Determination of the Sugar Content in Commercial Plant Milks by Near Infrared Spectroscopy and Luff-Schoorl Total Glucose Titration

Thirty-nine samples of plant milks (rice, soy, oat, barley, spelt, quinoa, almond, and a variety of wheat called kamut) were analyzed for their reducing sugars content by NIR spectroscopy, using the Luff-Schoorl official method as reference to build the calibration models. The amount of reducing sugars, expressed as grams of glucose/100 mL of beverage, ranged from 0.5 g/100 mL (soy) to 7.6 g/100 mL (rice). Both partial least squares (PLS) and interval-partial least squares regression (iPLS) were used to build multivariate calibration models, testing different spectra preprocessing methods. The performance in prediction of the best calibration model was evaluated on an external test set of nine randomly selected samples (RMSEP = 0.98 g/100 mL, R2 PRED = 0.84), and its statistical significance was assessed using a randomization t test based on Monte Carlo simulations. The results obtained suggest that NIR spectroscopy can be a valid alternative to the laborious reference titrimetric method for the determination of total glucose content in plant milks

Application of NIR spectroscopy and Luff-Schoorl methods for determining the sugar content of commercial plant milk

The present study reports on the use of near infrared (NIR) spectroscopy for the control of sugar content of plant milk. Total glucose content was selected as parameter for building the calibration set for the instrument. Thirty six plant milk samples were purchased from local supermarkets and included milk obtained from soy (15 samples), rice (11 samples), oats (6 samples), kamut (1), spelt (1), almond (1), Quinoa (1). In all samples total glucose was determined applying the Luff-Schoorl method. The apparatus was a Bruker NIR Multi Purpose Analyzer equipped with a Liquid Sampling Module, operated in an interval of frequencies ranging from 10000 to 4000 cm-1 with a resolution of 16 cm-1. On every sample 32 scans were carried out, at 40°C, with a thermostat time of 30 sec. Results of the Luff-Schoorl glucose determination were compared with the respective data obtained from the NIR instrument. All data were processed by the software included in the instrument proprietary computer program. The quality of the calibration model obtained was good (R2>0,996; RMSEE=0.18) and it was confirmed by a good cross-validation model (cross-validation R2> 0.98; RMSECV=0.34). Therefore the model was applied and fully validated on an independent test set of samples which confirmed the excellent prediction of the model. The NIR measurements proved to be reliable with high accuracy and the comparison of the Luff-Schoorl method and NIR evidenced that the actual sugar content of the purchased products can be slightly different from what is reported on the product label. From the data presented in this study it can be concluded that NIR spectroscopy can be applied successfully for the control of sugar content in plant milk products obtained from a wide variety of plant sources. Considered the practicability, the speed, and the low cost of the NIR measurements, the spectroscopic technique can be applied in in-line and on-line quality control of plant milk production. The NIR instrument appears superior under all aspects of reliability, robustness, precision and speed in comparison with the impractical, laborious, and non-robust classical Luff-Scoorl method