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 735

Temperature of

retort

Convective heating

of liquid

Conductive heating

of solid particle

0

0

20

40

60

80

100

120

10

Time (min)

Temperature (°C)

20 30

Figure 38.5.Heat penetration curves foods in container in relation

to retort temperature (♦), convective heating (
), and conductive

heating ().

Equation 33 derived from the heat penetration curve is used

to estimate the process time,B(minutes).

B=fhlog

jhIh

g

(33)

In order to understand the contents of this equation and de-

termine the process time (B), it is necessary to recognize heat

penetration parameters, retort CUT, other several inputs, and as-

sumptions of Ball (1923) as indicated in the following section.

Heat Penetration Parameters

The following data are normally obtained from the heat pene-

tration data and heating conditions for calculation purposes as

well:

fh—Heating rate index. It is the time required for the straight-

line portion of the heating curve to pass through one log cycle.

It is also the negative reciprocal slope of the heating rate curve.

jh—Heating rate lag factor. This is a factor that, when mul-

tiplied byIh, locates the intersection of the extension of the

straight-line portion of the semi-log heating curve and the ver-

tical line representing the effective beginning of the process

(0.58 CUT).

jch=

Tr−Tpih

Tr−Tih

(34)

Iih—Difference between the retort temperature and food tem-

perature at the start of the heating process (Tr−Tih).

Tr—Retort temperature.

Tpih—Pseudo-initial temperature during heating. It is the tem-

perature indicated by the intersection of the extension of the

heating curve (originalTpih) and the vertical line representing

the effective beginning of the process (0.58 CUT) (NewTpih).

Tih—Initial food temperature when heating is started.

CUT—Come-up time. In batch processing operations, the retort

requires some time for reaching the operating condition. The

time from steam on to when the retort reachedTris called the

CUT.

0.58 CUT—Effective beginning of the process. Implies 58% of

CUT from the start, which do not significantly contributes for

process lethality. Ball recognized that the last 42% of the CUT

only has a significant contribution on sterilization process.

g—Difference between the retort temperature (Tr) and the max-

imum temperature (T) reached by the food at the slowest

heating point (Tr−T).

Tic—Food temperature when cooling is started.

Tcw—Cooling water temperature.

Ic—Difference between the cooling water temperature and food

temperature at the start of the cooling process (Tic−Tw).

gc—The value ofgat the end of heating or beginning of cooling

(Tr−Tic).

fc—Cooling rate index. The time required for the straight-line

portion of the cooling curve to pass through one log cycle. It

is also the negative reciprocal slope of the cooling rate curve.

Tpic—Pseudo-initial temperature during cooling. It is the tem-

perature indicated by the intersection of the extension of the

cooling curve and the vertical line representing the start of

cooling.

jcc—Cooling rate lag factor. This is a factor that, when multiplied

byIc, locates the intersection of the extension of the straight-

line portion of the semi-log cooling curve and the vertical line

representing the start of the cooling process.

jcc=

Tcw−Tpic

Tcw−Tic

(35)

Formula methods make use of heat penetration data ob-

tained from a semi-logarithmic plot of the temperature difference

(Tr−T) on log scale against time (on linear abscissa).When the

temperature difference is greater than one, three-cycle log paper

is recommended (Fig. 38.6), and when the difference is below

one, four-cycle log paper is required.

Gathered time–temperature data may be plotted in two differ-

ent ways. In the first case, log (Tr−T) versus time is plotted on

the paper (spread sheet), and the slope is obtained from the lin-

ear portion of the negative reciprocal slope of the graph. The lag

factor is calculated from the intercept of the equation of the line.

In the second case, the semi-log paper is rotated 180◦(Jackson

plot) and the top line of the paper is marked 1◦less than retort

temperature (Tr− 1 ◦C), the next log cycle is marked 10◦less

(Tr− 10 ◦C), and then the third cycle 100◦C less (Tr− 100 ◦C).

Using this interval, the product temperatures are directly num-

bered without conversion on semi-log graphing paper to produce

the linear plot needed to determine heating rate index (fh)and

lag factor (jh) (Fig. 38.7).

The Retort CUT,fhandjhValues

Once heating medium (hot water or steam) is injected into the

retort, the retort is not reached immediately to the specified

operating temperature of 121.1◦C. The time from introduction of