The Freezing Curve of Foods
Below is a typical freezing curve of a food material (Figure 1). See also figure 9.3 in your textbook. The graph shows that foods do not begin cooling at exactly 0oC (32oF). Instead they become supercooled, after which there is an abrupt increase in temperature due to release of latent heat of fusion (also called latent heat of crystallization). This is followed by a gradual drop in temperature as water freezes. The numbers on the graph shows the percentage of water frozen over time. As the water freezes it causes a corresponding increase in concentration of solutes which keeps depressing the freezing point. If the food was pure water (Figure 2), the freezing point would remain constant and hence there wouldn’t be any drop in temperature after the freezing point is reached. Hence you would see a flat graph during the phase change from solid to liquid.
Fig. 1. Heating curve of a thin section of beef (Food Science by Norman Potter (1995)
Where you introduced to this (Figure 2) curve in physics? Here is a quick review. Figure 2 shows pure water being heated and converted to a water and then to steam. Notice that as energy is added to ice, the temperature rises. This is called sensible heat. It is heat that you can feel or sense. However, you notice that when the temperature reaches its melting point there is no more temperature increase until all the solid is melted. This is called the latent heat of fusion. Latent means hidden. Energy is being absorbed but you cannot sense it by observing a change in temperature. Instead of the energy being lost as heat, it is absorbed to break the chemical bonds that hold the ice together. Once all the solid is melted, temperature continues to increase until the boiling point is reached. Here again, we see that energy is being absorbed to allow for breaking of chemical bonds to produce a vapor. The heat that is absorbed during the phase change from liquid to gas is called the latent heat of vaporization.
Fig. 1. Heating curve of pure water
Changes During Freezing
Although changes may occur at a slower rate, the following effects are caused by freezing
- Precipitation of solutes
- Breaking of emulsions
- Denaturation of proteins
- Rupturing of cell walls, and hence textural damage
- Water and nutrient loss
- Freezer burn
Effect of Freezing Rate
Factors affecting freezing rate include compositional and non-compositional influences. Generally, foods with higher composition of fat and entrapped air will have lower conductivity. Non-compositional influences include air velocity, product thickness, agitation, degree of contact with the food and cooling medium, and packaging. Slow freezing normally leads to growth of ice crystals and rupturing of cell walls in fruits and vegetables. This causes them to become soggy. Rapid freezing retards the growth of ice crystals. Smaller crystals do not damage cell walls. This causes the texture and appearance of the products to be retained. Methods used for rapid freezing include air freezing, indirect contact freezing, and immersion freezing.
Air Freezing: Use of cold air, which may be still-air at -23oC to -30oC or cold air moving at high velocity. The air may move horizontally along a tunnel (blast freezing) or vertically beneath the food being conveyed (individual quick freezing or IQF).
Indirect Contact Freezing: Freezing of the food indirectly by heat transfer through a contact surface e.g. plate freezing where plates refrigerant is circulated around plates
Immersion Freezing: Direct contact of the food with the refrigerant by immersion. Types of refrigerants may include liquid nitrogen, liquid carbon dioxide, brines and syrups
Immersion Freezing with Cryogenic Liquids: Direct contact of the food with the liquified gases of cryogenic (“extremely cold”) liquefied gases (usually nitrogen or carbon dioxide)