HPLC Determination and FT-MIR Prediction of Sugars from Juices of Different Apple Cultivars during Fruits Development

Individual sugars were analyzed by high performance liquid chromatography (HPLC) in samples of apple juices obtained from the fruits of ‘Jonathan’, ‘Starkrimson’ and ‘Golden Delicious’ cultivars. Samples were harvested from the inside and the periphery of the crown, at different periods during fruits growth, from 7 to 144 days after full bloom (DAFB). Values from 0.42 to 14.33%, 0.29 to 4.06% and 0 to 4.28% were determined for fructose, glucose and sucrose, respectively. The values of fructose and glucose have increased significantly (p<0.05), starting with the seventh DAFB, regardless of the studied cultivar, while sucrose increased slowly at the beginning and then faster starting 65 DAFB. Fourier transform mid-infrared (FT-MIR) analysis confirmed the differences between juice samples, the region 900-1500 cm^-1 being the most specific to sugars signals. FT-MIR coupled to partial least squares (PLS) calibration models for predicting individual sugars of apple juices were developed. The optimal regions and pre-treatments of the spectra were 900-1500 cm^-1 and Savitzky Golay first derivative (d1) for fructose, 900-1200 cm^-1 and d2 for glucose and 900-1200 cm^-1 and standard normal variate for sucrose. In cross-validation, the PLS calibration models showed very good performance for fructose (R_cval²=0.95; standard error of cross-validation (SECV) =0.907) and acceptable for glucose (R_cval²=0.85; SECV=0.424), while for sucrose showed only satisfactory performance (R_cval²=0.75; SECV=0.561). For practical relevance, the FT-MIR predicted values were compared against the HPLC determined reference values in external validations tests. The best results were achieved for fructose (R_p²=0.94; RPD=4.9), while glucose (R_p²=0.84; RPD=2.61) and sucrose (R_p²=0.7; RPD=2.08) models reached satisfactory values.