High Pressure shift freezing (HPSF)

In pressure shift freezing, the food is cooled under high pressure to sub-zero temperatures.  If water is subjected to very high pressure and cooled, it may remain liquid at a temperature much lower than its freezing temperature at atmospheric pressure.

If now the pressure is suddenly released, rapid freezing will occur. This method of freezing is called high pressure-shift freezing. Because the freezing point decreases with pressure, phase change does not take place.

Application of high pressure during freezing can avoid products damage due to the instantaneous and homogenous formation of ice through the product.

High pressure shift freezing, are gaining attentions a freezing method for high quality or freezing sensitive foods. The high level of supercooling during high pressure shift freezing of peach and mango led to uniform and rapid ice nucleation through sample volume, which largely maintained the original tissue microstructure.

In case of salmon, high pressure shift freezing produced a large amount of homogenously distributed fine and regular intracellular ice crystals, which helped in the maintenance of muscle fibers in comparison to the frozen muscle structure.
High Pressure shift freezing (HPSF)

High Pressure assisted Freezing (HPAF)

High pressure freezing processes operate in pressure-resistant vessels with thermally isolated thermostatic circuits to reach temperatures below 0 ° C.

Packed foods are immersed in the pressure/cooling medium and frozen.

In high pressure assisted freezing process, water solidification is produced while pressure around the product is high.  During freezing, use of high pressure facilities supercooling and promotes uniform and rapid ice nucleation throughout the product on pressure release, producing smaller ice crystals.

Phase transition occurs under constant pressure, higher than atmospheric pressure while the temperature is lowered to below the corresponding freezing point.

HPAF is based on the fact that freezing point changes with pressure and the product can be stored at normal frozen storage conditions.

High pressure assisted freezing was particularly useful for freezing large pieces of food when uniform ice crystals are requires.

In high pressure assisted freezing, product are cooled under 200 MPa to -20  ° C without ice formation, then the pressure is released and the high supercooling reached ( approx. 20 °C) promotes uniform and rapid nucleation.
High Pressure assisted Freezing (HPAF)

High pressure processing in food

High pressure processing of foods inactivates microorganism, spores and undesirable enzymes and increases the shelf life. High pressure processing presents an alternative that retains food quality and natural freshness and extends shellfire.

Although discovered in 1899, it is relatively a new method and one still under development.

The motivation of using high pressure is based on chemical, physicochemical, physic-hydro dynamical and physic-hydraulic effects.

High pressure processing also commonly referred to as ‘high hydrostatic pressure’ or ‘ultra high pressure’ processing, uses elevated pressure with or without the addition of external heat, to achieve microbial inactivation or to alter food attributes.

Jams made by high pressure processing retain the taste and color of fresh fruit, unlike conventional cooked jams.

Other foods now include fish, meat products, salad dressing, citrus juices ,rice cakes and yoghurt. The technology provides food processor with an opportunity to process heat sensitive, value added foods with fewer additives and cleaner ingredients labels.

High pressure treated of grapefruit juice appeared on the market in 1991. The process is that high pressure of 200 MPa is applied to freshly squeezed grapefruit for 10 min at 5° C and then conducted with conventional manufacturing process.

The unique compression heating effect helps to reduce the severity of thermal effects encountered with conventional processing techniques. Other advantages of the technology include uniform pressure application, minimal heat damage to food and potential for altering functional properties of foods.
High pressure processing in food

High Pressure Processing

The primary aim of treating foods with high pressure processing in most cases is to reduce or eliminate the relevant foodborne microorganisms that may be present.

Applying high pressure uniformly throughout a food product is another method of non-thermal food preservation. This inactivates microorganisms, spores and undesirable enzymes, and increase the shelf-life of foods without the used of chemical preservatives.

The pH and water activity of foods can also significantly affect the inactivation of microorganism by high pressure processing.

Japanese is a leader in this technology. In Japan, the technique has been used since 1990 on some juice, jams and jellies.

Although it was discovered in 1899, treating foods with high pressure is a relatively mew method of preservation and one still under development.

High pressure processing or pascalization is named after Blaise Pascal, a 17th century French scientist who describe how contained fluids are affected by pressure.

Jams made by high pressure processing retain the taste and color of fresh fruit, unlike conventional cooked jams. High pressure processing is also used in yoghurts, salad dressings and citrus juices.

The draw back of this method it is costly to implement, but interest in the technique was revived during the 1980s and 1990s.

In recent years, food preservation strategies have been developed that combine high pressure processing with the use of anti-microbial food additives.
High Pressure Processing

Food Preservation by High Osmotic Pressure

Food Preservation by High Osmotic Pressure
Bacteria reach osmotic equilibrium by two means:

1. In hypertonic environments the volume of the protoplasts will shrink, and
2. In hypotonic environments the rigid wall will resist increase in protoplasts volume at a limiting volume of water; equilibriums results from turgor against the wall.

The rigid wall present in bacteria cells enable most bacteria to tolerate even extremely dilute environments. Osmotic equilibrium is achieved by development of turgor pressure against the wall. The wall of gram-positive micrococci can withstand 22 atm of pressure.

Although the walls of gram-negative rods have lower tensile strength, the wall is sufficiently strong to retain the turgor pressure if the cell is suspended in water.
Food Preservation by High Osmotic Pressure