On the assessment of citric acid in aqueous solution and fruit titratable acidity levels using SWNIRS : (a) citric acid solutions

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Shortwave (700 to 1100 nm) near infrared spectroscopy using diode array instrumentation suited to in-line or field portable instrumentation was considered in context of the assessment of titratable acidity (TTA) in intact fruit. A searching window method was used to optimise the wavelength window for model development (around 750 – 950 nm). In pure citric acid – water mixtures (mean + SD of 5.05 + 6.65 g citric acid /100 ml), a RMSECV = 0.34 and R = 0.999 was achieved, while if the matrix was varied with 0, 10 and 13% sucrose, a RMSECV = 0.59 and a R = 0.996 was achieved. Models developed on spectra collected of the cut surface of a lime fruit (mean + SD of 7.3 + 0.51 g citric acid equivalents/100 ml), possessed a RMSECV = 0.17, R = 0.93, and a SDR = 3.0 (where SDR = SD/RMSECV), comparable with prediction results (RMSEP = 0.16, R = 0.89, bias = -0.03, and SDR = 2.4). For intact lime fruit, model calibration results (RMSECV = 0.16, R = 0.92, and SDR = 3.1) were markedly better than prediction results (RMSEP = 0.30, R = 0.70, bias = -0.07, SDR = 2.1). For a low TTA product, peach (with spectra collected across fruit maturity stages; mean + SD of 0.88 + 0.17), model calibration results were relatively poor (RMSECV = 0.09, R = 0.83, SDR = 1.8), while in prediction the model failed (RMSEP = 0.104, R = 0.05, bias = 0.02, SDR = 0.9). We conclude that SWNIR is not appropriate for assessment of the acidity of intact low TTA fruit, and has limited use for high TTA fruit. The method has value for rapid assessment of the TTA of juice extracted from moderate and high TTA fruit.