Food Biochemistry and Food Processing (2 edition)

BLBS102-c38 BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come

744 Part 7: Food Processing

thermal process conditions is the key for the success of the HPP

and for regulatory approval.

The critical factors in the HPP include pressure, time to

achieve treatment pressure, time at pressure, depressurization

time, treatment temperature (including adiabatic heating), prod-

uct initial temperature, vessel temperature distribution at pres-

sure, product pH, product composition, product water activity,

packaging material integrity, and concurrent processing aids.

Although HP processing related research work has increased

tremendously in the last decade, there is serious lack of infor-

mation in this area to permit establishing a reliable process.

Pulsed Electric Field

Pulsed electric field (PEF) processing is a nonthermal method

of food preservation that uses short bursts of electricity for mi-

crobial inactivation and causes minimal or no detrimental effect

on food-quality attributes. PEF can be used for processing liquid

and semi-liquid food products. PEF processing involves treating

foods placed between electrodes by high-voltage pulses in the

order of 20–80 kV (usually for a couple of microseconds). The

applied high-voltage results in an electric field that causes micro-

bial inactivation. A series of short, high-voltage pulses breaks the

cell membranes of vegetative microorganisms in liquid media

by expanding existing pores (electroporation) or creating new

ones. The membranes of PEF-treated cells become permeable to

small molecules; permeation causes swelling and eventual rup-

ture of the cell membrane. The treatment is applied for less than

one second, so there is little heating of the food, and it main-

tains its “fresh” appearance, shows little change in nutritional

composition, and has a satisfactory shelf life (Castro et al. 1993,

Kozempel et al. 1998). Since it preserves foods without using

heat, foods treated this way retain their fresh aroma, taste, and

appearance.

RETORT TYPES FOR COMMERCIAL

APPLICATION

Retorts generally are either batch or continuous types. Their

configuration may be either vertical or horizontal. Horizontal

retorts are commonly preferred due to their ease of loading and

unloading facilities as compared to vertical retorts.

Batch Retorts

Steam Heating Retort

In this type of retort (static or rotary), saturated steam is used as

a heating medium. Latent heat is transferred from steam to the

cans when saturated steam condensed at the surface of the cans.

The saturated steam process is the oldest method of in-container

sterilization. Since air is considered an insulating medium, satu-

rating the retort vessel with steam is a requirement of the process.

If air enters in the retort, it forms insulation layer of the surface

of the can and prevents the condensation of steam and causes

underprocessing of the product. It is inherent in the process that

all air be evacuated from the retort by flooding the vessel with

steam and allowing the air to escape through vent valves. There

is no overpressure during the sterilization phases of this process,

since air is not permitted to enter the vessel at any time during

any sterilization step. However, there may be air-overpressure

applied during the cooling steps to prevent container deforma-

tion. This is because during cooling, the steam rapidly condenses

in the retort and the food cools more slowly and pressure in the

container remains high. When the food temperature becomes

below 100◦C, overpressuring is stopped and the food is allowed

to continuously cool up to 40◦C.

Water Heating Retort (Immersion and Spray Modes)

The water immersion process is the most widely accepted

method of sterilizing product using an overpressure process.

The water immersion process is similar to a saturated steam pro-

cess in that the product is totally isolated from any influence of

cooling air. The product is totally submerged in preheated water

at preset temperature. But it is different from saturated steam

in that air can be introduced into the vessel during sterilization.

The water is preheated in different chamber with steam to de-

sired temperature and pumped to retort chamber. Overpressure

is provided by introducing air (or steam) on top of the water. In

some instances, air is added to the steam (which then heats the

air). The heated air agitates the water as it flows to the surface

and serves to pressurize the process load. Because it is an over-

pressure process, the machine can handle most, if not all, of the

fragile containers. For cooling, the water present in the retort

passes through a heat exchanger, where it is gradually cooled by

fresh water circulating in the service side.

The water spray process is also an overpressure process, like

water immersion, except that the product is exposed to the influ-

ence of the overpressure air. In this retort, low amount of water

is stored in the bottom of the retort and circulated by a pump

with high flow rate and sprayed on containers. It is similar to

the saturated steam process in that steam is the driving force

for reaching the center of the load. But the water spray process

is different from saturated steam in that air can be introduced

into the vessel during sterilization. Overpressure is provided by

introducing air (or steam) into the retort. To overcome the insu-

lating effects of the air, the spray nozzles vaporize the steam and

mix the steam with the air. The condensates are automatically

evacuated by a drainer and can be returned to the boiler.

Steam-Air Heating Retort

The steam-air process is an overpressure process, like water im-

mersion, except that the product is exposed to the influence of

the overpressure air. Steam is directly injected inside the re-

tort chamber and distributed and mixed uniformly with air with

the help of a fan to prevent cold spots in the retort. It is a

“ventless” process, resulting in significant energy savings. Fur-

thermore, a precise control of the steam/air process results in

a shortened cycle time for maximum production and consistent

quality, and reduces the process time and optimal heat transfer

characteristics, which allow foods to retain more of their natural

qualities. The opening of the steam inlet valve is automatically