Dry Matter Assessment or Firmness – Alternatives to the Penetrometer

Though firmness has been popularly used to assess the quality of fruit, fruit dry matter is now increasingly being used instead. It is necessary to understand both traits to find the reason behind this shift in preference.

Dry Matter

When water is removed from produce, what is left are the solids, or the dry matter. The dry matter (DM) of plants is made up of many soluble compounds, called the soluble solid contents (SSC). In other words, the dry matter of fruit is a measure of the amount of nutrients and fiber present in it.

The bulk of SSC is made up of sugars such as fructose, sucrose, and glucose. Fruit sweetness depends on the total amount of sugar, which is estimated by degrees Brix. However, not all consumers like fruit that is only sweet; some balance of taste with sourness derived from acids is desirable. Thus, though DM correlates with SSC, it serves as a better estimate of taste than SSC. Research studies have found that consumers rate fruits such as apples and cherries – which have higher DM – as the tastiest.

Research has shown that the DM pre-harvest, at harvest, and post-harvest are related. Thus, DM measured pre-harvest or at harvest is a reliable indicator of the quality of fruit after storage.

Measuring DM

In the laboratory, DM is measured by drying fruits in a conventional or microwave oven, and calculating this dry weight as a ratio to the fresh weight (or weight of the fruit before drying).

Given the usefulness of DM as a measure of fruit quality and taste, other techniques are necessary to estimate it in the field.

Near infrared (NIR) spectroscopy is a non-destructive method to use on fruit to estimate their DM levels without affecting produce in any way. This technology is easily used with small handheld instruments. Two such tools are the F-750 and F-751.

The F-750 Produce Quality Meter that was developed through partnership with Central Queensland University can be used for all kinds of fruit. It measures and provides quantitative metrics for DM and TSS, and qualitative metrics for quality and taste. The accompanying software makes it possible to create models suitable for different varieties, which are then used to evaluate the fruits’ quality. Up to 25,000 readings can be stored on its drive and transferred to a computer for further analysis. The various advantages of F-750 are that it:

• Is suitable for use in the field for farmers to determine the fruit maturity to fix the right time for harvesting.
• Can estimate fruit quality of imported or stored produce.
• Measures fruit quality prior to processing.
• Is an instrument precise enough to be used in the laboratory. Many studies reporting research with its use have already been published.

The F-751 Avocado Quality Meter is similar to the F-750 but is designed to specifically evaluate the quality of avocados based on their dry matter content. Avocados do not ripen after harvest, so their harvest time needs to be optimised by field measurements to determine if they are ready.

Firmness

Firmness is the softness or hardness of a fruit. It is a measure of fruit texture that is used as an indicator of ripeness and fruit quality. Two of the common methods to measure firmness are the application of pressure and the deformation of the fruit.

Penetrometers

Some of the Penetrometers on the market which measure firmness are:

• Magness-Taylor’s Pressure Tester, based on a punch test. It is one of the most popular means of measuring firmness. However, studies have shown that there can be a degree of error in the readings depending on fruit size and position of fruit on the tree.
• Fruit Penetrometers with gauges that measure both pressure and compression.
• Drill-based Penetrometers with metal tips for probing the firmness of fruit digitally.

Only a handful of Penetrometers can be used for all kinds of fruits. Some are made to test soft fruits such as peaches and tomatoes, while others are needed for hard fruits and vegetables such as apples and pumpkins. All penetrometers:

• Are not accurate in estimating quality.
• Damage the fruit, leaving behind holes so that they have to be discarded.
• Are time-consuming to use.

NIR Instead of Penetrometers

Since penetrometers are destructive, much effort has been put into finding non-destructive alternatives to measure firmness.

• Most attempts have been ineffective, such as near infrared (NIR) spectroscopy, which has been tried because it is a fast method of testing firmness. However, it has been largely unsuccessfully with a variety of fruits.
• Some technology was not suitable for use in the field, even if they successfully test firmness, because the technology requires large instruments and machinery. Magnetic resonance imaging (MRI) is an example.
• One of the successful alternatives to penetrometers is Deformation Testers, which measure the force necessary to deform soft fruits, ascertaining their firmness.

DM is a Better Measure of Quality

There are many reasons why dry matter is now considered a better measure of fruit quality than firmness.

Determining Maturity

One of the main purposes of quality estimation is to determine maturity and the best time to harvest, and this is not possible when relying on firmness. Firmness is only one of the indicators of fruit quality and cannot be used independently to quantify maturity. Using DM, it is possible to optimize harvest time based on the ripening behavior of each fruit type and variety.

Indicator of Taste

While firmness as an indicator of ripeness is important, it does not provide any information on the taste, which depends on sugars and acids present in the fruit. DM, the measurement of the solid content of fruit, provides information on taste.

Technology

There is available technology which can precisely measure DM for any fruit or vegetable. This is already commercially viable and is in use in fields and laboratories. On the other hand, precise measurements of firmness are still not possible in the field with small handheld tools.

Advantage of Dry Matter

Dry matter is the more reliable measure of quality, as it an indicator not just of texture but also of fruit biochemistry. Moreover, the efficient, economical, and practical tools available to measure dry matter make judging fruit quality easy and accessible to all.

—

Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Source

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