Fast determination by Fourier-transform infrared spectroscopy of sugar-acid composition of citrus juices for determination of industry maturity standards

Abstract

Predictive models using attenuated total reflectance/Fourier-transform infrared (ATR-FTIR) spectra of citrus juices have been developed for determination of soluble solids (SSC), titratable acidity (TA) and the Californian-Australian citrus industry BRIMA index. Generic models, developed using partial least squares regression, included fresh juices (n = 326) from grapefruit, lemon, Mandarin (plus hybrids tangelo and tangor), oranges and interspecific hybrids. The models cover the ranges 4.0-17.6 °Brix, 0.08%-6.96% TA and -332-212 BRIMA units. Root mean square errors of prediction were 0.18 °Brix, 0.055% TA and 4 BRIMA, respectively. Spectral data acquisition time was 52s per sample. Further analysis of navel oranges (n = 1470) demonstrated that improved models that analyse juices from single species give better prediction errors (0.12 °Brix, 0.027% TA, 3.1 BRIMA units) than generic models. It is suggested that FTIR analysis is a suitable replacement for manual titration for TA in countries where regulations allow adoption of new technologies, while simultaneously measuring SSC and industry maturity standards.