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.
Effect of high pressure on biological material
1. Microorganism: High pressure sensitivity varies with type of microorganism. Pressure sensitive microbial cells are inactivated by 400 – 600 MPa. Spores resist pressure higher than 1000 MPa at ambient temperature. Gram positive bacteria are more pressure resistant than gram negative bacteria. Among gram positive bacteria, staphylococcus is one of the most pressure resistant and can survive treatment at 500 MPa for more than 60 minutes.
2. Vegetative cells: Baroduric or barotolerant organisms that can grow at pressure of 40 – 50 MPa and survive pressure > 200 MPa for prolonged period. 6 log cycles of coli can be reduced at pressure higher than 405 MPa for 10 minutes.
3. Microbial spores: Pressure resistant spore proteins are protected against solvation and ionization. The structure and thickness of spore coats are believed to account for its high pressure. High pressure can cause spore germination and subsequent high pressure treatment can cause inactivation of germinating cells. Bacillus spores treated at 25 MPa and 50°C for 30 minutes can cause germination of 50 – 64 % of initially inoculated spores. However, there is no possibility of sterilization of low acid food at low temperature.

Effect of high pressure mechanism action on microorganism:
Cell membrane is the most possible key site of disruption. Inactivation of key enzyme including those involved in DNA replication and transcription is also involved as a possible inactivating mechanism. High pressure can cause nuclear membrane disruption in yeast cell. Microbial death is attributed to permeability of cell membrane after high pressure treatment.

Effect of high pressure on chemical and biochemical reaction:
High pressure of 100 – 200 MPa causes destruction of oligomeric structure into their subunit, partial unfolding and denaturation of monomeric structure and protein aggregation.

Effect of high pressure on enzymatic reaction:
Inactivation of enzymes by application of high pressure is influenced by pH, substrate concentration, the subunit structure of enzyme and temperature during pasteurization. High pressure inactivation can be attributed to an alternation of intermolecular structure or conformation changes at active site. Some enzymes such as phosphate and peroxidase has been successfully deactivated during milk treatment. High pressure in the range of 100 – 300 MPa for 30 minues reduce proteolytic activity of fish muscle and enhance texture of fish muscle.
Hot water blanching causes thermal damage, leaching of nutrients and possible environmental pollution. However, high pressure treatment avoid mineral leaching accumulation of waste water and produce less effluent because less water is required than in hot water blanching. High pressure treatment at 900 MPa for 10 minutes reduce the peroxidase activity by 88 % in green pea.

Effect of high pressure on functional properties:
High pressure greatly influences ionic or hydrophobic interaction and hydrogen bonding (to lesser extent). Hydrogen bonding stabilize protein structure (α- helix and β-pleated sheet) and results in shortening of interatomic distances with corresponding decrease in volume. Therefore, high pressure treatment gives denatured effect and increase in volume.
Exposure to high pressure unfolds protein which results in alteration of functional properties of protein such as foaming, gelling and water binding capacity of protein. Protein aggregation and gelation occurs under pressurization as well as after release. Phase change in protein and lipid are accompanied by high pressure treatment. This offer opportunities to develop new products with unique rheological properties and mouth feel.
In plants, vacuoles and pores can be filled with surrounding fluid after which material can maintain its structural integrity and density. Some vegetable structure are resistant to pressure while others exhibit significant softening and color change after high pressure pasteurization. The effect depend on kind of vegetable or fruit, physical characteristics and maturity.
Fat crystallization increases with the extent of pressure treatment. Maximum changes are found in the pressure range of 300 – 330 MPa. It is used for ageing of ice-cream mixes and physical ripening of dairy cream in butter making.

Effect of high pressure on Sensory properties:
High pressure treatment can cause alteration of structure of starch and protein such that rich can be cooked in few minutes. Grapefruit juice when processed by conventional heat treatment have bitter taste due to limonene. This bitter taste can be prevented when treated with high pressure. This also provides fresh like flavor with no loss of vitamin C and extended shelf life.
High pressure treatment contributes structure of tomatoes to become tough, tissue of chicken and fish filet become opaque and pre-rigor beef tenderized.

Effect of high pressure on gelation
Gelation by high pressure is attributed to decrease in volume of protein solution. Rearrangement of water molecules around amino acid residues in pressure treated gels produce glossy and transparent gel compared to opaque gels obtained by heat treatment. Cohesiveness of gel increases with increase in applied pressure. Gel produced by high pressure is less gummy, retains all vitamins and amino acids, easily digested, retains color or yolk or white and is soft lustrous  as compared to heat induced gel

Effect of high pressure on combination treatment:
The combination treatment allows lower processing pressure, temperature and time of exposure. Pasteurization and sterilization of low acid food using high pressure is difficult without using some additional factor to enhance the inactivation rate. Factors such as heat, antimicrobials, ultrasound and ionizing radiation can be used in combination. Antimicrobial effect of high pressure can be increased with heat low pH, CO2, organic acids and bacteriocins such as nisin.

Potential application of high pressure treatment:
1. Decontamination of raw milk and milk products
2. Reduction of intensity of thermal processing for prepared chilled meat
3. Developing new food of high nutritional quality and sensory quality, novel texture and increased shelf life.
4. Pressure depresses freezing point. Thus can be used in pressure assisted freezing, pressure assisted thawing and non-frozen storage at low temperature under pressure.

Yordanov, D. G., & Angelova, G. V. (2010). High pressure processing for foods preserving. Biotechnology & Biotechnological Equipment, 24(3), 1940–1945.


About Author

Name : Pratiksha Shrestha

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.