Objective: To determine the effect of different cooling methods on the cooling rate of snap beans

Background: Cooling of fruits and vegetables is important after harvest in order to slow down respiration and other metabolic reactions that could lead to spoilage. Air and hydrocooling methods are commonly used to bring temperatures down to safe levels. Knowing how long to cool is important to know since this will affect the cost of your operation.

Cooling rates will be affected by a number of factors such as type of cooling medium (air versus water), amount and time of contact of cooling medium with food, surface area of food, heat transfer coefficient of food, and the temperature of the cooling water and food surface. Heat transfer in fruit and vegetable cooling is based on convection currents.  The heat heat transfer equation used is: 

Q = h x A x (Ts – Tw) (North Carolina State Extension, 1992)

where:
Q = the rate at which heat is transferred from a batch of produce, in units of Btu/hr (Note: BTU means British Thermal Unit. It’s the amount of heat required to raise the temperature of water by 1oF)
h = the heat transfer coefficient, in units of Btu/(hr ft2 °F) 
A = the total exposed surface area of the produce, in square feet, over which the heat transfer occurs
Tw = the temperature of the water (°F)
Ts = the temperature of the produce surface (°F)

From the equation you will see that cooling is faster when there is a bigger difference in the temperature of the food compared to the temperature of the cooling medium. Foods with larger surface area will also cool faster. For example 20 kg of cherries will cool faster that 20 kg of potatoes. Higher heat transfer coefficient will also speed up rate of heat transfer. The heat transfer coefficient (h) represents the amount of energy that will pass through the cooling medium per square foot when there is a 1oF difference between the food and the medium. h is dependent primarily on the velocity of the cooling medium and its properties such as density, viscosity, specific heat, and thermal conductivity (Singh and Heldman, 2017). Water will be a better cooling medium than air since it will absorb more heat (specific heat capacity = 4185.5 J/kg.K versus air, 1006 J/kg.K) and is a better heat conductor (thermal conductivity = 0.6w/mK versus air, 0.024 w/mK). Water or air moving at higher velocities will achieve more efficient cooling. 

Researchers have developed time-center temperature response graphs showing decimal temperature difference (DTD) versus cooling time (Fig. below). This allows processors to predict the cooling time once the DTD is known.

DTD = (T – W) ÷ (P – W)

where:
T = the target temperature (°F)
W = the temperature of the water (°F)
P = the starting temperature of the produce (°F)

Time temperature response curves. (A, kale, leafy greens; B, peas and beans; C, radishes, small beets; D, small apples and peaches; E, maize, apples and peaches; F, cucumber, large apples and peaches, G, cantaloupe, large egg plant)(Image: Fellows, 2009)

Notice that the cooling time is shorter for smaller produce.  

In this lab you will cool snap beans using three methods: immersion cooling, air cooling and hydro-air cooling. For each cooling application, you will periodically check and record the temperature and create a cooling curve in excel.   

Method

Preparation: Place snap beans in a convection oven set at 37oC, for 2 hours

Air-cooling 

  1. Weigh 300 grams of snap beans
  2. Place beans on lab counter top in a single layer
  3. Cool using a fan set at high speed
  4. Every 5 minutes, take the the temperature of a snap bean by inserting the probe into the center of the bean pod. Discard after testing. 
  5. Continue collecting temperature data until the temperature falls to 20oC

Hydro-air cooling

  1. Follow the steps above, except that every 2 minutes, use a spray bottle to apply a thin layer of moisture to the surface of the beans

Immersion cooling

  1. Immerse 300 g of snap beans in an ice water bath at 0oC
  2. Collect temperature readings as before

Lab Questions

  1. Draw a graph for each cooling method showing temperature versus time
  2. Explain the reason for differences you observe in cooling rates
  3. Calculate the DTD based on immersion method, to reduce the temperature to 20oC
  4. Use the DTD versus cooling time graph above to predict the cooling time. How does your actual results compare to what is predicted in the graph? What could account for the difference if any?

References:

  1. Fellows, PJ. (2009). Food Processing Technology: Principles and Practice. Cambridge, MA: Woodhead Publishing. 
  2. North Carolina State Extension (October 1, 1992). Hydrocooling postharvest technical series. Retrieved on February 15, 2018. From https://content.ces.ncsu.edu/hydrocooling
  3. Singh, RP and Heldman, DR. (2017). Introduction to Food Engineering, 5th edition. San Diego CA: Academic Press. 
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Courtney Simons
Courtney Simons
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Dr. Simons is a food science educator. He earned his bachelor’s degree in food science, and Ph.D. in cereal science at North Dakota State University.