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

38 Thermal Processing Principles 741

90% destruction of nutrient

40% destruction of nutrient

10% destruction of nutrient

Log

time

Temperature

C

B

A

D

E

F

G

H

I

Figure 38.9.A diagram of resistance of nutrient (- - -), microbial spores (–), vegetative bacteria (•••), and HTST heating principle.

For instance, in UHT sterilization process, milk is exposed for

2–5 seconds at high temperatures of 140–145◦C to kill bacteria

spores. UHT treatment limits browning of the milk, development

of cooked flavor, and denaturation of proteins. In HTST pasteur-

ization, milk is subjected to a temperature of 71.1◦C for 15–20

seconds. HTST or UHT approaches are not always beneficial

for conduction heating food products that heat relatively slowly,

exhibiting large temperature gradients between surface and cen-

ter of the container. However, they are best for liquid foods and

liquids containing small particles that can heat rapidly while sub-

jected to in-container thermal processing or in-heat exchangers,

as an aseptic processing.

Agitation Processing

Some retorts agitate the cans during processing in order to in-

crease the rate of heat penetration in cans. As compared to static

retorts, the process time may be reduced by 80% because the con-

tents are heated up faster and more evenly. Agitation processing

is mainly groped into axial and end-over-end (EOE) type (Fig.

38.10). EOE involves the containers being loaded vertically and

a crate rotating around a central horizontal axis. During axial

rotation, cans are rotated individually in the horizontal plane.

As the containers are agitated inside a retort, the contents of

the containers are mixed uniformly; this eliminates cold spots

and increase heat penetration rate. Mixing largely is due to the

movement of the headspace bubble during agitation, and to be

effective, there must be sufficient headspace in cans. Both small

and large headspace may result in lower heat transfer, leading to

underprocessing because of limitation of mixing of food com-

ponents in cans. In addition to headspace, fill of the container,

solid to liquid ratio, consistency of the product, and the speed of

agitation are the crucial factors to be standardized in agitating

processing. Unless and otherwise these conditions are properly

optimized for a given product and processing condition, the re-

quired fast heat penetration may not be achieved.

Aseptic Processing

The word “asepsis” implies the process of removing pathogenic

microorganisms or protecting against infection by such organ-

isms. It can be defined as a state of control attained by using

an aseptic work area and performing activities in a manner

that precludes microbiological contamination of the exposed

sterile product. In food processing, aseptic processing involves

a section that precludes microbiological contamination of the

Cans in vertical position Cans in horizontal position

Figure 38.10.End-over-end and axial agitation orientation of cans in retort cage.