Analyzing stone fruits ripeness: Linking sugar content to absorbance spectra

Stone fruits characterized by the large single seed in their center (also known as the stone or pit), stone fruits include apricots, cherries, raspberries, olives, and even coconuts.. It's important to harvest these fruits when they are at their peak ripeness since they won't ripen after being picked. When buying stone fruits, customers can check for ripeness by squeezing (a hard fruit indicates that it is not fully ripe) or smelling the fruit (the more aromatic the fruit, the riper it is) .

Although this could work for a consumer purchasing a few pieces of fruit, how can distributors sort fruits so that their shipments contain only the best, ripest fruits? Since the concept of ripeness is somewhat arbitrary and challenging to measure, it is frequently associated with a fruit's increased sugar content with ripening. A refractometer is a useful tool for measuring sugar content. The unit Brix, which is defined as 1 g of sucrose in 100 g of solution, is easy to use to determine sugar concentration. Unfortunately, extracting juice from each fruit is necessary for this sugar content measurement, which is detrimental because the fruit is no longer fit for sale to customers. For commercial operations who are interested in characterizing and sorting fruits of various types by the ton, non-contact near infrared spectroscopy can be used to measure brix accurately and efficiently.

Distinguishing fruits that are suitable for harvest is done based on reflectance-absorption measurements. The light source used can provide a Vis-Nir light range in the region from 450 nm to 1100 nm. The non-contact testing technique aims to predict the sugar content and ripeness of fruit, while also keeping the fruit intact for delivery to consumers. Ocean Insight (Ocean Optics) offers sturdy, reliable Vis-NIR Light Source (such as Krypton Light Source and Tungsten Halogen Light Sources) and Vis-NIR spectrometers (Ocean SR2 VIS-NIR, Ocean SR4 VIS-NIR, Ocean SR6 VIS-NIR, Ocean  HR2 VIS-NIR, Ocean HR4 VIS-NIR, Ocean HR6 VIS-NIR, Ocean FX-VIS-NIR, Ocean-HDX-VIS-NIR) that are suitable for this technique.

For example, test on different peaches with different degrees of ripeness. The peaches were stored in a refrigerator to prolong the timeframe for their ripeness but were left in a room-temperature environment for a minimum of four hours before measurements were taken to ensure the sample temperature had minimal effect of the recorded data. Each sample was individually placed on the turntable center and allowed to spin freely under the overhead lamp. 10 measurements of each peach sample were taken to quantify the average spectrum of each sample, create more data points for the algorithm creation, and determine if any measured spectra were outliers that could skew the algorithm. Each peach sample was then sliced in two opposite positions on the fruit, and the fruit of each slice was individually squeezed onto the refractometer window to determine the Brix value of each slice. The two Brix values for each peach sample were averaged. Because the spectra used were absorbance spectra, need to do reference measurement with white reference tile at a similar distance and angle as the peach samples. Ocean Insight (Ocean Optics) has researched and developed reflectance sample, which is ideal  for this reference measurement.