Sterilization of Pepper

In order to ensure high quality and freedom from microbial contamination, cleaned and dried pepper is subjected to sterilization. For this, several methods are available including hydrostatic/pressure sterilization, ozone sterilization, compressed carbon dioxide treatment, irradiation, microwave heating, alcohol vapour treatment, extrusion, steam treatment and fumigation.

Hot Air/Steam Sterilization

The spice is preheated to a temperature of 50–55°C and then sterilised by a combination of indirect heating and direct steam injection. It was found that the total counts, yeast, moulds, coliform counts etc. were considerably reduced. The sensory quality as well as volatile oil content remained unaffected ().

Another process involves exposure to elevated pressures and temperatures for a predetermined time in a series of chambers. The material to be sterilised is placed in the first chamber and pressurised, then transferred to subsequent chambers and exposed to respective sterilization regimes for set times. The sterilised material is subsequently depressurised in a chamber ().

A modified steam sterilization process consists of steam jacketed pressurised vessel with three temperature envelopes (50, 100 or 120°C) for different products. The pressurised system ensures no loss of oil or moisture and the product appearance is unchanged. Steam, compressed air and nitrogen gas may be used depending on the end product ().

A continuous steam sterilization method developed involve subjecting the spice to a rapid flow of superheated steam for a predetermined period of time followed by drying, rehumidification and packaging. Microbial levels as well as enzyme activity are considerably reduced to low levels and no significant oil or flavour loss was reported (). There are reports about the use of pulsed steam for the sterilization of pepper where the spice is subjected to rapid pulses of steam. The steam condenses on the surface of the spice and the contaminants are removed (). In countries where sterilization by radiation as well as by chemical methods are not permitted, steam sterilization is the best alternative.

Gamma Irradiation

Gamma irradiation is considered to be a superior method for sterilization and insect disinfestation due to its ability to treat items which have been prepacked. This irradiation treatment is a cold process and hence the loss of volatile or flavour components are practically absent (). Low dose of 1 kGy (KiloGray) is effective in controlling insect infestation whereas a dose of 10 kGy is required to reduce the microbial load to acceptable levels. Unlike fumigation, this process does not leave any harmful residues. The size and shape of containers have to be selected depending on the source geometry as well as the conveyor system employed. Codex General Standards for irradiated foods and recommendations of the International Code of practices for the operation of irradiation facilities gives guidelines for irradiation procedures. It is essential to follow Codex Standards to facilitate international trade.

The sensory evaluation of the irradiated pepper showed that the sensory attributes are not affected in pepper subjected to 10 kGy of radiation dose. No significant change in the pepper oil content or piperine was observed in the irradiated spice. Studies have indicated that 30 kGy of radiation was required for heavily contaminated pepper because 10 kGy of radiation produced product with aerobic bacterial count of 2–4 log cycles.

Several countries including USA, France etc. have approved irradiation as a sterilization process and have started the commercial application of this technology. Over 95 per cent of the gamma irradiation facilities operating in the world use cobalt-60 sources. These are highly suited for treating bulk materials. The radiation energy emitted by cobalt-60 continuously decreases at the rate of about 13 per cent per year. The source have to replenished regularly to maintain the throughput of the process. A gamma irradiation facility generally consists of four main parts- a source for radiation, a shielded room to expose and store the sources, product conveyer with control mechanisms and safety devices. Among this the product conveyer and control system is the most critical component. The speed of the conveyer is the only parameter for control of the process and hence an accuracy of 99 per cent is insisted upon. Gamma irradiation facilities are capital intensive and the unit cost of processing is highly sensitive to the scale of operation. The best way of achieving a minimum unit cost of processing is to operate round the clock at the maximum rated throughput that the facility is designed for. A gamma irradiator is simple to operate and maintain and a plant availability factor as high as 95– 97 per cent can easily be attained. Efforts should be made to utilise the full rated capacity of the gamma irradiator for economic operation.

Chemiluminescence, thermoluminescence and free radial dosimetry are employed as routine methods for the detection of irradiated pepper. Other methods include use of coloured indicators on the containers to be exposed to radiation, viscosity measurements of gelatinised pepper and intensity measurements of reflected signals of Electron Spin Resonance (ESR) spectrum.

Chemical Sterilization

Fumigation with ethylene dibromide for insect disinfestation followed by ethylene oxide or propylene oxide for destroying microbes are the most common chemical processes employed for the sterilisation of pepper. The effectiveness of sterilisation depends on the moisture content of the pepper, concentration of the gas, temperature and time of contact. The volatile and non-volatile contents of pepper are reported to be reduced on fumigation.

The major problem associated with the use of these fumigants are their explosive, toxic and irritant nature. They are unsafe for the workers and packaged items cannot be subjected to fumigation. Further it leaves behind harmful residues and affects the organoleptic properties. Because of all these, many countries have banned the use of fumigants.

Microwave Sterilization

Laboratory and industrial scale trials conducted on sterilization of pepper by microwave heating (2450 MHz) or high frequency heating (27.12 MHz) indicated that microbial counts were reduced by a factor of 10–103 which is sufficient for moderately contaminated spices. Microbicidal efficiency increased with increasing moisture content of the samples but resulted in loss of some of the essential oil constituents. Both the above methods of sterilization are comparable in all aspects.


Selections from the book: “Black Pepper. Piper nigrum”. Edited by P.N.Ravindran. Series: “Medicinal and Aromatic Plants — Industrial Profiles”. 2000.