Anomalous Expansion of Water and Food Chemistry
Water’s Anomalous Expansion
It is said that water has anomalous dilation. This occurs at 4 degrees Celsius, and between 0 and 4°C. Experience tells us that when we heat a body, it expands, and when cooled, it contracts. However, water does not always behave this way. When water freezes, it expands. That is, a mass of ice has a greater volume than the same mass of liquid water. This phenomenon is called the anomalous expansion of water.
The density of water varies with temperature, so that the maximum density (1g/cm3) is reached as liquid water at 3.98°C. Between 0 and 4°C, water contracts, reaching its minimum volume. From 4°C, water expands like other liquids as temperature increases.
Water Activity in Food
Water activity indicates the strength of the forces that bind water with other non-aqueous components. It represents the water available for the growth of microorganisms and for various chemical and biochemical reactions.
Desorption and Absorption
Food given at a constant temperature has a higher water content during desorption than during absorption. During water desorption, the water that was more or less bound to the food is not totally free. Food retains water inside capillaries. During absorption, the collected water has more difficulty entering due to pressure. The water vapor needed to penetrate the capillaries is larger than that needed to exit. Therefore, with the same moisture content, there is more water outside the capillaries during adsorption than during desorption.
Lipids in Food
Lipids are one of the most important immediate principles of food for several reasons:
- They are abundant in a large number of foods.
- They play an important nutritional role in the composition and permeability of cell membranes and walls.
- For technological reasons: some foods are emulsions, and some lipids are excellent amphiphilic stabilizers.
- They are susceptible to degradation, which causes alterations in the sensory characteristics of foods.
Phospholipids
Phospholipids are complex lipids containing phosphorus and have, as a core, a molecule of glycerol or sphingosine.
Hydrogenation Mechanism
The mechanism involved in the hydrogenation of a fat is the reaction of an unsaturated fatty acid and atomic hydrogen absorbed in the metal catalyst.
Autooxidation
Autooxidation is one of the main biochemical reactions of food spoilage. It implies the appearance of tastes and smells commonly known as rancid. If linoleic acid (an essential fatty acid) is involved, it results in the oxidation of fat. This can be mitigated with cooling, packaging, and proper storage.
Flavor Reversion
Flavor reversion is a special type of oxidation that occurs mainly in linoleic acid-rich oils (soybean and rapeseed oils) when exposed to air.
Antioxidants
Antioxidants are substances naturally present or intentionally added to food fats to delay the appearance of oxidation while maintaining their sensory characteristics.
Physical Procedures to Protect Against Oxidation
- Vacuum packing
- Oxygen-free atmosphere (inert atmosphere)
- Oxygen-impermeable material
Lipolytic Rancidity
The rupture of the ester bond of a lipid is called lipolytic rancidity (a phenomenon of lipolysis). It decreases the quality of fats in seasonings and fried foods.
Mixed Crystals
The crystals are held together by Van der Waals forces. In their growth, they form a three-dimensional network that gives rigidity to the product and can retain oily liquid inside, even without crystallizing.
Melting Point and Melting Zone
The melting point of a fat is regarded as the fusion point of the most stable polymorphism in an open capillary. It corresponds to the temperature at which all solids melt. The melting points of pure fats are very specific. However, since fats and oils are made of a mixture of lipids with different melting points, we refer to a melting zone rather than a net melting point. The melting zone is recognized as the fusion of the fat component that melts at the highest temperature.
Plasticity of Fat
The plasticity of fat is due to the presence of a three-dimensional crystal network within which liquid fat is immobilized.