6.1. Visual appearance
- Size: Fruit size can be measured with a sizing ring or calipers (Fig. 13). There is generally a good correlation between size and weight; size can also be expressed as number of units of a commodity per unit weight. Volume can be determined by water displacement or by calculating from measured dimensions.
- Shape: Ratios of dimensions, such as diameter-to-depth ratio, are used as indices of fruit shape (e.g. sweet pepper: Dag et al., 2007) (Fig. 14).
- Colour: The uniformity and intensity of colour are important visual qualities, as is light reflectance which can be measured by any number of dedicated meters. These devices measure colour on the basis of amount of light reflected from the surface of the fruit; examples include Minolta Colorimeter, Gardner, and Hunter Difference Meters. Internal colour and various internal disorders can be detected with light transmission meters. These devices measure light transmitted through the fruit. Fruit colour can be evaluated on the basis of pigment content, usually a function of quantity of chlorophylls, carotenoids and flavonoids.
- Defects (Fig. 15): Incidence and severity of internal and external defects can be evaluated on a five-point subjective scale (1 = none, 2 = slight, 3 = moderate, 4 = severe, 5 = extreme). To reduce variability among evaluators, detailed descriptions and photographs may be used as guides in scoring a given defect. An objective evaluation of external defects using computer-aided vision techniques appears promising. Internal defects can be evaluated by non-destructive techniques, such as light transmission and absorption characteristics of the fruit, sonic and vibration techniques associated with mass density, and nuclear magnetic resonance imaging.
Fig. 13. Measuring guava fruit size using digital calliper
Fig. 14 Left: large sweet pepper fruit from honey bee-pollinated greenhouse. Right: small fruit from a control, unpollinated greenhouse.
Fig. 15. Right: misshapen strawberries due to poor pollination. Left: regularly and well-shaped fruits resulting from satisfactory pollination.