Food Biochemistry and Food Processing (2 edition)

BLBS102-c39 BLBS102-Simpson March 21, 2012 14:20 Trim: 276mm X 219mm Printer Name: Yet to Come

39 Minimally Processed Foods 747

food safety systems in the world, outbreaks of illness that are

linked to consumption of fresh fruits and vegetables have been

reported. The Public Health Agency of Canada, Health Canada,

and the Canadian Food Inspection Agency have been coordinat-

ing to identify gaps, develop and enforce solutions to improve the

Government’s response to outbreaks, and to ensure Canadians

are provided with the highest levels of food safety protection.

Food quality and safety are not necessarily complementary. In

fact, they are opposing in some cases. The balance and optimiza-

tion of the two parameters has been a major challenge in food

processing. The use of intense and broad-spectrum technologies

such as thermal treatment for microbial inactivation and shelf

life extension are known to be effective but unfortunately result

in accelerated degradation of sensory, nutritive, and functional

quality of food products. There is also the aspect of excessive

energy use with some of the food processing technologies. More

recent technological advancements have allowed the application

of mild processing of food with the promise of safe products

without the downside of quality degradation. These technolo-

gies are termed as minimal processing technologies, and food

products obtained by their applications are referred to as min-

imally processed foods. Thus, minimally processed products

have not undergone very extensive transformation, particularly

thermal processing as compared to conventional foods. The aim

of each minimal processing technique is to minimize the rate of

deterioration within the food matrix.

OVERVIEW OF THERMAL PROCESSING

TECHNIQUES

Thermal processing involves the application of adequate heat

to inactivate undesired spoilage microorganisms, degrading en-

zymes, and antinutrients, and prepare the food product to be

consumed. It is robust and widely used in processing and pre-

serving foods. Sufficient heat destroys microorganisms and food

enzymes but also impacts both desirable (textural changes, pro-

tein coagulation, and release of aroma) and undesirable changes

(nutrient loss, loss of freshness) in the food. Thus, in practice,

designing appropriate thermal process typically involves consid-

eration of time–temperature combinations that are required to

inactivate the most heat-resistant pathogens and spoilage organ-

isms in a given food matrix. There is often the need to evaluate

heat penetration characteristics in a given food product, includ-

ing can or container in order to evaluate temperature distribu-

tions. The choice of process depends on the minimum possible

heat treatment that should guarantee freedom from pathogens

and toxins and give the desired quality and storage life.

High Temperature Short Time Process

The mechanism of heat destruction of microorganisms has been

attributed to thermally induced changes in the chemical structure

of proteins in the cells. The phenomenon can be described by a

first-order reaction equation.

dN

dt

=−koNe(−

RTE)

(1)

whereNis concentration,tis process time,kois the activation

factor,Eis activation energy, andTis absolute temperature.

Along with the microbial inactivation, other chemical reactions

that occur during thermal processing lead to degradation of vari-

ous quality attributes (sensory, vitamin C, thiamine, flavor, etc.).

These degradation reactions can also be described using kinetic

equations similar to Equation 1 with their relevant activation

factors and activation energies. Several studies have confirmed

that at high temperatures, the destruction rate of pathogenic

bacteria accelerated more rapidly than the degradation rate of

nutrients (Holdsworth 1985). Thez-value (defined as change

in the death or degradation rate based on temperature, i.e., the

change in temperature required for tenfold change in microbial

or chemical resistance) is typically around 10◦C for microbial

death, whereas the value for chemical degradation is around

30 ◦C. Thus, the interaction between safety and quality can be

examined by considering microbial death kinetics and chemical

degradation kinetics.

High temperature short time (HTST) process takes ad-

vantage of the differences in the microbial inactivation and

quality degradation kinetics to improve retention of nutrients

(higher quality) while ensuring adequate microbial inactivation.

Temperature–time combinations used for HTST may range from

about 72◦C for 15 seconds for pasteurization of milk to much

higher temperatures for shorter times. It is typically applied

at about 132–143◦C for different times depending on the re-

quiredFovalues (the amount of time a food is maintained at the

reference temperature of 121.11◦C; used as a benchmark and

indicating the severity of a process) to sterilize low-acid foods

and beverages. The use of temperatures beyond 138◦C is usually

referred to as ultra high temperature (UHT) processing. A major

limitation associated with HTST lies in its application to prod-

ucts that are highly viscous, solid, or that contain solid particles.

Safety considerations require that sufficient heat is applied to

sterilize the fastest moving (which is really the slowest heating)

particle in the system. This implies that other slower moving

particles are overprocessed, since they receive the high temper-

ature but not the short time, resulting in degraded quality of the

product. There are also limitations of heat transfer rate in solids,

which may lead to surface heating and reduced quality. These

limitations create a need for other technologies for effective de-

livery of energy to the product to enhance both safety and quality.

Aseptic Processing

Aseptic processing is an answer to the limitations of HTST. In

this technique, processed food products and package are brought

together aseptically (i.e., in a sterile environment). Liquid food

product is initially sterilized by HTST or UHT and then heat

is transferred to the solid particulate components as they are

transported by the liquid and filled into presterilized containers.

This revolutionary packaging system first appeared in US super-

markets in the 1970s (Fellows 1988, Holdsworth 1992, Welch

and Mitchell 2000). In 1989, the Institute of Food Technologist

selected aseptic packaging as “the most significant food sci-

ence innovation in the past fifty years.” An aseptic process for a

low-acid product containing particulates (potato soup) was filed