Blanching and it's techniques, Lecture notes of Food Science and Technology

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BLANCHING

Presented by: Miss Kulsoom Ibrahim

Learning Objectives:

To have better understanding of blanching.

To have keen knowledge of practicality of process.

To know the industrial insight of process.

To understand blanching equipment principle and processing consideration.

Mechanisms and Purposes of Blanching  (^) The major purpose of blanching is frequently to inactivate enzymes that would otherwise lead to quality reduction in the processed product.  (^) For example, with frozen foods, deterioration could take place during any delay prior to processing, during freezing, during frozen storage, or subsequent thawing.  (^) Similar considerations apply to processing, storage, and rehydration of dehydrated foods.  (^) Enzyme inactivation prior to heat sterilization is less important as the severe processing will destroy any enzyme activity, but there may be an appreciable time before the food is heated to sufficient temperature, so quality may be better maintained if enzymes are destroyed prior to heat sterilization processes such as canning.  (^) It is important to inactivate quality-changing enzymes, that is, enzymes which will give rise to loss of color or texture, the production of off-odors and flavors or breakdown of nutrients.  (^) Many such enzymes have been studied including a range of peroxidases, catalases, and lipoxygenases.

 (^) Peroxidase and to a lesser extent catalase are frequently used as indicator enzymes to determine the effectiveness of blanching.  (^) Although other enzymes may be more important in terms of their quality- changing effect, peroxidase is chosen because it is extremely easy to measure and it is the most heat resistant of the enzymes.  (^) Blanching causes the removal of gases from plant tissues, especially intercellular gas. This is especially useful prior to canning where blanching helps achieve vacua in the containers, preventing expansion of air during processing.  (^) In addition, removing oxygen is useful in avoiding oxidation of the product and corrosion of the can. Removal of gases, along with the removal of surface dust, has a further effect in brightening the color of some products, especially green vegetables.  (^) Shrinking and softening of the tissue is a further consequence of blanching. This is of benefit in terms of achieving filled weight into containers.  (^) It is important to control the time/temperature conditions to avoid overprocessing leading to excessive loss of texture in some processed products.  (^) Calcium chloride addition to blanching water helps to maintain the texture of plant tissue through the formation of calcium pectate complexes.  (^) A further benefit is that blanching acts as a final cleaning and decontamination process.  (^) The effectiveness of blanching in removing pesticide residues from the

Types and equipment of blanching: The two main approaches in commercial practice are to convey the food through saturated steam or hot water.

1. Steam Blanching: Conventional steam blanching consists of conveying the material through an atmosphere of steam in a tunnel on a mesh belt. Uniformity of heating is often poor where food is unevenly distributed, and the cleaning effect on the food is limited. However, the volumes of wastewater are much lower than for water blanching. Fluidized bed designs and individual quick blanching (a three-stage process in which vegetable pieces are heated rapidly in thin layers by steam, held in a deep bed to allow temperature equilibration, followed by cooling in chilled air) may overcome the problems of non-uniform heating and lead to more efficient systems.

2. Hot water blanching:  (^) The two main conventional designs of hot water blancher are reel and pipe designs.  (^) In reel blanchers the food enters a slowly rotating mesh drum which is partly submerged in hot water. The heating time is determined by the speed of rotation.  (^) In pipe blanchers, the food is contacted with hot water recirculating through a pipe. The residence time is determined by the length of the pipe and the velocity of water. 3. Microwave blanching:  (^) Microwave-assisted blanching (MAB) is recognized as a technology that reduces the time and energy required to achieve the inactivation of enzymes since it is a method that allows high temperature/short time treatment of solid foods through volumetric heating. Microwave blanching has been demonstrated on an experimental scale but is too costly at present for commercial use and several disadvantages related to products quality.