Dehydrating foods is a way of preserving them and reducing their bulk weight. By reducing bulk weight, transportation efficiency and cost can be reduced. Moisture content of dehydrated foods is generally between 1-5%. It is generally impossible to get a final moisture content of 0%. Factors affecting heat transfer during drying includes:

  1. Surface area
  2. Temperature
  3. Air velocity
  4. Relative Humidity
  5. Vacuum

Large surface area, high temperature, high air velocity , high vacuum, and low humidity increases the rate of dehydration. The relationship between moisture content and relative humidity can be displayed on a graph known as the moisture sorption isotherm. The higher the temperature, the greater the amount of moisture that is capable of being removed before the equilibrium is reached. At equilibrium, no further moisture loss is possible because the same amount of moisture that is lost to the environment is the same that is being reabsorbed back into the food. Dry foods are said to be hygroscopic. That means that they are capable of reabsorbing water unless the moisture content and relative humidity are at equilibrium. Dried foods will absorb moisture if the surrounding air is too moist. Therefore, dried foods must be properly sealed.    

Drying Curve

Drying is characterized by a rapid fall in moisture followed by a slow decline as the last moisture in the food become more and more difficult to be removed. This happens because of a number of reasons.

  1. As food is dried, the outer surface acts as an insulator making heat transfer to the center more difficult.
  2. The gradual removal of heat increases salt and solute concentration. As the concentration increase, the boiling point of water is increased. Therefore much more energy is needed to remove additional water
  3. Heat from the center has more distance to travel to the surface
  4. As the moisture approaches zero, more moisture is chemically bound to other food components like lipids, carbohydrates and proteins, making them difficult to remove
  5. In some cases, sugars and minerals are transferred to the surface as water migrates out of the food, leaving it sticky and then hard. We call this case hardening. This is usually avoided by gradual heating instead of using too high temperatures at the beginning of drying  
  6. As heating continues, the food approaches an equilibrium where the same amount of heat being lost is the same amount being reabsorbed. As this point is reached, further heating will have less and less effect on moisture removal 

Chemical Effects of Dehydration

  1. Enzymatic browning: Due to reaction of enzymes (polyphenoloxidase) with phenols, causing brown colors. This may be prevented by blanching prior to drying in order to inactivate enzymes
  2. Maillard reaction: Reaction of free amino acids and reducing sugars to form brown colors and flavors. This reaction is accelerated at high heat but can occur slowly at lower temperatures. Maillard browning is optimum at 20-25% moisture 
  3. Caramelization: Breakdown and polymerization of sugars on exposure to high heat to form colors and flavors
  4. Reduced water absorption: Proteins may be denatured and carbohydrates such as gums and starch hydrolyzed (split apart), reducing their functional ability to absorb water in the products in which they are used

Microbiological Effects of Dehydration

Drying achieves some microbial kill but cannot be relied on to destroy all vegetative cells. Spores will survive the drying process. Mild heat treatment such as when vacuum drying is used, will cause more microorganism to survive the drying process.  

Drying Methods

There are several types of drying methods, however they generally fall under one of three categories i.e. air convection, drum, and vacuum. 

  1. Air convection dryer: Use of hot circulating air in an enclosed system. As moist air is removed from the system, moisture is removed from the food
  2. Drum dryer: The food rotates in a thin layer on a circulating heated drum; normally heated from within by steam. At a point on the drum, the thin dry layer is scraped off
  3. Vacuum dryer: Dryers with the ability to dry at reduced pressures. These type of dryers are often used for sensitive foods, e.g. those prone to flavor loss. By applying vacuum, less heat is required to achieve the same results since low pressure reduces the boiling point of water. Vacuum drying is normally the most expensive drying method used.  

Freeze Drying

A unique type of vacuum drying is freeze drying. In this method, the food is frozen and then mild heat (heat of sublimation) is applied under vacuum to allow the food to sublimate from ice, directly to water vapor. Sublimation occurs below the triple point i.e., the temperature and pressure at which all states of matter (solid, liquid and gas co-exist). Due to the low temperature used, freeze-dried products maintain their color and flavor. Their size and bulk is also retained. Instead of shrinking, freeze dried products develop multiple pores like a sponge that allows water to escape. 

File:Phase-diag2.svg - Wikimedia Commons
Phase diagram of water

Dehydration and Water Activity

When foods are dried, their water activity is reduced. Water activity gives you an indication of how bound or free the water is in the food. Water activity ranges from 0 to 1, where higher values indicate more free water. It can be calculated as RH/100, where RH means relative humidity, or vapor pressure surrounding the food/vapor pressure of pure water. In the lab, water activity is determined quickly using a water activity meter.

At low water activity (0 – 0.25), water exists as a monolayer around the food material. This is also referred to as bound water because it is chemically bound to components such as lipids, proteins and carbohydrates. Between 0.25 and 0.75, water exists as a multilayer. It is therefore less tightly bounded and can be more easily removed. As the water activity approaches 1, it exists as free water. This is the easiest, and first to be removed during drying.

Courtney Simons
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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