Blanching

Blanching:

      Blanching is a unit operation prior to freezing, canning, or drying in which fruits and vegetables are heated for the purpose of inactivating enzymes, modifying texture, preserving color, flavor and nutritional value and removing trapped air.

      It is a mild heat treatment but is not a method of preservation. It is a pretreatment usually performed between preparation and subsequent processing.

      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

How to done blanching?

Blanching consists of heating the food rapidly to a predetermined temperature, holding for a specified time and then cooled rapidly to near ambient temperature or passing immediately to the next processing stages.

Purpose of blanching:

• The major purpose of blanching is frequently inactivate enzymes which otherwise would lead to quality reduction in the processed product.

Eg. With frozen products, deterioration could take place, during any delay of processing, during freezing or during subsequent thawing.

• Blanching causes  the removal of gasses from plant tissues specially intercellular gas

Eg. Especially useful prior to canning where help to achieve or create vacuum in container preventing expansion of air during processing.

• To bring about shrinkage in order to pack sufficiently high filling weights.
• It act as a final cleaning and decontamination process.

Eg. It remove pesticide residue, toxic constituent s, from the food material by leaching process during blanching.

• To prevent an increase in pressure developing In the can s during processing.
• To prevent loss of color and flavor.
• It also reduces microbiological content or vegetative cell content during processing.

Eg. Which is  useful for frozen or dry food where the chance of multiplying microorganism have during thawing or dehydration.

Processing condition:

1. Under blanching:
2. Marker enzymes

Factors that are necessary to considered during blanching process:

Following factor are must be considered for selecting time and temperature during blanching.

      Fruits and vegetables properties

-Thermal conductivity

-Degree of maturity]

      Overall blanching effect required for the processed product.

-achieving specified center temperature

-inactivation peroxidase

-treating the nutrient content

      Size and shape of the food pieces

      Method of heating and temperature of the blanching medium.

N.B: time and temperature combination may vary very widely for different food and different process and must be determined specially any situation holding time 1-15 minutes  at 70-100⁰C are normal.

Blanching equipment:

There are two type of equipment are used in blanching

1. Steam blanching.
2. Hot water blanching.

Steam blanching:

Steam blanching consist of a mesh conveyor belt that carries food through a steam atom tunnel ,the residence time of the food is controlled by the space of the conveyor and length of the tunnel .steam blancher are two type:

1. Individual quick blancher(IQB)
2. Batch fluidized-bed  blancher

Individual quick blancher (IQB):

Fig: IQB steam blancher

The equipment for IQB steam blancher (Fig. 10.1(a)) consists of a bucket elevator which 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 of À 1food. The complete inactivation of peroxidase is achieved with a minimum loss in quality, indicated by the retention of 76–85% of ascorbic acid.

It is three stage blanching process in which the vegetable pieces are heated rapidly in the thin layers by steam followed by holding in a deep bed where temperature  equilibration take places, after which the material is cooled down. the method has the advantages of short residence time improved yield and reduced wastewater.

Batch fluidized-bed  blancher:

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

4.5 m/s, which fluids 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 (Gilbertet 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.

Hot water blancher type:

1. Reel  hot water blancher:
2. Pipe  hot water blancher

Reel blancher:

Food enters a slowly rotating cylinder mesh drum which is partially submerged in hot water. The food is moved through the drum by internal flight. The speed of rotation and length control the heating time.

Pipe blancher:

Pipe blancher consists of a continuous insulated metal pipe fitted with feed and discharge ports. Hot water is recirculates 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. The blanchers have the advantage of a large capacity while occupying a small floor space. In some applications they may be used to transport food simultaneously through a factory.

v  Advantage and limitations of conventional steam and hot water blancher:

Equipment	Advantages	Limitations
Conventional steam blancher	Smaller loss of water soluble components       Smaller volume of waste       Lower disposal charges       Easy to clean and sterilize	Limited cleaning of food Higher capital costs       Uneven blanching       Some mass loss in food       Poor energy efficiency
Conventional hot water blancher	Lower capital costs       Better energy efficiency	Loss of water soluble components       Higher cost of water and disposal of effluent       Risk of contamination

Effect on food during blanching:

1. Nutrient content:

Some minerals, water-soluble vitamins and other water-soluble components are lost during blanching. Losses of vitamins are mostly due to leaching, thermal destruction and, to a lesser extent, oxidation. The extent of vitamin loss depends on a number of factors including:

·         the maturity of the food and variety

·         methods used in preparation of the food, particularly the extent of cutting, slicing or dicing.

·         the surface-area-to-volume ratio of the pieces of food

·         method of blanching

·         time and temperature of blanching (lower vitamin losses at higher temperatures for shorter times)

·         the method of cooling

·         The ratio of water to food (in both water blanching and cooling).

Losses of ascorbic acid are used as an indicator of food quality, and therefore the severity of blanching (Table 10.2).

2. Color and flavor:

Blanching brightens the colour of some foods by removing air and dust on the surface and thus altering the wavelength of reflected light. The time and temperature of blanching also influence the change in food pigments according to their D value (Chapter 1).Sodium carbonate (0.125% w/w) or calcium oxide are often added to blancher water to protect chlorophyll and to retain the colour of green vegetables, although the increase in pH may increase losses of ascorbic acid. Enzymic browning of cut apples and potatoes is prevented by holding the food in dilute (2% w/w) brine prior to blanching. When correctly blanched, most foods have no significant changes to flavour or aroma, but under-blanching can lead to the development of off-flavours during storage of dried or frozen foods (Chapters 15 and 21). Changes in colour and flavour are described in more detail by Selman (1987).

3. Texture:

       One of the purposes of blanching is to soften the texture of vegetables to facilitate filling into                         containers prior to canning. However, when used for freezing or drying, the time–temperature conditions needed to achieve enzyme inactivation cause an excessive loss of texture in some types of food (for example certain varieties of potato) and in large pieces of food. Calcium chloride (1–2%) is therefore added to blancher water to form insoluble calcium pectate complexes and thus to maintain firmness in the tissues.