Measuring the Sugar Content of Apples and Apple Juice by Near Infrared Spectroscopy

The sweetness of apples is usually measured by destructive inspection, which is time-consuming. As an alternative, a rapid instrument with high accuracy is very desirable. This paper reports an application using the NIR method to measure the sugar content of apples quantitatively. Samples containing mainly water showed that it was possible to measure the sugar content of fruit using the second derivative treatment (the background is removed) of the nearinfrared absorption spectrum. Measurements of the sugar content of four varieties of apples had correlation coefficients of 0.94 or more and a standard error of prediction (SEP) of 0.546°Bx at most. In the two kinds of apple juice, we obtained a SEP value of 0.439°Bx at most and correlation coefficients of 0.97 or more. We also found that 912 nm was an important wavelength for determining sugar content for both apples and juice. These results were in good agreement with calibrated values. NIR spectroscopy is a non-destructive measurement technique, and there is value in utilizing it for samples that are mainly water. Biotechnology applications are expected.