Principles of Food Sanitation

occurrence of values within each class. Class
limitations are selected to make the table easy
to read and graph. The frequency table of
microbial load from raw materials (Table 8–2)
displays how data are divided into each class.
To help visualize how these data are
arranged, one can graph it in the form of a
histogram. Figure 8–4 takes the information
from Table 8–2 and displays it graphically.
The histogram in Figure 8–4 depicts an
important curve common to statistical analy-
sis—the normal curve or normal probability
density function. Many events that occur in
nature approximate the normal curve. The
normal curve has the easily recognizable bell
shape and is symmetrical about the center
(see Figure 8–5). The area underneath the
curve represents all the events described by
the frequency distribution.
From Figure 8–5, the mean is the highest
point on the curve. The variation of the
curve is represented by the standard devia-
tion. It can be used to determine various
portions underneath the curve. This is illus-
trated in the figure where one standard devi-
ation to the right mean represents roughly
34% of the sample values. Consequently,
68.27% of the values fall within ±1 standard
deviation from the mean. Similarly 95.45%
fall within ±2 standard deviations. Virtually
all of the area (99.75) is represented by ±3
standard deviations. The information thus
far can be used to establish control limits in
order to determine whether a process is in a
state of statistical control.

Control Charts

Control charts offer an excellent method

of attaining and maintaining a satisfactory

level of acceptability. The control chart is a

widely used industry technique for on-line

examination of materials produced. In addi-

tion to providing a desired safety level, it can

be useful in improving sanitation and in pro-

viding a sign of impending trouble. The pri-

mary objective is to determine the best

methodology, given the available resources,

then to monitor control points. This varia-

tion can be classified as either chance-cause

variation or assignable-cause variation.

In chance-cause variation, the end products

are different because of random occurrences.

Quality Assurance for Sanitation 133

Table 8–2Frequency Table for Microbial Load
(CFUs/g)

Class in CFUs Frequency

0—100 5
100—1,000 10
1,000–10,000 22
10,000–100,000 13
100,000—1,000,000 3

20

0

5

Frequency^10

15

25

Microbial Load

0 − 102102 − 103103 − 104104 − 105105 − 106

Figure 8–4Histogram of microbial load (CFUs/g).

0.0

− 4 − 3 − 2 − 101234

0.2

0.4

0.6

0.8

1.0

Figure 8–5Normal curve.