GRAPE PACKING TRACEABILITY
Grapes (Vitis vinifera L.) are berry fruit belonging to the grapevine family (Vitaceae). Grapes come originally from south-west Asia.
Grapes are cultivated on vines (climbing shrubs). Dessert grapes are very carefully cut by hand and individual sick or damaged berries are removed. They are harvested at full ripeness, i.e. at the climacteric stage, as they do not post-ripen well after harvesting at the preclimacteric stage. The „Brix“ value is used to determine the time of harvest.
The berries on the panicles (commonly known as „bunches“) are usually covered by a whitish wax layer or bloom which can easily be wiped off and provides natural protection.
Before being eaten, dessert grapes should be washed thoroughly in order to remove any traces of spray.
Grapes are cultivated in the open, under glass (Belgium, Netherlands) and in plastic tunnels. Cultivation under glass and in tunnels allows harvesting times to be extended.
The following varieties may be distinguished:
White varieties (green, yellow to amber-colored varieties)
Dark varieties (red, blue to black varieties) which get their color from anthocyanins (oenin)
Waltham Cross grapes, so named because they were first exported to South Africa from the English town of this name. These have a stronger skin, are not so tightly packed on the bunch and are thus less susceptible to spoilage
Muscat grapes, which have a musky aroma, which occur as both white and dark varieties
Varieties with up to 5 seeds, although the seedless varieties are more popular
Quality / Duration of storage
Grapes awaiting transport should be sound, fresh, exhibit no foreign odors or flavors, be free of abnormal moisture, have no pressure marks or bruises, should not include any burst or shriveled grapes or any which have fallen from the bunch, nor exhibit any fungal growth or dried out stems. Grapes to be eaten fresh (dessert grapes) must have reached their full ripeness and color and must not be withered.
Seedless varieties are considered to be of higher quality and are therefore particularly popular with consumers.
Various sources state maximum duration of storage as follows:
Temperature Rel. humidity Max. duration of storage Source
-1 – 0°C 90% 4 weeks 
1 – 4°C 90 – 95% 8 weeks 
-1 – -0.5°C 90 – 95% 8 – 24 weeks (depending upon variety) 
Where controlled atmosphere transport is used, the transport and storage duration of the grapes may be extended. The following parameters apply in such a case :
Temperature Rel. humidity O2 CO2 Suitability for controlled atmosphere
1.1 – 2.2°C 90 – 95% 3 – 5% 1 – 3% Moderate
Dessert grapes are principally intended to be eaten fresh.
(Click on the individual Figures to enlarge them.)
Photo, black grapes
Figure 1 Photo, white grapes
Countries of origin
This Table shows only a selection of the most important countries of origin and should not be thought of as exhaustive.
Europe Turkey, Spain, Italy, France, Hungary, Romania, former Yugoslavia, Bulgaria, Greece, Cyprus, Belgium, Portugal
Africa South Africa
America Chile, Argentina, Brazil, USA
Australia Australia, New Zealand
Grapes are packaged in crates, fruit crates and cartons, each containing 10 kg. Particularly high quality grapes are individually wrapped or packaged in shaped trays in cartons lined with wood wool or in perforated polyethylene film in fruit crates.
Packaging should be treated against mold attack.
Symbol, general cargo
Means of transport
Ship, aircraft, truck, railroad
Refrigerated container with fresh air supply or controlled atmosphere.
Since grapes are highly sensitive to impact due to their relatively thin skins, they must be handled with appropriate care.
The required refrigeration temperature must always be maintained, even during cargo handling.
In damp weather (rain, snow), the cargo must be protected from moisture, as there is otherwise a risk of premature spoilage.
3.29 m3/t (boxes) 
4.18 m3/t (cartons) 
4.00 – 4.25 m3/t (boxes) 
2.80 m3/t (10 kg cartons on pallets) 
The stowage factor depends very much on weight categories and the packaging units used.
Stowage space requirements
Cool, dry, good ventilation
Marker pen, oil crayon
Because of its considerable impact- and pressure-sensitivity, packages of this cargo must be secured in such a way that they are prevented from damaging each other. Spaces between packages or pallets must be filled, to prevent slippage or tipping. By selecting the correct packaging size or cargo unit (area module or area module multiple), holds can be tightly loaded (without spaces).
Risk factors and loss prevention
Grapes require particular temperature, humidity/moisture and ventilation conditions (SC VII) (storage climate conditions).
A written cooling order must be obtained from the consignor before loading is begun. This order must always be complied with during the entire transport chain.
The following Tables merely constitute an estimate of appropriate temperature ranges. Temperatures may deviate from these values, depending on the particular transport conditions.
Designation Temperature range Source
Travel temperature -1.5 – 0°C 
approx. 0°C 
Since the storage or transport temperature may vary depending upon the variety, exact refrigeration instructions should be obtained from the consignor. Where the grapes have a high sugar content, temperatures may be maintained slightly below freezing. Grapes with a low sugar content, on the other hand, require temperatures above freezing.
In order to prevent or limit the extent of postharvest water loss and consequent reduction in quality, the cargo must be refrigerated as rapidly as possible. The temperature of the cargo on loading should not be > 5°C.
The cargo and holds/containers should be precooled prior to loading.
Chilling damage begins at hold temperatures of approx. -2°C and generally results in total loss of the cargo.
Grapes require particular temperature, humidity/moisture and ventilation conditions (SC VII) (storage climate conditions).
Designation Humidity/water content Source
Maximum equilibrium moisture content 90% 
If shriveling of the grapes is to be avoided, relative humidity should be approx. 90 – 95%. However, relative humidity ranges of 85 – 90% are recommended in order to avoid moistening of the packaging materials, such as wood wool, and consequent mold growth on the grapes.
The cargo must be protected from seawater, rain and condensation water as moistening of the cargo and packaging materials increases the risk of spoilage.
Shriveling and drying of the grapes may be avoided by packaging them in perforated polyethylene film inside the fruit crates.
Understanding the Grape for Proper Packaging
Grapes have some traits that require understanding in order to be packaged properly.
January 14, 2019
KEYWORDS cold chain packaging / food packaging technology / fruits and vegetable packaging
Richard (Dick) Lindenmuth, president and CEO of Styrotek (styrotek.com), a California-based company that designs and manufactures EPS packaging for table grapes as well as other foods and pharmaceuticals, discusses grape packaging challenges and solutions.
Why is extending grapes’ shelflife a challenge?
Richard Lindenmuth: Unlike some other fruits, like pears and bananas, that ripen after harvest, grapes do not. They immediately begin to deteriorate — which is exacerbated by varied temperatures from the moment they are picked in the vineyard to the time they reach the grocer’s aisle, and ultimately, the consumer. We know that high temperatures and low humidity cause water loss from the grapes and stems, which ultimately increases the rate of deterioration. Getting the grapes in a cooler temperature and providing them packaging that allows the cooler temperature to be maintained preserves grape quality. This becomes very important to extending shelflife. Maintaining the cold chain with proper cooling and storage, using expanded polystyrene (EPS) grape shipping containers, shelflife can extend up to 120 days vs. less than 10 days with corrugated boxes.
What are benefits involved in extending grapes’ shelflife (for grocers, consumers, etc.)?
Lindenmuth: Extended shelflife is real economic for grocery stores, saving them money because of less damage and waste. Pre-harvest grape season begins May 1 and goes until December — and sometimes into January. More than 99 percent of grapes commercially grown in the U.S. come from California. The 2015-2016 grape season had a record setting crop value at $1.76 billion.
EPS table grape shipping containers are second to corrugated boxes with a 35 percent market share. About 50 percent of the grapes are shipped in corrugated boxes with a shelflife of less than 10 days and 20 percent waste.
For table grapes that will be used locally within 10 days, corrugated is fine. But, for farmers and grocers who want to either hold the grapes locally for a longer period or may be shipping them across country, EPS containers can extend that time up to 120 days. (When it comes to exporting grapes, they are all shipped in EPS packaging for protection and extending shelflife.)
Preserving the grape off the vine from field to the consumer’s table is important. That’s why farmers developed EPS packaging. It was a group of San Joaquin Valley grape growers who founded Styrotek 40 years ago. Today, Styrotek is still owned by California Central Valley growers who understand the critical need to preserve the shelflife of fruit.
Can you explain how packaging and shipping grapes in expanded polystyrene helps?
Lindenmuth: We know that grapes begin to deteriorate immediately at harvest. So, the aim of post-harvest treatment is to limit the rate of deterioration, and that can be done by maintaining the cold-chain process using EPS packaging — enabling quality grapes to make it to the grocery store aisle — so ultimately, consumers have fresh, juicy grapes when making their decision to buy or not to buy.
EPS is cost-effective because of its versatility and durability. Produce may stay in the same container from field to cold storage to transport and final destination. The produce is protected from damage and is shielded from moisture and/or heat. And, because the containers are lightweight, fuel costs for transport are less.
What were the results of the UC Davis study regarding EPS packaging and shelflife?
Lindenmuth: A published UC Davis study said that packing grapes in EPS containers has many advantages. Specifically, EPS does not absorb water; is a good insulator; and does not lose strength in refrigerated storage. Moreover, they are lightweight and can be forced-air cooled as quickly as corrugated.
EPS does not absorb water. Much of the weight loss of packaged produce is due to water absorption by paper and corrugated packaging.
EPS is a good insulator. The combination of its good thermal insulation and lack of moisture absorption maintains grape quality.
EPS retains its strength in high-humidity cold storage; corrugated boxes lose strength as they gain moisture in storage and transportation. EPS can stack two to three pallets high for long periods of time; corrugated can only stack two boxes high for short periods. The strength loss causes the boxes to sag after about two weeks, resulting in crushed and damaged grapes.
EPS boxes can be forced-air cooled as quickly as corrugated. The EPS boxes are packed with less interior packaging, allowing airflow through boxes, which helps with the cooling process.
EPS boxes are lightweight at about 1.1 lbs., which reduces trucking weight; corrugated weighs more about 1.8-1.9 lbs.
Aside from the economic upside of extended shelflife for produce such as grapes, what environmental benefits might result?
Lindenmuth: There is a lot of misinformation out there, and many people are not aware that EPS material is 100 percent recyclable and can be repurposed into a wide range of products — from bike helmets and picture frames to wine shippers. A California nonprofit uses recycled EPS foam to make sustainable surf boards. EPS leaves a smaller footprint on the planet than other comparable materials; reduces waste in the supply chain; is ozone-friendly; and uses less fuel for transport. Many of our customers have a commitment to reduce or eliminate waste completely. EPS packaging helps companies achieve environmental priorities.
Do you see consumers as more interested in environmental impact regarding not just produce, but all of their purchases?
Lindenmuth: The EPS industry across a broad range of industries, including EPS insulation used for non-food items such as car bumpers, is growing and expected to go from $15 billion to around $22 billion over the next few years. So there will be more emphasis on going green and becoming more sustainable. Clearly, there seems to be a systemic shift in the recycling infrastructure and establishing a circular economy for EPS.
Recycling is an important part of sustainability and is increasing, along with the growth in new collective programs in the industry. EPS is not organic, so it has to go through a process. Shipping EPS containers that are full of air to a recycling facility can be expensive, which is one of the reasons why people are not doing it. But, one of the biggest positives in the industry over the next three or four years is new technology that recycles expanded polystyrene and massively reduces its transportation costs.
There’s a new method we are going to be part of, an experiment in Canada, where a company called Polystyvert Inc. has invented a process where they can take our recycled material through a chemical process; restore it back to its original state; then send it back to the manufacturers of chemical resins — who will then make it into resin and ship back to us to manufacture new products. [This is] a complete, full-circle recycling using a decentralized model that helps reduce transportation costs.
If so, what changes in the food/beverage industry might be ahead?
Lindenmuth: I do see that there’s more use of the containers (Fish Boxes, etc.) to display the fruit and other products in the stores, right in the EPS box. Therefore, there are more and more requests for printing artwork and the brand name on the containers. We will begin to do this this year ... Holiday Grapes, Goblin Grapes, etc.
Explain how EPS differs from Styrofoam.
Lindenmuth: The EPS material is not Styrofoam and is manufactured differently, in that the EPS manufacturing process does not use CFCs, HCFCs, HFCs or formaldehyde — which are harmful to the ozone layer. Styrofoam—a registered trademark brand of another company — has never been used to hold food.
What are some applications for this packaging beyond grapes?
Lindenmuth: Our packaging can be used for many kinds of berries, such as strawberries and blueberries. We also supply our packaging for other types of food, as well as to the pharmaceutical industry. We invested in new technology and equipment. We have the ability to manufacture products quickly with no additional permits needed and have the supply chain. And, most importantly, we have a committed workforce to do the work.
Table Grape Packing Process
Table Grape Harvest, Packing & Much More!
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Post published:July 13, 2021
What an exciting trip to visit a table grape harvest and packing operation at Illume Ag in the Coachella Valley! The grapes shown here are the Midnight Beauties, which are black seedless grapes.
Just to add a little bit of context, the Coachella Valley is the desert located in Southern California. I visited this operation in the middle of June, when the temperatures were well into the hundreds. The benefit of working within the grapevines is the extra shade, but there is no escaping the humidity!
Where are Grapes Grown in California?
When is Grape Season in California?
Table Grape Harvest & Packing
Table Grape Harvesting
Table Grape Packing
Questions about Table Grapes
Where are Grapes Grown in California?
Table grapes (not wine grapes) are grown in two main regions throughout California: the Coachella Valley (southern California) and the San Joaquin Valley (central California). The example provided here is from a growing operation in the Coachella Valley.
When is Grape Season in California?
Between the two main growing regions in the state, table grapes grown in California are available from May to January, with the season starting in the Coachella Valley and ending in the San Joaquin Valley. So, make sure you check your grape packaging to see that you are buying California grown grapes when/where possible!
Table Grape Harvest & Packing
For a detailed overview of the entire harvesting and packing process, check out the video below! In it, you can also see what the bunches look like before harvest compared to the bunches you buy at the grocery store!
Table Grape Harvesting
The table grape harvesting process is as follows:
The harvesters take carts into the rows of grapevines to harvest. These carts contain multiple bins that will be filled as the workers harvest. (At this particular operation, they use bags to cover the bins and then switch them out once they get dirty.)
They clip the bunches from the grapevines using clippers, and then thin out the bunches based on quality aspects (e.g., not the right color, pruned, has some sort of physical defect such as cracking). One really notable difference regarding the table grape harvesting process is the slowness and attention to detail. The workers are carefully selecting the grapes that are ready for harvest, and then removing the fruit that does not meet quality specifications.
Once all of the buckets have been filled, they take them to a designated packer.
Harvesting Table Grapes
Table Grape Packing
Once the carts have been filled, the containers go to a designated packer in the field. Each packer has a set packing table, where they pack the table grapes into bags and then into boxes/containers. In this case, you can see that they are packing the bags into both cardboard boxes and Styrofoam containers. This is based on the customer, and in this case, the Styrofoam containers were for international customers.
Table Grape Packaging
One thing that is consistent regardless of the actual box type, is the set up of the box. There is a bag, a liner and then after all of the bags of grapes have been included, a Sulfur Dioxide pad is included, the bag is closed and so is the outer container. The Sulfur Dioxide pad acts as a fungicide, which is very commonly used with conventionally grown table grapes, and the use is included on the label for each bag of grapes, as well as the label for the outer packaging. (Click here if you would like to learn more about why and how Sulfur Dioxide is used.)
Table Grape Packing Process
During the packing process, the packers are doing a second quality check. They have a set of clippers and will remove any fruit that does not meet quality specifications that the harvester might have missed.
The packers then determine which/how many clusters go into each bag.
Eight bags fill up a box, are weighed to make sure they fulfill the weight requirement for the customer and then are palletized.
Once the pallets are complete, they are covered and then will be taken to the cooler to await transportation to the customer.
Packing Table Grapes
Packing Table Grapes
I would like to send a big thank you to Illume Ag for allowing me to visit and to share this content!
Questions about Table Grapes
How are table grapes harvested?
They are harvested using clippers, and the workers remove any excess/undesirable fruit.
Are any tools used to harvest table grapes?
Yes, small clippers are used.
Do all of the grapes get harvested at the same time or are there multiple passes through the vineyards?
All table grapes are harvested at the same time. Any fruit that may not be ready for harvest or are already passed their harvest time are left on the vines.
How do they know when table grapes are ready to be harvested?
This is specific based on the grape cultivar being grown, but in general, it is based on the Soluble Solids Concentration (SSC) and color of the fruit. The actual time of harvest is determined based on taking a sample of the fruit in each block and using the results to determine if the fruit is ready for harvest. These are referred to as maturity indices, and you can learn more of the specifics related to table grapes on the UC Davis Postharvest website.
Are table grapes packed in the field?
Yes, they are commonly packed in the field (as you saw in the example above). There are also other examples of them being packed in packing houses, but it is a similar process and no wash water is used.
Are any tools used during the table grape packing process?
Yes. Packing tables and clippers are used. The packing tables are used to pack the grapes, and the clippers are used to remove any fruit that might have been missed during the harvesting process.
Why are Sulfur Dioxide pads used with table grapes?
To help prevent pathogen growth and to keep the green stems greener for longer. The green stems are an important marketing aspect, as consumers associate them with freshness.
Are Sulfur Dioxide pads used with organic table grapes?
No, they are not allowed to be used with organic grapes. (Click here to read more about the use of Sulfur Dioxide and the associated rules.)
Does the grape bag/packaging reflect the use of the Sulfur Dioxide pads?
Yes, all grape bags (and any other form of direct packaging) is required to include that Sulfur Dioxide has been used. In the example above, the use of Sulfur Dioxide is included on each individual grape bag, as well as on the boxes.
Do grapes continue ripening after harvest?
No, they do not continue ripening after harvest and are considered non-climacteric fruit.
How should I store grapes at home?
Grapes should always be stored in the fridge.
How should I wash grapes at home?
The FDA recommends that you only wash them with water. You should also make sure and wait to wash them until right before you are going to eat them. This will help them last for longer.
Most of South Africa's table grapes are exported to the EU and the UK. In recent years, prepackaged table grapes are now preferred in many European supermarkets. This increased demand has resulted in stringent quality standards, including the specification of punnet mass. Locally, table grapes are packed manually using seasonal labour, who often have limited formal education. Punnets must conform to upper and lower mass limits, but many deviations occur due to human and machine error. Check-weighing proved effective in reducing out-of-specification punnets, but human and machine errors were still problem factors.
Despite concerns about packaging material wastage [1, 2], the display space devoted to pre-packed fruit and vegetables in European supermarkets, and the sale of pre-packed products in the United Kingdom, have increased -by 70% and 71% respectively during the past decade . Most of South Africa's table grapes are exported to these countries . Currently, only 30% to 40% is pre-packed in transparent 500g containers known as punnets, but the growing demand is driving a change towards punnet-packing [5, 6, 7].
Punnet-packing is labour intensive, and quality assurance is a major challenge. The supplier must take into account a 2% to 3% mass loss due to a reduction in moisture content during shipping , to ensure a net mass of at least 500g for each punnet upon arrival overseas, when packing to the minimum system. Alternatively, an average mass of 500g per batch of 1,200 punnets with a tolerable negative error of 15g per punnet must be ensured when packing to the average system [9, 10]. Sampled quality inspections are performed at the inland pack houses and on arrival overseas [11, 12]. Under-mass punnets may lead to the rejection of a pallet containing 1,200 punnets. The upper mass limit depends on the producer, and an optimal ratio of productivity-to-oversupply beyond specification - referred to as 'giveaway' - needs to be found.
Several systems are available for packing grapes into punnets. All systems rely on humans to do the work, and all have some means to assist the packer to achieve the correct mass. None of these are, however, entirely error-proof. In an effort to eliminate underweight punnets and to limit 'giveaway', producers have started implementing external check-weighers, as well as replacing old systems with new ones featuring mandatory internal check-weighing.
During the 2011/2012 packing season, four different systems employed by three large grape pack houses were evaluated. The four systems are described in the following section. The data sources and data gathering methodology are explained in section 3. The results are presented and discussed in section 4, followed by the conclusion in section 5.
In order to ensure complete customer satisfaction in a mass production environment, the quality of all products must conform to the required quality standards [13, 14]. In the grape packaging industry, all possible steps are taken to ensure quality during production , but in the end, the quality of finished products is dependent on humans. The only way to ensure that all products leaving the pack house comply with minimum quality standards is through 100% final product inspection . Check-weighing can, however, be used to inspect one of the key quality parameters, that of punnet mass.
Punnet packaging systems were evaluated during this study. Three were existing systems with external check-weigher modules added before the beginning of the packing season, and one was a new system with integrated internal check-weighing only. Internal check-weighing refers to the built-in functionality of a punnet packaging system to perform check-weighing on punnets; and external check-weighing refers to the addition of an automated external check-weighing module to the existing system. Table 2.1 gives a summary of the four systems.
Table 2.1: Punnet packaging systems implementation
The different punnet packaging systems and check-weighing methods are discussed in the rest of this section, as well as the causes of error. For the systems with an additional external check-weighing module, the systems are discussed in their basic form without the external check-weigher. Figure 2.1 shows the symbols used in the process flow diagrams to describe the systems.
2.1 Punnet packaging system: No internal check-weighing
The generic scale system using the hand-packing method is described in Figure 2.2. It is often referred to as the guess-and-cut method, and can be difficult for labourers with a limited formal education to master. According to studies, this method could be cognitively exhausting for workers with limited education due to the monotony of the task  . The generic scale system is the most labour intensive method, and requires rather intensive management to be successful . This method does not have any technology-supported built-in check-weighing functionality, as the only feedback is the mass shown on the scale display. Packers are instructed which mass values to use for upper and lower mass limits, and are assumed to conform.
2.2 Grape Punnet packaging system: Internal check-weighing
Many of the existing punnet packaging systems incorporate some type of internal check-weighing system. These internal check-weighing systems rely on, or make assumptions about, the human operators and the dynamics of the grape supply, therefore making them prone to errors .
In addition to the normal upper and lower mass limits, the three systems in this section also have a target mass parameter. The system will aim to produce punnets with a mass closest to the target. The local storage combination system and computer-supported scale system make use of dynamic target mass adjustment. In the local storage combination system, the mean fill-mass is kept as close to the target as possible by dynamically adjusting the upper limit to push the band of permitted mass either upwards or downwards towards the target mass. In the computer-supported scale system, both the lower and upper limits are simultaneously adjusted to keep the allowable bandwidth constant .
The local storage combination system consists of a vertical storage rack with a microcontroller-based scale on each side, featuring dynamic target mass adjustment. Figure 2.3 describes the operation of this system. The two sides are integrated using common in-process storage. For optimal operation, three people are required on each side. The three workers on each side switch position throughout the day and cross-check each other's work .
Because of the monotony of the task, errors are bound to occur . This creates a need to remove the possibility of human error from the system . This system therefore has an internal poka-yoke type error-recognition system that recognizes an incorrect punnet before it is finished , and that is independent of lapses in the operator's attention span . After the contents of the two complimentary punnets have been added together, the internal check-weighing function verifies that the net mass of the punnet is within specification, and a green go-ahead light is activated on the display. However, it is still dependent on the operator/packer to execute his task as instructed.
The conveyor system is a computerised horizontal conveyor combination system, described in Figure 2.4. The system runs at high speed, and packers need to be constantly alert not to miss a bunch or pick up the wrong bunch. This system's internal check-weighing is based on combinations made by the computer, and assumes that packers do not make mistakes and that no loose berries fall from bunches when picked up from the cups. The check-weighing function gives no feedback to the packers.
Grape Conveyor system process flow diagram
The computer-supported scale system is a new entry into the South African market. It is a computer-based scale-assisted hand-packing system with dynamic target mass adjustment, as described in Figure 2.5. Each scale dynamically adjusts its target mass and assists the packer by means of LEDs to achieve approximately the desired punnet mass. When a punnet with a mass outside the lower or upper limits is removed from the scale, the functional type internal poka-yoke  check-weigher locks the user interface, and a supervisor code must be entered to continue operation.
2.3 Punnet packaging: Causes of error
According to Kolarik, there are two types of manufacturing errors: machine errors and human errors . Most machine errors occur as a result of the incorrect set-up of a system. The various causes of error are discussed in this section. It is clear that the punnet packaging systems reviewed have some inherent opportunities for human error.
All the systems discussed in sections 2.1 and 2.2 require an initial set-up at the beginning of the day, or after being restarted during the day. With the generic scale system, the scales need to be tared with an empty punnet in order to ensure the correct net mass. The set-up of the other systems investigated is somewhat more complex, requiring setting up of the lower and upper mass limits as well the target mass.
Most machine errors occurred when scales were not tared correctly during the initial setup, or when the tare button was accidently pressed during operation. When using the generic scale system, only one scale is affected, and in most cases results in below-mass specification punnets. If the operator does not notice this, he or she could unknowingly beproducing below-mass specification punnets . When using the local storage combination system, a tare error on one or both of the scales will result in wrong combinations. If only one scale has a tare error, the other scale's built-in check-weighing system will show that many combinations have an out-of-specification net mass, and the operator will realise that errors are occurring and alert a supervisor. A tare error in the computer-supported scale system will continue producing under-mass specification punnets if not noticed by the packer or supervisor.
With the conveyor system, berries that fall from bunches when handled are left behind in the cups, and can result in underweight punnets. Berries protruding from punnets may get dislodged before the punnet is closed, resulting in under-mass punnets.
The Yerkes-Dodson human performance curve depicted in Figure 2.6  shows that performance is related to the levels of arousal, stress, and anxiety experienced. It is difficult to determine the level of stress or arousal that results in acceptable performance because it differs between individuals . In the pack house setting, the level of stress and anxiety can become rather high, especially just before lunch and later in the afternoon when the amount of grapes in the pre-cooler is perceived to be an insurmountable task for the day. (The general practice is that grapes should be packed on the same day that they are picked.) Workers become exhausted by the monotony and by their standing position . Human errors mostly occur during these periods.
Figure 2.6: Yerkes-Dodson human performance curve 
Kolarik defines a human error as an out-of-tolerance action, where limits of acceptable performance are defined by the system . Two types of human error apply to grape punnet packaging: errors of omission, where part of a task is omitted; and errors of commission, where a task is performed incorrectly. In all four punnet packaging systems, packers repeat the same actions over and over throughout the day, and with the local storage combination system in particular, they tend to develop shortcuts for some actions in order to work faster. The built-in check-weighing function is often judged to be an optional extra step that can be omitted to save time without affecting the operation of the system. When performance is below acceptable limits, the probability for human error increases , and it often happens that a punnet is accidently put into a wrong position on the rack, or that a command to put a punnet on to the rack is confused with a command to add two punnets together. With the conveyor system, the same problem occurs when packers add wrong bunches together, or when loose berries are left behind in the cups. Although the computer-supported scale system locks the user interface in the event that a punnet with out-of-tolerance mass is removed, it is still up to the packer to recheck the mass of the punnet, and sometimes this step is omitted with the out-of-tolerance punnet put on the conveyor line instead. These two types of errors combined could result in a large number of under- and overweight punnets.
From this discussion it is clear that the current solutions for packing punnets are not mistake-proof, and that success ultimately relies on the human operators performing their task correctly and not circumventing built-in error-correction functionality. These operators, however, are mostly seasonal workers with a limited formal education and low level of numeracy [27, 28]. This proves to be a challenge in the mistake-proofing process. An automated 100% mass check of the final product could ensure that no punnets with out-of-tolerance mass go through the process undetected.
2.4 External check-weighing module and integration
An external, independent check-weigher can be added to an existing system with minimal change to the infrastructure . The check-weigher used in the evaluation is a modular system comprising a load cell-and-conveyor belt combination with control panel and controller, as well as a separate faulty package rejection unit to remove the punnets with out-of-specification mass. The unit is shown in Figure 2.7.
Figure 2.7: Automated external check-weighing module
Although the check-weigher is fully automated, it still relies on a human to set it up and to ensure that the air pressure for the pneumatic rejection unit is adequate.
3. DATA COLLECTION
During the 2011/2012 packing season, data from the four different systems described above was collected by collating the check-weigher data and by extracting stored data from the computer controlled system.
For validation of the results, a comparison was made with information from the company's UK operation. Upon arrival in the UK, the open-top punnets are flow-wrapped. During this process the punnets are check-weighed [1, 11, 29]. Two sets of data from the 2010/2011 and 2011/2012 packing seasons were obtained from the UK.
4. RESULTS AND DISCUSSION
To facilitate comparison, 2,5% was subtracted from the net mass at production to compensate for moisture loss during shipping.
4.1 Punnet mass distribution
Data collected and analysed during the 2011/2012 season, with more than 65,075 punnets of grapes packed, display distinct differences between the packing systems studied. The generic scale packing technique shows a distribution skewed towards the upper limit in Figure 4.1, with an intuitive implication of increased giveaway.
Figure 4.1: Generic scale system punnet net mass distribution
The local storage combination system, shown in Figure 4.2, displays a skewed distribution towards the lower limit. It indicates that the system's algorithm, which provides a target mass independent of the upper and lower limits, has a marked effect, with a positive influence on giveaway.
Figure 4.2: Local storage combination system punnet net mass distribution
The conveyor distribution system, as shown in Figure 4.3, indicates that this system also has a skewed distribution towards the lower limit. The distribution is also wider relative to the other systems tested, with a marked tail towards the upper limit.
Figure 4.3: Conveyor system punnet net mass distribution
From Figure 4.1, the net mass of punnets when packed using generic scales has a mean (µ) of 513,3g, a standard deviation (σ) of 7,2g, and a mode of 515g. The clearly cut tails at 498g and 528g correspond to the mass limits of the external check-weigher. The punnet-packing process is totally random when using this system. Punnets can be made up of any arbitrary bunches as long as the resulting mass falls between the upper and lower limits. Although the limits for the packers were the same as on the check-weigher, 10,3% of the day's 13,042 punnets were rejected by the external check-weigher.
Figure 4.2 depicts the net punnet mass distribution for the local storage combination system. The mean (µ) is 509,8g, the σ is 9,2g, and the mode is 503g. This skew distribution can be attributed to the dynamic target mass function of the system, which aims to make a punnet combination closest to the target mass. The target mass was the same as the lower limit on the system - 497g. Therefore, at a mere 6g over target, the mode shows that the targeting functionality has an observable effect. The upper limit of this system was set higher than the generic scales because the operators were still new to the system, and a larger mass bandwidth makes it easier to use in the beginning. The check-weigher's limits were set up accordingly. Of the day's 12,847 punnets, 4,8% were rejected by the external check-weigher. This shows that the local storage combination system's internal check-weighing function is relatively effective, but that human error still exists.
The net punnet mass distribution for the conveyor system is shown in Figure 4.3 with µ 521g; σ 17,5g, and mode 496g. This system's standard deviation is approximately double that of the generic scale and the local storage combination systems. As it functions as a combination system with a limited storage area on the conveyor, throughput can be increased by extending the bandwidth. During data collection, the bandwidth was set to 71g to increase throughput further. Nevertheless, the mode of the distribution shows that combinations closer to the target mass of 491g indeed have higher priority. Of the day's 17,940 punnets, 4% were rejected by the external check-weigher.
According to its log, the computer-supported scale system produced 21,246 punnets on the particular day, with an average net mass of 502,2g, marginally higher than the 502g target mass. The dynamic target mass adjustment function implemented in this system is therefore effective. As the system does not have an external check-weigher, the punnet mass distribution is not available. However, the internal check-weigher uses the same system parameters to perform its task.
It therefore becomes clear that the check-weighers detected under-mass punnets caused by human or machine errors. In the next section, the feedback data from the UK is used as validation for the effectiveness of the check-weighers.
4.2 Validation by means of punnet mass feedback from UK
The punnet mass data from the UK sketches a clear picture of the effect of external check-weighing (generic scales, local storage combination system, and the conveyor system) and mandatory internal check-weighing (computer-supported scale system). The two sets of data from the UK are shown in Figure 4.4. During the 2010/2011 season, the generic scale, local storage combination, and the conveyor systems were the same as in the following year, but without integrated external check-weighers. In the last pack house a generic scale system was replaced by a computer-supported scale system for the 2011/2012 packing season. Since 2010 the emphasis on minimum and maximum punnet mass has increased, and has resulted in a lower and smaller allowable mass bandwidth. Lower limits were reduced to embrace the average system. The upper limits were set by the company as a target to reduce giveaway.
Figure 4.4: UK punnet mass feedback
Feedback punnet mass data from the UK for the generic scale and local storage combination systems are depicted in Figure 4.4. From the graph it is clearly seen that 2011/2012 had improvements on every level following the addition of the external check-weighers. The large number of overweight punnets can be attributed to the pack house using a higher upper limit than in the UK. This was either a communication error, or the pack house manager deliberately decided to keep the mass bandwidth larger in order to increase the throughput. Although there has been a significant decrease in underweight punnets, they account for 2,8% of the total checked in the UK.
For the conveyor system, the feedback results are shown in Figure 4.4. Again, an increase in punnets with the correct mass is observed, but it is small compared with the increase seen for the generic scale and local storage combination systems. The small decrease in under-mass punnets could be because open top punnets were used, resulting in moisture loss greater than the 2,5% used for calculating the lower mass limit. The high number of over-mass punnets was a result of the high upper limit set for the system.
The feedback results for the computer-supported scale system are also shown in Figure 4.4. It is important to note that the 2010/2011 results were for a generic scale system that was replaced by a computer-supported scale system for the 2011/2012 season. A 12,2% increase in punnets of the correct mass was observed as a result of the decrease in over- and under-mass punnets. In spite of the 10,2% decrease, 7,7% of the punnets were still over-mass. A possible reason could be that the supervisor's code was entered to reset the internal check-weighing function warning without re-checking the punnet mass, if they realised that it was an over-mass error.
The continued occurrence of underweight punnets during the 2011/2012 season is attributed to human and machine errors, as described in section 2.3. As stated there, even though the check-weighing module runs fully automated, it still needs to be set up by a human. A typical example of this was when the air pressure for the rejection unit was not turned on during set-up, and although the check-weigher detected an under-mass punnet, the rejection unit could not remove it from the production line.
Proximity sensors are used to determine when there is a punnet on the load cell. A typical machine error occurred when one of the sensors was misaligned after being hit by a punnet. This caused the check-weigher to miss a large number of punnets - some of which possibly had an out-of-limit mass.
4.3 Check-weigher cost analysis
In 2011 a survey was conducted on the occurrence of under-mass punnets and the associated penalty costs for rejected pallets . Based on this, the cost implications for implementing check-weighing at the pack houses studied are calculated. The cost of having to repack an entire pallet, together with the loss of production and sale of another pallet, is estimated to be R10,000 (2011 prices). The cost of an external check-weigher module is R90,000, and the cost of a computer-supported scale system is R1,500,000. The total cost savings from a decrease in under-mass punnets is calculated using the total number of pallets produced at the end of the 2011/2012 season at each of the pack houses, and the percentage decrease in under-mass punnets for each. The payback period for the external check-weighers at each pack house was calculated in years or 'packing seasons'. The calculations are shown in Table 4.1.