Objective: To determine the effect of sugar concentration on freezing temperature

Background

Freezing is a common method used to preserve food. In the freezing process, water is converted from liquid to solid. When this happens, free water is no longer available for chemical reactions and microbial growth. Hence, food spoilage due to microbial activity is delayed. “Delayed” is the correct term to use here since microbes such as bacteria are not killed by freezing. They can resume growth and activity when the food is thawed. That is why frozen foods must be kept frozen to ensure food safety. Although microbes are effectively controlled during freezing, enzymes are less affected and may be active even at freezing temperatures resulting in changes in color, texture, flavor and aroma over time. Food texture may be the most damaged of the sensory properties. This is particularly true if the food is frozen slowly. Slow freezing can cause textural damage due to the growth of ice crystals in foods such as ice-cream, and frozen fruits and vegetables. When ice crystals are large in ice-cream, you experience a grainy texture. This usually happens in home-made ice-cream that is frozen in the kitchen freezer. Fruits and vegetables that are frozen slowly tend to get soggy and drain water. This is because the ice crystals grow large enough to damage the cell walls so that they can no longer hold its content intact. 

Due to the negative effects caused by slow freezing, food manufacturers prefer to freeze foods rapidly using different freezing methods. These methods may include blast freezing with cold air, or immersion freezing with cryogenic (“extremely cold”) fluids (liquid nitrogen and carbon dioxide) to temperatures of 0oC or below. The lower the temperature, the longer the food can be preserved.

One factor affecting final temperature after freezing is the amount of dissolved solids in the food i.e. salt and sugar content. These solids reduce the melting point of water causing it to freeze at lower temperatures, hence improving the keeping quality of the food material. In this lab you will observe the effect of sugar on the temperature of food after freezing. 

Materials and Equipment

  1. Four 300 ml beakers
  2. Four digital thermometers
  3. Sugar
  4. Water
  5. Digital lab scale

Method

  1. Collect four 300 ml beakers or available containers about the same size
  2. Prepare 250 ml of sugar/water solutions with sugar concentrations of 0% (no sugar), 5%, 15%, 35% sugar respectively, and pour them into each beaker. Make sure to label them accordingly.
  3. Place a digital thermometer in each container. Note: Turn off the battery power on the thermometer during freezing
  4. Allow to sit for at least 40 minutes in the freezer
  5. Remove beakers from the freezer, check and record temperatures. Note: You can also store the beakers in the freezer overnight to observe larger differences in temperature due to freezing 
Sugar solutions (0-35%) in freezer  
Beakers (Left to right – 0%, 5%, 15%, and 35% sugar solution) after removal from freezer overnight (12 hours). Notice how freezing temperature (oC) drops with increasing sugar concentration.

Laboratory Questions

  1. Record the temperature of the solutions and draw a bar graph showing sugar concentration versus temperature (5 points)
  2. Based on the graph what was the effect of sugar concentration on the temperature of the sugar solution? (2 points)
  3. Explain the reason for the difference in temperatures. (3 points)
  4. Using your sense of touch, observe the texture of the frozen solutions. Do you notice any difference in hardness? Record your observations and discuss how sugar affects the ability of water to freeze. (5 points)
  5. Based on your observations, what effect do you think sugar content would have on the toughness of ice-cream? (5 points)
Courtney Simons
Administrator
Courtney Simons is a food science professor. He holds a BS degree in food science and a PhD in cereal science from North Dakota State University.
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