Blanching is a thermal pretreatment applied primarily to vegetables and fruits before freezing, canning, or dehydration. The primary objective is enzyme inactivation, particularly of peroxidases, polyphenol oxidases, and lipoxygenases, which would otherwise cause quality deterioration such as off-flavors, texture softening, and color changes during frozen or dehydrated storage. The peroxidase test is used as an indicator of adequate blanching because peroxidases are among the most heat-stable enzymes; a negative peroxidase test confirms sufficient enzyme inactivation.
Hot water blanching uses temperatures of 75-95°C and is the most common method, especially for leafy vegetables and cut products. It offers uniform heat transfer and relatively low capital cost. Steam blanching uses saturated steam at atmospheric pressure and is preferred for products where water-soluble nutrient retention is critical, as leaching losses are minimized. Individual quick blanching (IQB) uses a two-stage process: a high-temperature short-time exposure followed by a tempering period to equilibrate heat. Microwave and ohmic blanching are emerging alternatives.
Blancher designs include belt blanchers for continuous processing of loose products, rotary drum blanchers for gentle handling of fragile items, and pipe blanchers for particulate products in liquid streams. Critical process control factors include time and temperature (product-specific, typically 1-10 minutes), piece size and geometry (smaller pieces require shorter times), and throughput rate. Cooling after blanching is essential to stop the thermal effect and is usually achieved by cold water sprays or immersion.
Nutrient losses during blanching result from leaching of water-soluble vitamins (vitamin C, B vitamins), thermal degradation, and oxidation. Vitamin C losses can range from 10-30% depending on product and method. Steam blanching generally retains more nutrients than water blanching. Recovery of blanch water for nutrient recovery or energy optimization is practiced in modern processing plants. Wastewater from blanching has a high biological oxygen demand (BOD) and requires treatment. Blanching inactivates enzymes responsible for enzymatic browning and precedes pasteurization or commercial sterilization in many processing lines.
