Measuring quality

Quality assessments can be either objective or subjective. Objective measurements involve measuring something, whereas subjective measurements rely on individual judgements.

Objective measurements

Objective quality attributes are those that can be measured. They include size, weight and colour as well as internal quality factors such as sugar and acid content and nutritional value.

The easiest-to-use objective measurements are quantities such as weight, length and diameter. These are commonly used in grading equipment to automatically sort products by size. Colour can also be measured objectively using various devices. Objective measurements also include nutritional content—sugars, acids, fibre, antioxidants and various other compounds that may be beneficial to human health.

Firmness

The firmness of vegetables can also be measured objectively. Penetrometers are usually used for measuring the firmness of relatively homogenous fruit such as apples and stonefruit. They can also be used to provide an objective measurement of the firmness of vegetables such as zucchini, squash, and eggplant but are less useful for a product such as a capsicum or tomato.

Although they can be handheld, better results are obtained when the device is mounted in a drill press or similar.

For unevenly textured products, different types of equipment may be used. Values are not absolute, but can provide an objective measure of differences in product firmness resulting from, for example, packaging methods or storage temperature. Devices range from a fairly simple meter which measures compression (mm) of a vegetable when a standard weight is placed on top to sophisticated texture analysers that can test crispness, viscosity or other aspects of product texture.

A manual (left) and automated (right) penetrometer for measuring firmness of vegetables.

Firmness can also be measured non-destructively by recording the mm of compression with a given weight over a standard time period (left) or recording the force required to rupture the product with a texture analyser (right)

Colour

Colour can be measured objectively using a chroma meter. This records the reflected spectrum of light in terms of its hue, lightness and chroma. Research papers often report colour in terms of L*a*b*. This is the CIE Lab colour scale, where ‘L’ indicates lightness (0 – 100), ‘a’ indicates red through to green and ‘b’ indicates yellow through to blue. It is often measured with a Minolta chroma meter, which is a standard piece of equipment in many postharvest laboratories.

The Minolta chroma meter (left) is used to objectively measure colour. This is open reported in the CIE Lab colour scale (centre) or as 'Hue angle' – which is an integration of ‘a’ and ‘b’ values. The device can be used in the laboratory or field (right).

While the chroma meter is a standard measure, there are now an increasing number of apps that can measure colour using the camera in a mobile phone or tablet. So long as lighting is consistent, these may be used to record colour values in a variety of scales including RGB (red – green – blue, as used on computer screens) and CMYK (cyan, magenta, yellow and black, as used in printing) as well as various other colour systems.

Software such as Adobe Photoshop can also be used to analyse colour in digital photographs. Again, if lighting is consistent, images, or parts of images can be easily analysed and reported as RGB, CMYK or even Lab. Photoshop has the advantage that it is possible to analyse the average colour of a whole vegetable—such as a whole cucumber or head of broccoli—rather than measuring a single point.

Sugars

Sugars are generally considered more important in fruit than in vegetables. However, many vegetables do contain significant amounts of sugars and these contribute to their flavour.

The percentage of sugars in juice can be estimated using a refractometer. This measures the percentage of soluble solids, also known as °Brix. Soluble solids are closely related to sugar content in juicy products. However, it is not a reliable measure of sugar content for sweet corn, as the milky liquid in corn kernels contains many non-sugar compounds that are detected by the refractometer.

Digital (left) and optical (right) refractometers can be used to measure soluble solids in juice, which is generally a good indicator of sugar content.

Average % soluble solids content of different vegetables. Bars indicate maximum and minimum values. (Data compiled from numerous references).

Subjective measurements

Subjective measurements rely on judgements made by the assessor. They can include shape, colour, blemishes and general acceptability. Various devices can be used to increase the objectivity of such judgements. These include size guides for blemishes, visual grading scales and written descriptors.

Subjective quality assessments can include any attribute of the vegetable. Subjective assessments can include factors such as shape, colour, acceptability; even the presence of blemishes, disease or bruising is usually assessed subjectively, although some of these may include objective elements.

As subjective assessments rely on the judgement of the observer, there is clearly a wide scope for variability. Visual aids can be used to reduce variability and ensure assessments are consistent. Aids can include descriptive grading scales, colour chips, templates indicating size and pictures indicating what is acceptable and what is not.

For example, a specification may require a product to have spots or marks not exceeding a total of 2cm², or a discoloured area affecting 4cm². A guide such as shown below, printed on clear material, may be used to assess whether product will be acceptable. It may also be useful to have photographs of examples, or even a diagrammatic representation, to make it easy for packers to assess what is within or outside the specification.

For products that are variable in size, it may be more appropriate to specify defects in terms of the percentage surface area affected. For example, major defects may only cover 1% or 2% of the surface area, whereas a more minor issue such as ground spot may affect 20% of the surface.

However, it can be very difficult for an observer to accurately estimate the percentage surface area affected by defects or discolouration, especially if it is a cumulative total of several defects.

Diagrams, which show exactly what a certain percentage of the surface area of a vegetable looks like, based on calculated values, can improve consistency in how products are evaluated and avoid disputes between clients and suppliers.

This is particularly important as vegetables are mostly round, not flat, so surface area can be significantly larger than some may expect. This means an allowable defect of 2% surface area, or discolouration affecting 20% surface area, may actually be greater than some assessors imagine.

Size guide for area of defects.

Diagrammatic representation of 4cm² of green colour on a red capsicum, and blemishes totalling 0.5cm² on a greenhouse cucumber.

Diagrammatic representation of 1% blemish

Diagrammatic representation of 2% blemish

Diagrammatic representation of bleaching affecting 25% of surface area of beet, or yellowing affecting 10% of curds.

Areas are calculated from the surface area of the product, with the spot shown as the assessor would see it.

Wilting can be one of the most difficult things to assess with any degree of objectivity, yet many specifications require products to be ‘without wilting or limpness’. Again, wilting may be better expressed using a diagram.

Photographic quality grading scales may also be used for overall appearance, acceptability, yellowing, or other defects. Grading scales that are developed and accepted by both client and supplier, particularly if combined with a ‘product description language’ describing each type of defect, have the potential to greatly improve communication within supply chains.

Scale for assessing wilting of gai lan (Chinese broccoli).