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

742 Part 7: Food Processing

final sealed product. During aseptic processing, the product is

exposed to desired treatment temperature to eliminate food mi-

croorganisms and finally packed in sterile container in an aseptic

environment. This processing method is more efficient for liquid

products and liquid products containing small particles. Aseptic

processing sterilizes food and beverages in a way that puts the

least amount of thermal stress on a product, so nutrients and nat-

ural flavors, colors, and textures are maintained while sterility is

ensured.

The aseptic process begins with sterilization of both the pro-

cessing system and the filler. Generally, this is done with hot

water or saturated steam. Food is then pumped into the aseptic

processing system for sterilization. The flow of food is con-

trolled via a timing pump, so that no food flows too fast or too

slow.

This process consists of heating, holding, cooling, and packing

steps. First, the product is heated and held for some time to attain

desired degree of sterilization. Once the food leaves the hold

tube, it is sterile and subject to contamination if microorganisms

are permitted to enter the system. The best way to keep the

product sterile is to keep it flowing and pressurized. Heating step

is followed by cooling of the product. The next step is moving the

product into an aseptic surge tank to hold the product just before

packaging. Meanwhile, the packaging material is sterilized from

the other side of the aseptic environment. Packaging material is

sterilized by liquid hydrogen peroxide at a high temperature.

After the food and package are sterilized, the sterile package is

filled with the food, closed, and sealed in a sterile chamber.

The direct and indirect heating processes used in aseptic pack-

aging can affect the final taste of the food. Indirect heating, the

food comes into contact with either a metal plate or tube, which

can give food, such as milk, a burnt flavor. There are three

types of indirect heating processes: plate heat exchangers, tubu-

lar heat exchangers, and scraped-surface heat exchangers. The

former two are mainly used for liquid foods and the latter one is

for more viscous and particulate foods to prevent fouling due to

high temperature effect. All use a physical separation between

the product and the heating medium and transfer heat through

either a plate or tube to the product.

Direct heating uses steam injection or steam infusion and

minimizes the burnt flavor of the product by letting the food

come into direct contact with the heat source. With steam in-

jection, product and steam are pumped through the same cham-

ber; while with steam infusion, the product is pumped through a

steam-filled infusion chamber. In all these heat exchangers, since

the product is directly exposed to heating medium or subjected

to thin profile mode of heating, it allows faster heat penetra-

tion within short period of time, which destroys microorganisms

with limited effect on quality of the product. That is why aseptic

processing is considered as HTST processing in terms of opti-

mization of quality with desired degree of safety. However, for

particulate foods, the rate of heat penetration in the center of the

slowest heating point of the product should be determined to en-

sure desired degree of pasteurization/sterilization. In particulate

foods, first the heat is exchanged between the heating medium

and liquid food, and then transferred to the particle. In this case,

it is difficult to measure the temperature of the moving particle.

The time–temperature profile in the particle can be estimated

using mathematical models. Based upon the data of the model

used, the particle should be held at appropriate temperature for

sufficient time to achieve the required sterilization value at the

center.

Thin Profile and Retort Pouch Processing

Quality optimization is mainly achieved through enhancing the

heat transfer rate from the heating medium to the product. Any

resistance to rapid heat penetration slows down the heat pene-

tration rate and exposed the product for longer heating time.

The nature of packaging material, thickness, and its overall

thermal conductivity determine the heat transfer rate. In con-

ventional thermal processing, metal cans and glass containers

are the main type of containers used. However, the thickness

of these containers limits fast transfer of heat between heating

medium and product. Modification of geometric configuration

of the packaging materials from material thickness and geom-

etry point of view is additional opportunity to optimize quality

through improving heat transfer rate. Foods in rigid polymer

trays or flexible pouches heat more rapidly, owing to the thinner

material and smaller cross-section of the container.

Much of the retort pouch development was conducted by the

U.S. Army Natick Research and Development Center for use

in the Meal Ready-to-Eat (MRE), having relatively light weight

as compared to conventional metal containers. However, the

thin thickness of the pouch and its flat shape allows fast heat

penetration and contributes much on improvement of quality of

foods as compared to conventional packaging materials. The re-

tort pouch is a flexible, heat sealable container that is thermally

processed like a can and used to produce shelf stable, commer-

cially sterile food products. It is constructed of a 3-ply laminate

composed of an outer layer of polyester film, a middle layer of

aluminum foil, and an inner layer of polypropylene. The layers

are bonded together with a special adhesive. The tri-laminate

material provides seal integrity, toughness, puncture resistance,

printability, and superior barrier properties for long shelf life. It

also withstands the rigors of thermal processing up to 135◦C.

Retort pouches are filled with wet foods sealed and then heat-

treated in steam/hot water retort kettles to achieve commercial

sterilization (for shelf-stable foods) or pasteurization (for re-

frigerated foods). Because processing time typically is faster in

the pouch than in metal, glass, or rigid plastic containers, the

product tends to end up with better quality and safety. Pouches

also offer lower shipping and storage costs (pouch material is

lighter than cans, and pouches take up less storage space than

cans); easier, safer handling (no can openers or thawing time

required); and reduced product waste and reduced volumes of

disposed packaging waste material as compared to cans.

Some of the disadvantages include a lack of physical dura-

bility and slow production rates due to slow filling and sealing

rate as compared to cans or glass jars. Furthermore, it needs an

overpressure processing to protect the integrity of the packages

during processing. Pouches are more easily punctured; they can

overwrap for safe distribution.