Submitted to: Journal of Texture Studies
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/14/2000
Publication Date: 12/1/2000
Citation: LU, R., GUYER, D.E., BEAUDRY, R. DETERMINATION OF SUGAR CONTENT AND FIRMNESS OF APPLES USING NIR DIFFUSE REFLECTANCE. JOURNAL OF TEXTURE STUDIES. v. 31(6):615-630. Interpretive Summary: Currently, fruit are sorted based on color and size or weight but not on internal quality. Sorting for internal quality is a top concern in the fruit industry because poor, inconsistent quality fruit have caused the industry to lose competitive marketing advantages. Firmness and sugar content are the two most important parameters in determining apple quality. .An experiment was conducted to investigate the potential of a light-based sensing technique called near-infrared or NIR for nondestructive measurement of the sugar content and firmness of apples and to evaluate the effect of apple peel and variety on NIR predictions. Results showed that the NIR technique gave excellent predictions of sugar content with average prediction errors of 0.41% and 0.58% for peeled and unpeeled apples, respectively. The effect of apple variety on sugar predictions was small and could be minimized through statistical modeling. Relatively lower correlations were obtained between NIR predictions and Magness-Taylor firmness measurements (a standard destructive method used to measure fruit quality). Improved prediction results were obtained when a different firmness index was used. The findings of this research will lead to the further development of a NIR-based sensor for nondestructive determination of fruit quality. We can use the NIR technique to sort fruit for sugar content and firmness. Unlike current destructive methods that render fruit unmarketable, the NIR technique is nondestructive, rapid and safe and, therefore, would allow every fresh fruit to be inspected to ensure virtually 100% quality assurance for the consumer. The outcomes of this research should also help USDA Agricultural Marketing Service in developing objective fruit grading standards.
Technical Abstract: The objective of this research was to study near-infrared (NIR) diffuse reflectance technique for determining the sugar content and firmness of apples and to ascertain the effects of apple peel and variety on the prediction of these two quality attributes. The diffuse reflectance of 'Empire', 'Golden Delicious', and 'Red Delicious' apples with and without peel was measured for the spectral region between 800 nm and 1700 nm. Statistical models were developed using principal component analysis/regression (PCA/PCR). Excellent predictions of the sugar content in peeled apples were obtained, with the correlations of prediction between 0.93 and 0.97 and the standard errors of prediction (SEP) between 0.37 and 0.42 Brix for the three apple varieties. With the presence of apple peel, the SEP, on average, increased by about 0.17 Brix. Variety did not have a large effect on the model performance on sugar content predictions. Low correlations of prediction (r=0.38 to 0.58 and SEP=6.6 to 10.1 N) were obtained between NIR measurement and Magness-Talyor firmness for both peeled and intact fruit. Firmness predictions were improved when NIR reflectance was correlated with the slope of the Magness-Taylor force-deformation curves.