Tea Processing: Rolling and Fermentation


The aim of rolling is to establish proper condition for an enzymatic oxidation (fermentation) of catchin and polyphenols by polyphenolase in the presence of atmospheric oxygen. Rolling twist the leaf, break the cell structure and expel the juices, catchin and enzymes. Catchin and enzymes are originally separated in fresh leaf by cell compartment. During rolling, both the polyphenols and enzymes polyphenolases are brought together and are exposed to the atmospheric condition that begins the oxidation process. Read more

Non Thermal Preservation of Food-Pulsed Electric Field

In this method of food preservation, electric energy at low power is collected over an extended period and stored in capacitor. That same energy is discharged, almost instantaneously at high level of power. When exposed to electric field, plasma membrane of cell becomes permeable to small molecules and the permeation causes swelling and eventual rupture of cell membrane. Rupture of cell membrane depends on factors such as intensity of electric field, number of pulses and duration of pulses. The induced transmembrane potential larger than natural potential of cell membrane cause rupture of cell membrane. Generation of pulse electric requires two devices; a) Pulse power supply and b) Treatment chamber. Read more

Non Thermal Preservation of Food- High pressure processing (HPP)

High pressure presents unique advantage over conventional food processing including application at low temperature which improves the retention of food quality. High pressure treatment are independent of product size and geometry and their effect is uniform and instantaneous. The food is compressed by uniform pressure from every direction and then returns to original shape when pressure is released.
Processing operation: A sterile container filled with food is sealed and placed in the pressure chamber for pressurizing. Ethylene Vinyl Alcohol (EVOH) and Poly vinyl Alcohol (PVOH) films are recommended for packaging food for high pressure treatment. Also existing multilayer plastic and some aluminum packages may be used. No deformation of the package occurs because the pressure is uniform. The basis of applying high pressure to food is to compress the water surrounding the food. At room temperature, volume of water decreases.  Food is subjected to high pressure for a specified time period. The holding time in the pressure vessel depends on type of food and process temperature. At the end of processing time, the chamber is decompressed to remove treated batch. Read more

Non Thermal Preservation of Food – Dielectric Heating

Dielectric heating is also called microwave heating or high frequency heating. Dielectric energy is form of electromagnetic energy. They are transmitted as wave which penetrate food and are then absorbed and converted into heat.
Principle: the majority of food contain substantial proportion of water. The molecular structure of water consist of negatively charged oxygen atom separated from positively charged hydrogen atom and this form an electric dipole. When a microwave or radio frequency electric field is applied to a food, dipoles in water and in some ionic components such as salt attempt to orient themselves to the field (in a similar way to a compass in a magnetic field). Since the rapidly oscillating electric field changes from positive to negative and back again several million times per second, the dipoles attempt to follow and these rapid reversals create frictional heat. The increase in temperature of water molecules heats surrounding component of food by conduction and or convection. The depth of penetration of microwaves energy is determined by
1. Dielectric constant
2. Loss factor of food Read more

Non Thermal Preservation of Food – Ohmic Heating

Food can be heated by either direct or indirect methods.
Direct heating: In this method of heating, heat is generated within the product. Example; Dielectric heating, Ohmic heating, Pulse Electric Field (PEF), High Pressure Technology (HPT).
Indirect heating: This method of heating relies on heat that is generated externally being applied to the surface of food mostly by radiation, convection and conduction. Example; Infrared heating.
Non thermal preservation of food includes food preservation by following methods.
1. Ohmic heating
2. Dielectric heating
3. Pulse electric field (PEF)
4. High pressure treatment (HPT)
5. Oscillating magnetic field (OMF)
Ohmic heating:
operates by direct passage of electric current through food product with heat generated as a result of electric resistance. In conventional heating, heat travels from heated surface to product by conduction and convection paths. Ohmic heating is also called “resistance heating” or “electro heating”. Here alternating electric current is passed through a food and electrical resistance of the food causes the power to be translated directly into heat. As the food is an electrical component of heater, it is essential that its electrical properties (resistance) are matched to the capacity of heater. Read more