LESSON 8 Flashcards
Principle of preservation by food dehydration
microbial growth, chemical and enzymatic reactions occur only if sufficient free water is present. When the water activity of foods is lowered there is a direct impact on microbial growth as well as chemical & enzymatic reactions.
Water activity of foods ranges..
from 0 to 1.0
Water activity of dehydrated foods is in the range of..
0.2 to 0.6
Microorganisms cannot grow at aw below…
0.6
Chemical reactions (e.g. Maillard browning) can begin to occur at aw of..
0.3
Dehydration definition
removal of as much water from the food as possible in order to impart a long storage life.
Concentration
some of the water is removed from the food in order to concentrate the food constituents. Concentrated foods are not inherently shelf-stable and require the use of other forms of food preservation
Changes in Food during Dehydration
- Cell/tissue shrinkage
- Case hardening
- Chemical changes; Maillard Browning, poor rehydration, protein denaturation, loss of volatile substances
Factors affecting dehydration
Surface area, temperature, air velocity, humidity of drying air, atmospheric pressure and vacuum
Sun drying
Sun drying is mostly used in dry, warm climates. This is a very slow drying method (several days). It is mostly used for fruits, vegetables, and fish. An advantage of this method is the fact that it is quite inexpensive; however, disadvantages include long drying periods (up to several weeks) and the risk of invasion by insects, birds, rodents, and microorganisms.
The appearance is shrunken and has poor rehydration capacity.
Spray Drying
Spray driers are used to produce the greatest quantities of commercially dehydrated foods. Spray driers are restricted to use with liquid foods since the principle of the operation is the introduction of the food as a spray of small droplets into a high velocity stream of warm air. Because droplet sizes are small, drying rates are very rapid and high quality dehydrated food products can be produced. Foods most commonly dehydrated by spray driers include skim milk, coffee, tea and eggs.
Tray (Air) Drying
Food placed on trays or racks is exposed to heated air at a set velocity. This type of drying can be quite fast and requires heated air with a relative humidity (%RH) lower than that of the product to be dried. Water soluble components migrate to surface of food and are deposited as water evaporates. The dried food has relatively poor re-hydration properties, is shrunken in appearance and is very dense. Products dried this way are: pasta, vegetables, fruit, spices.
Drum Drying
During the process of drum drying, food paste is applied to a heated drum in a thin layer to promote rapid drying. As the drum rotates, it picks up a thin film of food material that dries rapidly. The dried food is scraped off the drum near the end of a full rotation of the drum. Dehydrated mashed potatoes and some ready-to-eat breakfast cereals are dried this way. Some popular low fat snack foods and potato chip like products contain drum dried potato flakes as the primary ingredient.
Freeze Drying
During freeze drying, water is removed from food in the frozen state without transition through the liquid state.
Sublimation
Water is removed from food in the frozen state without transition through the liquid state. Sublimation of water is accelerated under vacuum conditions. Application of heat from radiant heaters within the shelves of the freeze drier provides the energy required for sublimation to occur.
Vacuum microwave or radiant energy vacuum (REV)
The technology consists of a combination of vacuum (in order to keep the temperature low) and microwaves (for ultra-rapid energy transfer), producing high quality products with less nutrient loss, better flavour retention, and less colour change
Deep Fat Frying
During deep fat frying, the high temperature of hot oil causes water in the food to evaporate rapidly, often accompanied by substantial pick up of oil by the food being dried.
Extrusion (cooking) Drying
Slurry of food is passed though a tube, under pressure, that is heated by steam. The moist heat causes starch gelatinization and cooking of the product. Product is forced though a narrow opening (a die which can produce a product with a variety of shapes) at the end of the tube and escaping steam causes the dehydrating product to puff. Many ready-to-eat breakfast cereals and snack foods are produced this way.
