Non-destructive measurement of malic acid content of sapodilla fruit using visible near infrared (VisNIR) spectroscopy with variations in storage temperature

Sapodilla fruit (Manilkara zapota) is a climacteric fruit having a short shelf life. To extend shelf life, cold temperature storage is often used. During storage, one of the parameters that could change is malic acid. Malic acid can be measured non-destructively using visible-near infrared (Vis-NIR) spectroscopy. However, measurement using VisNIR spectroscopy has limitations, one of which is the sensitivity of the instrument towards temperature. The temperature of the fruit influences VisNIR spectral data, therefore prediction of malic acid using VisNIR is challenging. This study was conducted to study fruit temperature effect on the malic acid measurement of sapodilla fruit using VisNIR spectroscopy. Partial least square regression (PLSR) combined with spectral preprocess (Savitzky-Golay smoothing, normalization, Savitzky-Golay 1st & 2nd derivatives, standard normal variate (SNV), detrending, and multiple scattering correction (MSC) was used to build calibration model. Fruits were stored in three storage temperature i.e. 8°C, 13°C, and 29°C. Spectra of fruit were taken directly after stored in cold condition and after immersed in a water bath at 30°C to obtain normal temperature. Calibration models were built using cold temperature spectra at 8°C and 13°C and normal temperature spectra at 8°C, 13°C, and 29°C. Determination of calibration coefficient (R²c) of fruits measured in cold condition after stored at 8°C and 13°C were 0.89 and 0.87 obtained using MSC spectra. R²c of fruits measured after submersed in a water-bath after stored at 8°C and 13°C were 0.88 and 0.87 obtained using detrending and MSC spectra. Moreover, R²c of fruit stored in room temperature of 29°C was 0.92 obtained using mean normalization spectra. In general, PLSR models for determination malic acid of sapodilla were not significantly different between spectra of fruits which were cold and were heated to normal temperature. However, the PLSR model was the highest for fruits stored in room temperature (29°C). The results showed the potency of VisNIR spectroscopy for malic acid determination of sapodilla. Moreover, the result also showed that storage temperature and fruit temperature affected PLSR models developed using VisNIR spectroscopy.