Both enzymes and microorganisms have a high heat sensitivity and can be destroyed by exposure to the temperature of boiling water at 70-105°C. This heat treatment process used prior to freezing is called blanching must be applied for a time sufficient to inactivate enzymes which cause off-flavors, off-texture, off-color, and loss of nutritive quality. The development of the enzymes and microorganisms in certain fruits and vegetables (e.g. beans) is stopped by blanching before freezing; otherwise these products lose their green color and become brown. Blanching not only serves to inactivate enzymes but also partially precooks the tissues, which renders the cell membranes more freely permeable to moisture, leading to higher freezing rates. In meat products, enzyme development is stopped directly by freezing. The advantages of blanching can be listed, including:
• inactivation of undesirable enzymes in plant materials,
• destruction of microorganisms, and
• stabilization of texture, color, aroma, flavor and nutritional quality.
However, some disadvantages of blanching that may occur are:
• some loss of texture, color, flavor, and nutritional quality,
• formation of a cooked taste,
• some loss of soluble solids, and
• use of large amounts of water and energy.
A blancher’s capacity depends on process, time and product. At 3 minutes process time the blancher is capable of handling 6.7 ton/h of peas and/or 3.1 ton/h of whole green beans. This is an important factor when planning lines for a combination of products.
Commercially, a large number of domestic and industrial blanchers using either hot water or steam for food freezing applications are used. The following are the most common ones:
imersion type blanchers,
belt type blanchers,
open screw type blanchers, and
rotary type blanchers.
Figure 7.2 shows an industrial belt type steam blancher which is of completely stainless steel construction. It is commonly used for a wide range of vegetables such as spinach, peas, carrots, turnips, and potatoes. During the past decade a new concept of integrated blanching and cooling was introduced as one of the most promising technologies due to lower energy consumption and high-quality products. In this integrated blancher, the main idea is that cold products enter the blancher-cooler unit, and if cold products also leave the unit, no heating energy is in principle needed for the process. In this system small amounts of water and energy are used because of the use of a countercurrent flow of products and reuse of the water. This system is recommended mainly for leafy vegetables. Poulsen (1989) explained this technique in detail with examples. Figure 7.3 exhibits a cross-section of the double-spray system. In the system the lower spray nozzles are placed in a staggered arrangement in every second row so that the turbulence produced lifts and turns over all parts of the vegetables. In such a blanching-cooling system, there are five main steps, in which the vegetables are carried through by a conveyor belt, including:
• Step 1: The vegetables are heated to approximately 70°C by spraying hot water over them; then hot water is collected under the belt and recirculated in a five-zone counterflow arrangement.
• Step 2: The vegetables are heated quickly to 90°C by spraying recirculated hot water
over them; the water is collected under the belt.
• Step 3: The temperature is kept constant without water spraying.
• Step 4: The vegetables are cooled in a counterflow by cooling water.
• Step 5: Before freezing, the vegetable temperature is reduced further while at the same time surplus water is removed by evaporation.