Basic to advance in Blanching

 

Blanching

Blanching serves a variety of functions, one of the main ones being to destroy enzymic

activity in vegetables and some fruits, prior to further processing. As such, it is not

intended as a sole method of preservation but as a pre-treatment which is normally carried

out between the preparation of the raw material and later operations

(particularly heat sterilization, dehydration, and freezing).

Blanching is also combined with peeling and/or cleaning of food, to achieve

savings in energy consumption, space, and equipment costs.

A few processed vegetables, for example, onions and green peppers, do not require

blanching to prevent enzyme activity during storage, but the majority suffer considerably

loss in quality if the blanching is omitted or if they are under-blanched. To achieve adequate

enzyme inactivation, food is heated rapidly to a pre-set temperature, held for a pre-set

time and then cooled rapidly to near ambient temperatures. The factors which influence

blanching time is:

• type of fruit or vegetable

• size of the pieces of food

• blanching temperature

• method of heating

Theory

The theory of unsteady-state heat transfer by conduction and convection, which is used to

calculate blanching time.

The maximum processing temperature in freezing and dehydration is insufficient to

inactivate enzymes. If the food is not blanched, undesirable changes in sensory

characteristics and nutritional properties take place during storage. In canning, the time

taken to reach sterilizing temperatures, particularly in large cans, may be sufficient to

allow enzyme activity to take place. It is, therefore, necessary to blanch foods prior to

these preservation operations. Under-blanching may cause more damage to food than the

absence of blanching does, because heat, which is sufficient to disrupt tissues and release

enzymes, but not inactivate them, causes accelerated damage by mixing the enzymes and

substrates. In addition, only some enzymes may be destroyed which causes increased

activity of others and accelerated deterioration.

The heat resistance of enzymes is characterized by D and z values.

Enzymes which cause a loss of eating and nutritional qualities in vegetables and fruits

include lipoxygenase, polyphenol oxidase, polygalacturonase, and chlorophyllase. Two

heat-resistant enzymes that are found in most vegetables are catalase and peroxidase.

Although they do not cause deterioration during storage, they are used as marker enzymes

to determine the success of blanching. Peroxidase is the more heat resistant of the two, so

the absence of residual peroxidase activity would indicate that other less heat-resistant

enzymes are also destroyed. The factors that control the rate of heating at the center of the

product  and can be summarised as:

• the temperature of the heating medium

• the convective heat transfer coefficient

• the size and shape of the pieces of food

• the thermal conductivity of the food.

Blanching reduces the number of contaminating micro-organisms on the surface of

foods and hence assists in subsequent preservation operations. This is particularly

important in heat sterilization, as the time and temperature of processing are

designed to achieve a specified reduction in cell numbers. If blanching is inadequate, a

larger number of micro-organisms are present initially and this may result in a larger

number of spoiled containers after processing. Freezing and drying do not substantially

reduce the number of micro-organisms in unblanched foods and these are able to grow on

thawing or rehydration.

Blanching also softens vegetable tissues to facilitate filling into containers and

removes air from intercellular spaces which increases the density of food and assists in

the formation of a head-space vacuum in cans.  

Equipment  

The two most widespread commercial methods of blanching involve passing food

through an atmosphere of saturated steam or a bath of hot water. Both types of equipment

are relatively simple and inexpensive. Microwave blanching is not yet used commercially

on a large scale.  

Steam blanchers

In general, this is the preferred method for foods with a large area of cut surfaces

as leaching losses are much smaller than those found using hot-water blanchers.

At its simplest a steam blancher consists of a mesh conveyor belt that carries

food through a steam atmosphere in a tunnel. The residence time of the food is

controlled by the speed of the conveyor and the length of the tunnel. Typically a 

tunnel is 15 m long and 1–1.5 m wide. The efficiency of energy consumption is 

19% when water sprays are used at the inlet and outlet to condense escaping steam. 

Alternatively, food may enter and leave the blancher through rotary valves or hydrostatic

seals to reduce steam losses and increase energy efficiency to 27%, or steam

may be re-used by passing through Venturi valves. Energy efficiency

is improved to 31% using combined hydrostatic and Venturi devices (Scott et al., 1981).

In conventional steam blanching, there is often poor uniformity of heating in the

multiple layers of food. The time-temperature combination required to ensure enzyme

inactivation at the center of the bed results in overheating of food at the edges and a

consequent loss of texture and other sensory characteristics. Individual quick blanching

(IQB) which involves blanching in two stages, was developed to overcome this problem

(Lazar et al., 1971). In the first stage, the food is heated in a single layer to a sufficiently

high temperature to inactivate enzymes. In the second stage (termed adiabatic holding) a

deep bed of food is held for sufficient time to allow the temperature at the center of each

piece to increase to that needed for enzyme inactivation. The reduced heating time (for

example 25 s for heating and 50 s for holding 1 cm diced carrot compared with 3 min for

conventional blanching), results in an improvement in the efficiency of energy

consumption to 86–91% (Cumming et al., 1984). The mass of product blanched per

kilogram of steam increases from 0.5 kg per kilogram of steam in conventional steam

blanchers to 6–7 kg per kilogram of steam, when small-particulate foods (for example

peas, sliced or diced carrots) are blanched.

Nutrient losses during steam blanching are reduced by exposing the food to warm air

(65ºC) in a short preliminary drying operation (termed ‘pre-conditioning’). Surface

moisture evaporates and the surfaces then absorb condensing steam during IQB. Weight

losses are reduced to 5% of those found using conventional steam blanching (Lazar et al.,

1971). Pre-conditioning and individual quick blanching are reported to reduce nutrient

losses by 81% for green beans, by 75% for Brussels sprouts, by 61% for peas, and by 53%

for lima beans and there is no reduction in the yield of blanched food (Bomben et al.,

1973).

The equipment for IQB steam blanching consists of a bucket elevator

that carries the food to a heating section. The elevator is located in a close-fitting tunnel

to reduce steam losses. A single layer of food is heated on a conveyor belt and then held

on a holding elevator before cooling. The cooling section employs a fog spray to saturate

the cold air with moisture. This reduces evaporative losses from the food and reduces the

amount of effluent produced. Typically the equipment processes up to 4500 kg h power -1 of

food. The complete inactivation of peroxidase is achieved with a minimum loss in

quality, indicated by the retention of 76–85% of ascorbic acid.

Batch fluidized-bed blanchers operate using a mixture of air and steam, moving at

approximately 4.5 m s power -1, which fluidizes and heats the product simultaneously. The

design of the blanching chamber promotes continuous and uniform circulation of the food

until it is adequately blanched. Although these blanchers have not yet been widely used at

a commercial scale, they are reported to overcome many of the problems associated with

both steam and hot-water methods (Gilbert et al., 1980). The advantages include:

• faster, more uniform heating

• good mixing of the product

• a substantial reduction in the volume of effluent

• shorter processing times and hence smaller losses of vitamins and other soluble heat

sensitive components of food.

A continuous fluidized-bed blancher is described by Philippon (1984).

 Hot-water blanchers

There are a number of different designs of blancher, each of which holds the food in hot

water at 70–100ºC for a specified time and then removes it to a dewatering-cooling

section. 

In the widely used reel blancher, food enters a slowly rotating cylindrical mesh drum

which is partly submerged in hot water. The food is moved through the drum by internal

flights. The speed of rotation and length control the heating time. Pipe blanchers consist

of a continuous insulated metal pipe fitted with feed and discharge ports. Hot water is

recirculated through the pipe and food is metered in. The residence time of food in the

blancher is determined by the length of the pipe and the velocity of the water. These

blanchers have the advantage of a large capacity while occupying small floor space. In

some applications, they may be used to transport food simultaneously through a factory.

Developments in hot-water blanchers, based on the IQB principle, reduce energy

consumption, and minimize the production of effluent. For example, the blancher-cooler

has three sections: a pre-heating stage, a blanching stage, and a cooling stage. The

food remains on a single conveyor belt throughout each stage and therefore does not

suffer the physical damage associated with the turbulence of conventional hot water

blanchers. The food is pre-heated with water that is recirculated through a heat

exchanger. After blanching, a second recirculation system cools the food. The two

systems pass water through the same heat exchanger, and this heats the pre-heat water

and simultaneously cools the cooling water. Up to 70% of the heat is recovered. A

recirculated water-steam mixture is used to blanch the food, and final cooling is by cold

air. Effluent production is negligible and water consumption is reduced to approximately

1 m3 per 10 t of product. The mass of product blanched is 16.7–20 kg per kilogram of

steam, compared with 0.25–0.5 kg per kilogram in conventional hot-water blanchers.