Ohmic heating, also known as electrical resistance heating or Joule heating, passes an alternating electrical current through a food product, generating heat internally by the electrical resistance of the food. The heating rate depends on the electrical conductivity, which varies with temperature, ionic content, and product composition. Ohmic heating provides uniform and rapid volumetric heating without the temperature gradients typical of conventional heat exchangers, making it ideal for viscous liquids, particulate foods, and products containing large solid pieces.
Electrode materials for ohmic heating must be non-corrosive and food-grade; titanium, stainless steel, and platinized titanium are commonly used. The electrical conductivity of foods increases with temperature and ionic strength, but fat and oil phases have very low conductivity, potentially causing uneven heating. Frequencies of 50-60 Hz are standard, but higher frequencies (up to 20 kHz) can reduce electrolytic reactions at electrode surfaces. Applications include sterilization of fruit pulps, processing of liquid egg, and blanching of vegetables.
Microwave heating uses electromagnetic radiation at 915 MHz or 2450 MHz (the ISM band) for industrial applications. The heating mechanism involves dielectric polarization: water molecules and polar compounds align with the oscillating electric field, generating heat through molecular friction. Penetration depth is frequency-dependent and decreases with higher moisture content and ionic strength. At 2450 MHz, penetration depth in high-moisture foods is typically 1-3 cm, while at 915 MHz it reaches 3-10 cm.
A key challenge in microwave processing is the formation of hot and cold spots due to non-uniform field distribution. Mode stirrers, rotating turntables, and multiple feed ports are used to improve uniformity. Combined microwave and conventional heating (hybrid systems) can overcome surface overheating and center undercooking problems. Applications include tempering of frozen meat and fish, microwave-assisted drying, pasteurization of prepared meals, and microwave vacuum drying for high-quality dried fruits and vegetables. Novel heating methods offer alternatives to conventional pasteurization and commercial sterilization. Unlike UHT processing, ohmic heating can process particulate foods without surface overheating.
