SORTING OF FRUIT

 

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 machines.

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.

 

Case study

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.

Packhouse A:

  1. As is the case with most packhouses where GPB has been involved, sorting takes place as follows:

    1. Pre-sorters:

      They pick juice fruit off the sorting line and have only to decide whether the fruit moving past them is indeed juice fruit.

    2. Main sorters:

      They pick all fruit that does not adhere to Class 1 quality.

    3. Class 2 sorters:

      They pick all fruit that does not adhere to Class 2 quality.

  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.

     

    Packhouse B

  3. Here sorting is approached as follows:

    1. Pre-sorters: Is it a juice fruit? YES/NO

    2. 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

  4. The extent of defects principally determines the decision making process at sorting. The process is simplified to a single YES/NO decision.

  5. 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.)

  6. The time from decision making to start of the reaction to pick a selected fruit off the sorting table is 0,25 seconds.

  7. The duration of the fruit handling action (picking the fruit up and placing it on the feedbelt) is 0,6 sec.

  8. For Packhouse A the time for decision making and implementation is therefore 1 s, and for Packhouse B 1,2 s.

  9. 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.

  10. Assume:

    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

  11. 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 sec

    Total time used: 43,4 sec

  12. 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 sec

    Total time used: 46,4 sec

  13. From this theoretical analysis, it seems that there is approximately 10% more pressure on the sorters at Packhouse B than on the sorters at Packhouse A.

  14. 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.

 


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