Blanching

Introduction

• Most vegetables and some fruits are blanched prior to further processing operations

• Blanching is a mild heat treatment, but is not a method of preservation.

• It is a pretreatment usually performed between preparation and subsequent processing.

• Blanching consists of heating the food rapidly to a predetermined temperature, holding for a specified time, then either cooling rapidly or passing immediately to the next processing stage.

Purpose

• The major purpose : inactivate enzymes, reduce number of m.o

• Effect : reduce quality

Blanching causes

• cell death and physical and metabolic chaos within the cells.

• leads to enzyme destruction as well as damage to the cytoplasmic and other membranes, which become permeable to water and solutes

• loss of turgor pressure

• Water and solutes may pass into and out of the cells, a major consequence being nutrient loss from the tissue.

• cell constituents, which had previously been compartmentalised in subcellular organelles, become free to move and interact within the cell.

Effect of blanching on cell tissues

S, starch gelatinised; CM, cytoplasmic membranesaltered; CW, cell walls little altered; P, pectins modified; N, nucleus and cytoplasmic proteins denatured; C, chloroplasts and chromoplastsdistorted

Role of Blanching

• Storage product

• Packaging

• Further benefit

A. Storage product

• loss of colour or texture, production of off odours and flavours or breakdown of nutrients --- enzymes activities peroxidases, catalases and lipoxygenases.

• Peroxidase and catalase : indicator of blanching effectiveness

• peroxidase is chosen due to easy to measure and it is the most heat resistant of the enzymes

• complete inactivation of peroxidase may not be necessary and retention of a small percentage of the enzyme following blanching of some vegetables may be acceptable

B. Packaging

• Blanching causes the removal of gases from plant tissues :---- helpsachieve vaccuum in the cans, preventing expansion of air during processing and hence reducing strain on the containers and the risk of misshapen cans and/or faulty seams., removing oxygen is useful in avoiding oxidation of the product and corrosion of the can, removal of surface dust affects in brightening the colour of someproducts, especially green vegetables.

• Shrinking and softening of the tissue :--- benefit in terms of achieving filled weight into containers, possible to reduce the tinplate requirement in canning, facilitate the filling of containers.

• controling the time/temperature : ----- avoid overprocessing, leading to excessive loss of texture

• Calcium chloride addition to blanching water helps to maintain the texture of plant tissue through the formation of calcium pectatecomplexes.

C. Further benefit

• final cleaning and decontamination process.

• Removing pesticide residues from the surface of vegetables

• Reductions in microorganism content : ---very useful in frozen or dried foods where surviving organisms can multiply on thawing orrehydration and in before heat sterilisation if large numbers of microorganisms are present before processing.

Controlling factors

• Factors :– fruit or vegetable properties, especially thermal conductivity, which

will be determined by type, cultivar, degree of maturity etc.;– overall blanching effect required for the processed product, which

could be expressed in many ways including: achieving a specified central temperature, achieving a specified level of peroxidaseinactivation, retaining a specified proportion of vitamin C;

– size and shape of food pieces;– method of heating and temperature of blanching medium.

• Time/temperature combinations vary very widely for different foods and different processes and must be determined specifically for any situation. Holding times of 1–15 min at 70–100 C are normal.

Efek pada pangan

• Nutrisi

• Warna dan flavour

• Tekstur

Nutrisi

• 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).

Warna dan flavour

• 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

• 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. Whencorrectly 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.

Tekstur

• 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 oftexture 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.

Blanching Equipment

blancher–cooler

IQB steam blanching

counter-current blancher

Advantages and limitations of conventional steam and hot-water blanchers

Prediksi waktu blanching

PINDAH PANAS PADA BLANCHING: UNSTEADY STATE

• pindah panas yang suhunya berubah karenabahan dipanaskan atau didinginkan.

• Proses sangat rumit, melibatkan penyelesaianmenggunakan persamaan Fourier yang ditulisdalam bentuk diferensial parsial dalam tigadimensi.

persamaan

• faktor suhu, biot number, fourier number