Blanching (cooking)

Application

Blanching is used both in home kitchens and as a pre-treatment in the food industry, primarily to deactivate enzymes that cause browning, textural changes, and off-flavors. Key enzymes targeted include lipoxygenase, polyphenoloxidase, polygalacturonase, and chlorophyllase, with catalase and peroxidase serving as indicators of blanching success due to their high thermal resistance. The process expels air trapped in plant tissues, which is essential before canning to prevent container strain and faulty seams.

Removing intercellular gases also improves color retention and texture, as seen in pears. Blanching facilitates peeling of fruits, vegetables, and nuts; for almonds and pistachios, it softens the seed coat for easy removal. Steam peeling generates less environmental pollution and lower peeling losses than chemical or manual methods. Additional uses include enhancing drying rate, decreasing microbial load, removing pesticide residues, increasing extraction of bioactive compounds, surface cleaning, removing damaged seeds, killing parasites and their eggs, and reducing oil uptake.

Technology

Traditional blanching uses a water bath or saturated steam, with food heated briefly then cooled by cold water or cold air. Industrially, foods move on a continuous conveyor belt through preheating, holding, and cooling stages. Hot water blanching immerses vegetables in water at 70 to 100 °C, offering uniform heating that allows lower temperatures but requires longer times; water is recirculated via heat exchangers to reduce costs. Steam blanching injects saturated steam at about 100 °C onto food on a conveyor, reducing leaching of water-soluble compounds and being preferred for smaller or cut-surface foods.

It is more energy-efficient and allows shorter processing times, preserving color and flavor, though evaporation may lower product yield. After heating, foods are chilled by cold water or cold air; air cooling prevents nutrient leaching but causes evaporation and mass loss, a monetary disadvantage. Emerging technologies include ohmic, infrared, microwave, and radio frequency blanching.

Time and temperature

Recommended blanching times and temperatures depend on food type, size, and shape. Over-blanching causes excessive loss of nutrients, aromatic compounds, and softening. Under-blanching may fail to inactivate all enzymes and can release additional enzymes from plant tissue, increasing overall enzymatic activity and accelerating spoilage.

Disadvantages

Blanching times and temperatures are insufficient to destroy all microorganisms, so it is typically a pre-process before freezing, canning, dehydration, or heat sterilization. Hot water blanching leads to leaching of water-soluble nutrients such as vitamins, minerals, proteins, sugars, and flavor compounds, with the degree of loss depending on food composition, water-to-food ratio, and temperature. Heat-sensitive compounds like ascorbic acid, thiamin, and many aromatics are particularly affected. Industrial water blanching produces effluent containing water-soluble pollutants that require treatment before discharge, increasing capital costs and driving development of newer technologies.