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Vacuum steam blanching

Vacuum steam blancing is carries out in an end-over-end conical jacketed cooker.

Vegetables are loaded are loaded into cooker and a slow rotation started as pressure is

reduced to about 10 kPa. Vacuum is broken with saturated steam aand the vessel

brought to atmospheric pressure, which is held for about one minute. Vegetables are

then vacuum cooled by again reducing pressure in vessel. The liquid in vessel flash boils

under reduced pressure cooling the vegetables.

Advantage of vacuum steam blanching is the rapid heating of vegetables by

condensation of steam and hence liberation of latent heat of condensationin tissue.

Because for the high temperature short time blanching, this products firmer textures

blanched vegetables, resulting in superior frozen products. System is energy and effluent

efficient. This type of blanching is also used commercially in mushroom industry where

the shrinkage of blanched mushroom is not as great as with water blanching. System is,

however, only a batch operation and good vacuum pump is required to achieve low

pressure.

Hot water is also used in a tunnel type system. In this blanching system, water is sprayed

over the product at various stages of heating, holding and cooling as the product is

carried on a conveyor through tunnel. Here product is exposed initially to a pre heat

section. Then the product is sprayed with hot water at desired blanching temperature for

an established period of time. Following blanching stage, product passes through a pre

cool stage and lastly to final cooling stage. In the same manner as rotary blanching

system, time duration and temperatures required for blanching are accomplished by

control of water temperatures and by control of exposure time. In case of tunnel type

system, time is controlled very specifically by speed of conveyor through tunnel.

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Use of microwaves is tried as an alternate to blanch vegetables. This system is

specifically used for blanching of corn, potatoes and fruits. However, there is no

commercial plant, initiated with microwave power. Results of reported work differ widely,mainly because microwave effect is not as easy to quantify as methods using a tank of

bolling water or a steam cabinet.

In this method, vegetables are heated with a mixture of steam and flue gases from

natural gas burners. Advantage of the system is to reduce amount of effluent. However,

an appreciable reduction in yield was noticed, which would create difficulties for

commercial acceptance of system.

This is modified by making product entry and dischange through a water seal system.

Removable cover is also water sealed and lifted on a gantry to allow blanchers to be

easily cleaned.

Hot gas blanching using combustion gas as the heat transfer medium has been

successfully applied to spinach and other vegetables. The main advantage of the method

is that there is practically no effluenr since there is no steam condensation. In fact the

product generally loses considerable water in the process and the hot gas steam must be

humidified. Since partial dehydration can be accomplished in hot gas blanching. The

method is particularly well suited for products that are subsequently to be dried. The

method is not well suited for products such as corn since the high temperatures

employes can cause surface browning