Detection and quantification of apple adulteration in diluted and sulphited strawberry and raspberry purées using visible and near infrared spectroscopy

Abstract

Adulteration of sulphited strawberry and raspberry purées by apple is a commercial problem. Strawberry (n=31) and raspberry (n=30) purées were prepared from Irish-grown fruit and adulterated at levels of 10-75% w/w using cooking apples. Visible and near infrared transflectance spectra were recorded using a 0.1 mm sample thickness. Classification and quantification models were developed using raw and scatter-corrected and/or derivatised spectral data. Classification as pure strawberry or raspberry was attempted using soft independent modelling of class analogy (SIMCA). The best models used spectral data in the wavelength ranges 400-1098 nm (strawberry) and 750-1098 nm (raspberry) and produced total correct classification rates of 75% (strawberry) and 95% (raspberry). Quantification of apple content was performed using partial least squares (PLS) regression. Lowest predictive errors obtained were 11.3% (raspberry) and 9.0% (strawberry). These results were obtained using spectral data in the wavelength ranges 400-1880 and 1100-1880 nm respectively. These results suggest minimum detection levels of apple in soft fruit purées of approximately 25% and 20% w/w for raspberry and strawberry respectively.