In fruit packing, attention is often focused on whether the finished package reaches the target weight. From an economic point of view, however, it is equally important how close each successive punnet is to that value.

If the weight is too low, the result is an underweight package. If more product than necessary enters the package, it becomes overweight, also known as product giveaway.

Both situations are undesirable. Underweight packages may create problems during final product control and require correction. Overweight packages, on the other hand, contain product that is given to the customer without generating additional revenue.

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What is underweight?

Underweight occurs when the actual amount of product in the package is lower than the value required for a given process or the adopted packing standard.

In practice, this may mean that the package has to be stopped, checked again, topped up with fruit or removed from the process for correction.

If the problem occurs frequently, it affects not only the weight of individual punnets but also the efficiency of the entire line. Operators have to perform additional tasks and the finished product may require reweighing.

Underweight may result in:

• the need to recheck packages,
• manual topping up of fruit,
• stopping or slowing down the line,
• greater operator involvement,
• problems during final product inspection,
• customer complaints or rejection of a batch if weight requirements are not met.

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What is overweight?

Overweight is the opposite situation. The package meets the minimum product quantity requirement, but contains more fruit than intended.

From a quality control point of view, such a package may not create a problem. From the perspective of production economics, however, every additional blueberry is product for which the packing facility receives no additional payment.

If the label specifies a certain product weight, the customer pays for the package, not for every additional gram inside it.

For this reason, overweight is often less visible than underweight, but it can generate very significant losses over the course of a season.

One extra gram is not much – until it is multiplied by the number of packages

The scale of the problem is easiest to understand with a simple example.

Assume that a line produces 20,000 packages per day and the average overweight is only 3 grams per package.

• 20,000 packages × 3 g = 60,000 g,
• which means 60 kg of additional product every day.

Over 30 working days, this already amounts to 1,800 kg of blueberries supplied above the intended package weight.

This is not damaged or discarded product. It reaches the customer, but the packing facility receives no additional revenue from it. This is why a few grams of overweight may be difficult to notice in a single punnet while becoming very costly across total production.

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Which is more expensive – underweight or overweight?

There is no single answer that applies to every packing facility because the two problems generate different types of cost.

Underweight is usually more visible. It creates an immediate operational problem. The package has to be corrected, the operator must react and, in some cases, production may need to be stopped.

Overweight is often less noticeable, but it continues all the time. If every punnet receives a few extra grams of fruit, losses increase with every hour of production.

For this reason, from an economic point of view, a small but constant overweight can be particularly costly. The line operates normally, the product passes inspection, yet the cost remains hidden in the amount of blueberries being used.

Why not simply set a lower target weight?

A natural reaction to excessive overweight might be to reduce the target value. However, this approach has its limitations.

Every weighing process has a certain amount of variation. If the average weight is set too close to the lower limit, the number of underweight packages will increase.

The goal is therefore not simply to reduce the average weight. It is much more important to reduce the variation between successive packages.

The more repeatable the process is, the closer the average weight can be set to the required value without increasing the number of underweight packages.

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Repeatability is more important than one perfect weighing result

When evaluating the performance of a weighing machine, it is not useful to focus only on one package that weighs exactly as intended.

What matters much more is the result of the next several dozen or several hundred packages.

If one package is close to the target while subsequent punnets differ significantly from it, the operator must maintain a larger safety margin. This usually means setting a higher average weight to reduce the risk of underweight packages.

If, however, the results remain closely grouped around the target value, it is possible to reduce the average overweight without increasing the number of incorrect packages.

What can cause unstable package weights?

Not every difference in weight is caused by an error in the weighing system itself. The process is also influenced by the way the product is supplied to the weighing machine and by the behavior of the fruit during dosing.

It is worth checking:

• how evenly the blueberries are supplied,
• the stability of product flow to the weighing machine,
• the settings of fast and fine dosing,
• vibrations transferred to the weighing system,
• correct taring of the scale,
• cleanliness of the weighing components,
• whether any mechanical component is touching or supporting the weighing section,
• changes in fruit size and the weight of individual berries,
• the type of package being used.

The heavier an individual berry is, the more difficult it also becomes to make a very small weight correction. If, for example, only 1 gram is missing from the target weight and the next blueberry weighs several grams, adding a single berry will automatically create some overweight.

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Incorrect taring can create a constant error

Correct taring of the weighing system is particularly important. If the scale is tared incorrectly, the resulting error may then be repeated across subsequent packages.

In such a situation, the value displayed on the screen may appear correct even though the actual product weight differs from the expected value.

For this reason, after taring or changing the package, it is worth periodically checking finished punnets on an independent checkweigher or scale. This makes it possible to detect a systematic error before it affects a large production batch.

Changing the package requires the process to be checked again

A new punnet may have a different weight from the previous one. If this change is not taken into account during taring or process configuration, the difference may be transferred to the final result.

Therefore, after changing the type of package, it is important to check not only its guidance and filling but also the values obtained during weighing.

Correct package adjustment is also important from a mechanical point of view. A fruit stream that is too wide or incorrect punnet positioning can cause blueberries to fall outside the package. More information can be found in our article “How to Prevent Blueberries from Spilling During Package Filling?”.

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How can overweight be reduced without increasing the number of underweight packages?

The best solution is not to continuously reduce the target value, but to improve the repeatability of the entire process.

1. Ensure the product is supplied as evenly as possible.
2. Correctly adjust fast and fine dosing.
3. Check the tare regularly.
4. Verify the actual weight using an independent scale.
5. Analyze a series of packages rather than a single result.
6. After changing the variety, fruit size or package, check the settings again.
7. Monitor not only the number of underweight packages but also the average overweight.

This approach makes it possible to gradually reduce the amount of excess product in the packages without shifting the problem in the opposite direction and creating underweight packages.

It is worth analyzing kilograms, not just grams

During everyday work, an operator may see only a difference of a few grams. A production manager should look at the same result over a day, a week and the entire season.

A simple way to calculate excess product is:

number of packages × average overweight = total amount of product supplied above the target value

By multiplying the result by the price per kilogram of blueberries, the actual cost of overweight can be estimated.

Such a calculation often shows that even a small improvement in process accuracy and repeatability can produce a noticeable economic benefit.

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Weighing accuracy affects the actual efficiency of the packing facility

Line efficiency should not be evaluated solely on the basis of the number of packages produced per hour.

If achieving high speed requires significant overweight or causes frequent underweight packages that need manual correction, the actual benefit may be much smaller.

For this reason, efficiency should be analyzed together with dosing accuracy, the amount of rejected product, product losses and operator working time. A similar approach is important when choosing blueberry sorter capacity for daily fruit volume.

Automation helps control the cost of every package

In a large packing facility, even a small improvement in process repeatability can make a significant difference because it is repeated across tens of thousands of packages.

Automatic weighing and dosing help maintain more repeatable process conditions than manually adding fruit, especially with large numbers of packages and many hours of production.

Automation does not eliminate the need for control. Settings should be matched to the current product, package and expected weight, and results should be checked periodically.

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Summary

Underweight and overweight are two different problems. Underweight usually attracts attention more quickly because it requires operator intervention and can create problems with the finished product. Overweight is less visible, but every additional portion of blueberries reaches the customer without generating additional revenue.

For this reason, the goal of a correctly adjusted process should not be limited to avoiding underweight packages. Reducing average overweight by improving the repeatability of dosing and weighing is equally important.

The most important factors are stable product feeding, correct taring, verification of the actual weight and analysis of a larger number of successive packages. The smaller the variation in results, the closer the process can operate to the required value without unnecessarily giving away product.

Combined with automatic sorting, this makes it possible to use the raw material more efficiently and improve the performance of the entire packing facility. More information about Green Sort solutions can be found on the blueberry sorter page.