Quality evaluation of processing potatoes by near infrared reflectance

The percentage dry matter and reducing sugar content are prime quality parameters for assessing the potential of potato tubers to produce acceptable yields of processed products possessing good textural and colour quality attributes. A study was conducted to develop a near infrared (NIR) calibration for dry matter which would be unaffected by storage time and crop year, and to assess whether NIR could be used to screen tubers with high reducing sugar content from tubers with more acceptable levels. Tubers from commercial growers of French fry processing potatoes were sampled over two years for dry matter and one year for sugars. Tubers used for development of the NIR calibration were stored at 8°C, while those for sugars were stored at 8, 5 and 2°C. A small coin of potato flesh was excised from each tuber, its NIR spectrum obtained on a scanning spectrophotometer and its specific gravity and dry matter, or its reducing sugar content, determined. Correlation plots indicated that wavelengths that correlated well for specific gravity correlated well for dry matter. The standard error of performance for NIR determination of percentage dry matter varied from 1.3 to 1.5%. Calibrations for dry matter were robust and were not affected by time or conditions of storage nor by changeover from one crop year to another. It appears, however, that adjustments to the NIR calibration would be required at harvest time. Coefficients of variation (CV) in determining dry matter by NIR were 5–6%. Spectral averaging appears to have potential for improving on this CV. NIR in reflectance mode was unable to act as a screening tool for unacceptable sugar content in tubers.