Soups-n-Salads is a sizeable buffet restaurant chain with 57 locations in the Pacific Northwest,

offering its customers a choice of 5 different soups and 12 gourmet salads. As part of a continuous

effort to present its customers with food items that are fresh, delicious, and safe, Soups-n-Salads

follows rigorous quality control procedures developed by their inhouse Six Sigma team. Part of

this effort addresses the problem of temperature management for food items.

Danger Zone. Bacterial growth on food is a serious public health hazard. The U.S. Department

of Agriculture estimates that as many as 9,000 deaths and 6.5 to 33 million illnesses yearly are

directly linked to foodborne bacteria and other microorganisms. To survive and reproduce,

bacteria need time, moisture, and a favorable temperature. One of the most critical factors for the

successful prevention of bacterial growth is storing food at the right temperature. Long term

practice and lab experiments have shown that bacteria grow and multiply faster in temperatures in

the 40 – 140 °F range (5 – 60 °C). This range is known as the Danger Zone. Safe food storage

practices and regulations require from grocery stores and restaurants to take all the necessary steps

to avoid a prolonged exposure of food in the danger zone. Generally, cold temperatures (below

40 °F) will significantly slow down bacterial growth, and hot temperatures (above 140 °F) will

destroy most of the bacteria. Bacteria do not affect the taste, smell, or appearance of a food.

Therefore, if a food has been left in the danger zone for more than 2 hours should be discarded,

even if it smells or looks good.

Soups and Salads. Safe storage and display of salads at a buffet restaurant involves keeping the

salads in containers that will protect them from possible exposure to insects or rodents, avoiding

cross-contamination from other food, proper refrigeration, isolation from cleaning agents, practicing stock rotation (i.e., consuming the older stock first), and other practices. Temperature

management for salads is relatively simple, due to the fact that most salad items remain at a low

temperature until they are consumed or disposed of. Keeping soups at a safe temperature can be

more challenging, since they need to be heated when they are cooked, chilled when they are stored,

and reheated when they are about to be consumed by the customers. Soups pass through the danger

zone twice, and this transition needs to be as fast as possible, to prevent bacteria from having the

time or the opportunity to grow. The Process Flow Diagram shown in Exhibit 1 illustrates the

temperature management procedures for soups at Soups-n-Salads restaurant.

Statistical Quality Control. Soup temperature at each Soups-n-Salads restaurant is monitored

every half hour during the period 11:30am - 10pm, for a total of 22 measurements per day. Two

restaurant employees per day share two daily shifts recording the temperatures of 12 different soup

containers (bain-maries). A temperature log of the most recent 30 days is maintained, as required

by the Health Department. An effort is made to keep the soup temperature above 150 °F, just to

be on the safe side. Out of the 12 soups measured every half-hour, four are randomly chosen to

be included in a control chart showing the average soup temperature of the sampled bain-maries.

Exhibit 2 shows the first few observations of sampled temperatures corresponding to two

consecutive days. Occasionally all four soups in the sample have temperatures that are unusually

low and some other times just one or two temperatures are low and the rest are OK. Depending

on whether a sample has similar low temperatures or just large differences among the temperatures

it consists of, the managers of Soups-n-Salads may be able to get a head start in trying to figure

out a possible cause. For example, if the temperature in all soups is dropping it might be an

indication that the employee responsible for stirring the soups was not making the rounds, whereas if only one has a low temperature it might relate to a machinery malfunction. Exhibit 3 shows a

cause-and-effect (fishbone) diagram, created in order to transcribe a number of possible causes for

a low soup temperature, as they have been known to the managers through long experience.

Boiling hot

hot

cold

Cook soup. Temperature at

boiling is slightly above 212 °F

Divide soup into shallow

containers for rapid cooling.

Place soup in plastic bags and

Refrigerate within 2 hours

Danger Zone,

Temperature

45

Heat soup fast.

Raise temperature

to 160 °F

4

EXHIBIT 1: Process Flow Diagram showing the temperature management procedures for

soups at Soups-n-Salads Restaurant.

5

Preheat the bain-

marie before use

Keep the soup hot in a bain-marie.

Operate at the highest setting. Stir

soup frequently. Take temperature

every half hour. Maintain above 140

°F. Keep soup in the bain-marie for

a max of 1 hour, then dispose

Store in refrigerator and use

within 4 days. Keep

temperature below 40 °F

Sample temp1 temp2 temp3 temp4

1

149.8 148.9 150.2 149.3

2

154.1

150.4 153.2 155.8

3

151.2

155.4 152.2 153.9

154.1 151.9 153.9

153.3

153.1 157 156.7 153.2

EXHIBIT 2: Sampled temperatures for two consecutive days (first few observations) People

Employees

don't care

Soup is not

stirred often

Power outage

Environment

Equipment

Thermometer not

measuring properly

Instruments are old

and need replacement

Original heating

not high enough

Bain-marie preheating

not high enough

Procedures

EXHIBIT 3: Cause-and-effect (fishbone) diagram.

Bain-marie not

working properly

Poor

maintenance

Soup

temperature not

high enough

Refrigerator

temperature too low

Using statistical software, the Six Sigma team produced an R chart and an x-bar chart for the soup

temperature. Examination of the R chart showed that the process was out of control with respect

to the variability on the 27th sample. At that time, one of the four sampled soups was getting much

colder than the other three. Later, the Six Sigma team produced a new R chart, along with a

corresponding x-bar chart, both shown in Exhibit 4. Range

X-bar

2

10

8

O

157

155

153

151

149

2

3

4

5

6

7

8

9

10

MA

0

Range Chart for temp

0

10

10

d₂

1.128

1.693

2.059

2.326

2.534

2.704

2.847

20

2.970

3.078

x

Subgroup

20

X-bar Chart for temp

30

d3

.853

Subgroup

.888

.880

.864

.848

.833

.820

.808

.797

As

30

EXHIBIT 4: R chart and X-bar chart (second round of control charts).

40

AMA

D3

40

0

0

0

0

.076

.136

50

.184

.223

UCL =

CTR= 4.12

LCL =

50

D₂

3.267

2.574

2.282

2.114

2.004

1.924

1.864

1.816

1.777

UCL =

CTR= 153.78

LCL =