Chroma meter technology provides accuracy

CONTACT DETAILS: +27 21 809 3418/ +27 84 430 6555/
DURATION: Two years
LEAD INSTITUTION: ARC Infruitec-Nietvoorbij
Beneficiary: The deciduous fruit sector

It is extremely important to maintain fruit quality for local and export markets, which can be ensured by controlled atmosphere (CA) storage and maturity indexing. The best time for harvesting is indicated by fruit colour development and this is why standard industry colour charts have always been used for maturity indexing. However, the Unifruco charts are not manufactured any more – they tend to fade and discolour over time and different variations exist. These are some of the concerns that prompted research at the ARC Infruitec-Nietvoorbij, in which the chroma meter method was evaluated as a viable alternative.

Comparative maturity indexing tests

Kobus van der Merwe and his research team set up the green and yellow spectrum according to South African export regulations. A Konica Minolta model CR-400 Chroma Meter was calibrated and correlated with the selected chart colours. Statistical analysis indicated a strong correlation between the chart colours and chroma meter measurements.

‘Golden Delicious’ and ‘Granny Smith’ apples and ‘Forelle’ and ‘Packham’s Triumph’ pears were used for maturity indexing trials. Initially, ‘Golden Delicious’ apples were kept at -0.5°C in different storage conditions (CA, dynamic CA and Smartfresh™). After five months, three individuals scored samples with the colour chart. Results were compared with chroma meter readings. Comparative maturity tests continued on all the pome fruit cultivars in DCA storage. Fruit quality and maturity levels were recorded three times at harvest; and at three, five, seven and nine months, with an additional two-week shelf life period at 20°C.

The three individuals who used the colour chart method to determine fruit maturity, each obtained different results. By contrast, chroma meter measurements were consistent throughout. The results not only indicate that people perceive and interpret colour differently, but also prove the chroma meter method of colour measurement more accurate and reliable.

Chroma meter usage and benefits to industry

  • A calibrated chroma meter measures fruit skin background colour accurately and consistently, which minimises mistakes in colour interpretation by humans.
  • It is a digital measuring system, which provides data in a format designed for statistical analysis.
  • The device is fairly small and mobile and can be used at any point in the supply chain.
  • Unlike the colour chart method, chroma meter functioning is not dependent upon human colour perception or a specific operator.
  • The chroma meter method reduces labour and saves time, which increases productivity.
  • A very expensive chroma meter was used for testing, but more affordable models are now available. This will help to shift the focus from research laboratories to commercial use.
  • Independent tests are now being done on plums and pome fruit in the red spectrum.
  • If the chroma meter standard of colour measurement is found to set a benchmark for the fruit industry, it should be considered for inclusion in South African export regulations.