) Time (hrs) Avg. Refrig. Air Max. Refrig. Air Min. Refrig. Air Figure 6. Cheese Temperature Results Avg. Air Avg. Corner Avg. Center Temperature (F) Time (hrs) 8

9 The cheese was evaluated by three faculty members from the UW Center for Dairy Research after being held in the experimental chamber for 90 days, and compared to the cheese aged at constant temperature. This experts panel was composed of experienced cheese tasters who have served as judges in numerous international cheese competitions. The unanimous decision was that the temperature cycling did no harm. Some slight problems with the control block occurred due to yeast and/or bacterial spoilage around the edges, but this spoilage did not occur with the cycled cheese. The consensus was that the cycled cheese might have not been aged quite as much as that stored at constant temperature. This is not unexpected, as the average temperature (42 F) was slightly below the control block (45 F). Further testing was not recommended as there were no signs of any problem and the differences found were expected to be much smaller than batch to batch differences in cheese. In addition to this experts panel, informal testing was done with two other groups: approximately 30 people present at the UW ESERC seminar and a similar number present at the ECW Board meeting. For these tests, the cheese was cut into thin sticks about 4 inches long. One end of this stick was the exposed face of the test block, which experienced the maximum temperature variation, and the other end was near the core, which experienced the minimum temperature variation. None of these tasters was able to detect a difference in flavor or texture between the two ends of the cheese sticks. Conclusions The model developed by Zehr predicts spatial variation in cheese blocks reasonably well. The demand control of the refrigeration system tested produced daily temperature variations of about 6 F in the center of a 40-pound block of cheese and 15 F near the exposed surface. These temperature variations had no discernable effect on cheese quality. The taste, texture, and appearance of the cheese aged with temperature cycling were indistinguishable form cheese aged in constant temperature storage. A larger scale experiment conducted in a warehouse is recommended. 9