Looking at the fruit trays in the supermarket or at the
greengrocer, it is easy to assume that all the fruit has been sorted
according to equal weight, colour, size and quality by modern
It is not true that all labourers can be replaced by
machines. Although sophisticated sorting machines are used in the fruit
industry, men and women still play a vital role in the sorting of fruit
into different degrees of quality. They are called the sorters.
GPB Consulting was approached to re-evaluate aspects of
productivity at two citrus packhouses.
It is a known fact that the number of sorters required is
directly proportional to the expected fruit quality and also the decision
made by the packhouse manager concerning final quality policy.
- As is the case with most packhouses where GPB has been involved,
sorting takes place as follows:
They pick juice fruit off the sorting line and have
only to decide whether the fruit moving past them is indeed juice
- Main sorters:
They pick all fruit that does not adhere to Class 1
- Class 2 sorters:
They pick all fruit that does not adhere to Class 2
- All three sorter types mentioned above only have to make the
decision of "Yes" it is the specific fruit type, or "No" it is not. The
main sorters sometimes have to make an addisional decision, ie should
the stem be cut, or not.
- Here sorting is approached as follows:
- Pre-sorters: Is it a juice fruit? YES/NO
- Main sorters: Is it a Class 1 fruit? YES/NO
or Is it a Class 2 fruit? YES/NO
or Is it a Class 3 fruit? YES/NO
and Does the stem have to be cut? YES/NO
- The extent of defects principally determines the decision making
process at sorting. The process is simplified to a single YES/NO
- The reaction time to make a decision (ie the time it takes for the
brain to process a signal), eg "Is it Class 1?" – YES/NO, is given as
0,2 seconds. The more decisions there are to be made, the longer it
takes for the brain’s reaction time, because each incoming signal must
be processed by the brain to give the outcome. In the case op Packhouse
B where there are 3 outcome possibilities, the reaction time to make
decisions is 0,4 seconds. (There is no linear relationship between the
time to make a decision and the number of decisions to be made.)
- The time from decision making to start of the reaction to pick a
selected fruit off the sorting table is 0,25 seconds.
- The duration of the fruit handling action (picking the fruit up and
placing it on the feedbelt) is 0,6 sec.
- For Packhouse A the time for decision making and implementation is
therefore 1 s, and for Packhouse B 1,2 s.
- It must be taken into account that people get tired and require rest
periods. Compulsory (when a person is tired) and hidden (busy with own
activity) rest periods take up approximately 25% of the working time.
Throughput per packline per shift = 156000 kg
Juice fruit = 10%
Thus fruit over main sorting tables = 140400 kg
Shift duration = 9 hours
"Rest" time as above = 25%
Thus, "working" time = 6,75 hours
- At Packhouse A where there are 6 main sorting tables (juice fruit
already picked), 11700 kg fruit moves past 2 sorters per side on each
sorting table per shift of 405 minutes, approximately 145 fruit past 2
sorters per minute, where each sorter has to make the decision of
whether it is a Class 1 fruit or not. If assumed that 50% is Class 1
fruit, it follows that each sorter has to physically handle 36 fruits,
ie to sort 72 fruits will take a total of 43,4 sec.
Decision making = 73 x 0,2 sec = 14,6 sec
Transfer of 36 fruits = 36 x 0,8 sec = 28,8
Total time used: 43,4 sec
- At Packhouse B with 5 sorting tables and 3 sorters per side, 14040
kg of fruit moves over each table side per shift of 405 minutes, which
is equivalent to 35 kg of 175 fruits per minute. Theoretically each
of the 3 sorters make 3 decisions each minute about each of the 58
fruits moving past her. It is assumed that 50% of the fruit is Class 1,
thus each sorter physically handles 29 fruits, which means that the
sorting time is as follows:
Decision making = 58 x 0,4 sec = 23.2 sec
Transfer of 29 fruits = 29 x 0,8 sec = 23,2
Total time used: 46,4 sec
- From this theoretical analysis, it seems that there is approximately
10% more pressure on the sorters at Packhouse B than on the sorters at
- It is preferable to expect a sorter to make only one decision, eg is
it right or wrong ("go/no go")? In this way the complexity of sorting is
reduced and the effectiveness of the sorter increases.