5: High Pressure Processing

Question 1

What are the types of high pressure processing?

Answer 1

• Batch: Suitable for solid and liquid. Product packed in a flexible container is placed in a pressurized chamber. A pressure transmitting medium fills the chamber ant pressure is exerted for a certain amount of time before depressurizing and unloading.
• Semi-continuous: Suitable for pumpable (liquid products) .

Question 2

Provide some suitable materials to be used as pressure-transmitting medium.

Answer 2

• Water, ethanol, glycol, silicone oil, castor oil .

Question 3

What are the process principles behind HPP?

Answer 3

Le Chatelier’s principle:
When a system at equilibrium is disturbed, the system will adjust its state to counteract the effect of the change. HPP stimulates reactions that involve a decrease in volume but dampens reactions that involve an increase in volume.

Isostatic rule:
When sample directly contacts the pressure medium, the pressure transmits instantaneously and uniformly throughout the sample.

Question 4

What are the packaging requirements for solid foods to be treated with HPP?

Answer 4

(HOOF)
- High strength
- Oxygen impermeable
- Opaque
- Flexible

Question 5

Describe the effects of HPP on Food Safety.

Answer 5

• Bacteria: HPP disrupts integrity of cell membrane and modify its properties, causing cell morphological changes (e.g., compression of gas vacuoles, condensation of nucleoid, seperation of cell membrane from cell wall, cell protein aggregation.)
• Bacterial spores: Mild HPP (50-300 MPa) activates nutrient germinant receptors and forces spores to germinate, after further increase in pressure (>800MPa) to inactivate the germinated cells. Hurdle technology, high temperature or pressure cycling can increase efficiency of spore inactivation.
• Fungi: More susceptible to HPP than bacteria
• Virus: Non-enveloped is more pressure resistant.
• Prions: Combine high pressure and temperature to inactivate
• Parasites: very sensitive to pressure.

Question 6

What are the factors affecting the barosensitivity of microbes?

Answer 6

• pH: Acidic pH enhance HPP inactivation of most MOs
• Aw: Low Aw stabilizes protein and protect MOs against inactivation but also inhibit the recovery of injured cells
• Temp: High temp promotes MO inactivation, except virus
• Growth Temp and Phase of MO: Stationary phase MO more resistant, high growth temp increases HP-resistance.
• Antimicrobial agent: synergistic effect
• Food composition: Fat, protein, sugar, salt and mineral provides baroprotection
• HPP parameters: Magnitude of pressure, holding time, rate of compression/decompression

Question 7

What are HPP’s effects on food quality?

Answer 7

• Color: Oxidation of food, e.g. oxidation of myoglobin, enzymatic browning.
• Texture: Alters the shape and surface of starch which decrease gelatinization temperature and enthalpy. Saves energy.
• Sensory: High pressure caused significant undesirable aroma changes
• Yield: Increase yield - e.g., oyster shucking, reduce weight loss of sausage, increase yield of cheddar cheese.

Question 8

Why is meat treated with HPP more tender?

Answer 8

• HPP changes structure of myofibrillar protein, which changes the gelation properties, leading to increased tenderness of meat/surimi.

Question 9

State the applications of HPP.

Answer 9

• HPP freezing
• Thawing: Increased pressure reduces melting temperature
• Non-frozen storage
• Extraction: Pressure facilitate protein denaturation, increase permeability of cell membrane and increase solubility of extractives