NIR Spectroscopy to Assess Edible Fruit Coatings & Maturity

Growers and suppliers need a non-destructive method of measuring quality parameters, as these must be measured repeatedly, both before harvest and through storage. A recent set of experiments on kiwifruit tested the edible fruit coating Ca-chitosan, and whether near-infrared spectroscopy is a viable means of determining maturity, harvest time, ripening, and storability. Find out what the scientists recommend in this study of “baby kiwifruit” [A. arguta cv. Saehan].

Extending Kiwifruit Storage Time

Kiwifruit are popular for both their flavor profile and health benefits. Kiwifruit are climacteric, harvested when they are mature before ripening. They ripen post-harvest when starch is converted to sugars, measured as soluble solids content (SSC). As the ripening proceeds, there is an associated drop in firmness and acidity.

Kiwifruit are perishable and have a shelf life of only 1-2 weeks, depending on the levels of SSC, respiration, and microbial spoilage. Ongoing respiration in climacteric fruits hastens ripening and leads to moisture loss, while microbial infection spoils fruits by causing decay, changing both appearance and taste.

In an effort to decrease respiration, moisture loss, and microbial infection while slowing down physiological disorders, many stakeholders employ edible fruit coatings. Chitosan, a natural material derived from arthropod exoskeletons combined with calcium chloride (Ca-chitosan), has successfully extended the shelf lives of many fruits. Chitosan forms a thin antimicrobial, nontoxic, and edible film that protects fruits from fungal infection. Calcium preserves firmness by maintaining cell turgor, tissue firmness, and membrane integrity. It also increases resistance to diseases and lipid catabolism.

Non-destructive Measurement of Fruit Quality with NIRS

Though Ca-chitosan coating had been used on other fruits, its effect on the postharvest quality of kiwifruit was, until this study, unknown.

To understand the effect of the coating it is necessary to test fruit quality parameters like dry matter (DM), SSC, firmness, and acidity. In the supply chain, much of this testing is carried out using conventional destructive methods. However, near-infrared spectroscopy (NIRS) is emerging as a convenient, non-destructive way to estimate external and internal quality parameters. Portable NIRS devices are widely used to estimate fruit maturity and fix harvest time on farms. NIRS devices are also used for ripening and quality control in the postharvest stage. However, no recommendations exist for predicting harvesting maturity, ripening, and storage capacity for baby kiwifruit.

Kim, Park, Shin, Muneer, Lerud, Michelson, Kang, Min, and Kumarihami, horticulturists from three Korean Universities in connection with a representative from Felix Instruments – Applied Food Science endeavored to test Ca-chitosan’s efficiency as a coating using the F-750 Produce Quality Meter. The team also explored the device’s predictive power in maturity, ripening, and shelf-life.

Testing NIR Spectroscopy for Small Kiwifruit

Baby kiwifruit were harvested twice on September 24th (1st harvest) and October 8th (2nd harvest), 108 days after bloom.

Harvest Maturity: To predict harvest maturity, scientists tested 100 evenly-sized kiwifruit in good condition from the 1st harvest. The fruit were stored at three temperatures, 1°C, 15°C, and 25°C, for an hour to stabilize spectral response. Then, the spectra were collected non-destructively by scanning the fruits with the F-750 Produce Quality Meter.

Following this, 2cm cores from the scanned portion of the same fruits were oven-dried to estimate dry matter content. The researchers used these values as the reference DM values for calibration of the F-750.

Another 100 kiwifruit, stabilized at the same three temperatures, were scanned for SSC calibration. These fruits were then treated with ethylene and allowed to ripen artificially. After 4-5 days, the researchers estimated the SSC of the fruits. They finally correlated these values with the associated spectra to evaluate the efficacy of preharvest scanning to predict sweetness due to ripening. 

Evaluating Ca-chitosan Treatment: Fruits from the second harvest were used to test pre-treatment with Ca-chitosan and examine postharvest quality parameters.

The treatment was applied pre-harvest by dipping the fruits three times in the mixture. At harvest, the treated kiwifruit were stored in plastic packages for 7 and 14 days at 5oC and 95% relative humidity (RH). At the end of the storage, the fruits were treated with ethylene. The fruits were then stored for a further 5 days before the SSC was measured.

Kiwifruit Storage Changes: The scientists then evaluated SSC, titrable acidity, and firmness in nine replicates to see the change in quality due to storage, using destructive methods. 

The second derivative of the spectra was analyzed by Partial Least Square Regression Models and correlated with the reference value of the quality parameters in each experiment.

Internal Quality Parameters

The collected NIR spectra were loaded into the F-750 Model Builder Software to predict various quality parameters, as shown in Figure 1. Based on the findings derived from the spectra, the scientists made several important recommendations.

Kiwifruit Harvest Maturity

There was variation among the spectra collected from the fruits, which can be seen in the estimated DM values. About 71% of the fruits had 21-22% DM, 10% fruits had <21%DM, and 18% fruits had >23% DM. Since all kiwifruit do not mature simultaneously in a crop, being able to use non-destructive means of estimation is crucial on farms.

Predicted DM showed a correlation of R2=0.74 to actual reference values, as shown in Figure 2.

SSC increased by 1° Brix and firmness decreased by 3.7N in the fruits harvested at different dates, showing that ripening had already begun before the 2nd harvest. 

The predicted and actual SSC values showed a correlation of R2= 0.48. Also, the DM content at harvest showed a clear and positive relationship to SSC after ripening. The predicted DM’s correlation with actual SSC was R2=0.65, see Figure 3. When DM was 20%, 21%, and 22%, SSC was 9.1° Brix, 9.6° Brix, and 11.2° Brix, respectively. Fruits with higher DM content at harvest were sweeter after the artificial ripening process.

The scientists concluded that the model created to analyze non-destructively-collected NIR spectra was able to accurately predict DM and SSC.

Ca-Chitosan Treatment

The scientists found that the edible fruit coating Ca-chitosan influenced the metabolism and respiration of pre-harvest fruits.

Treated fruits had a higher SSC content (9.5° Brix) and DM content (22.3%) compared to untreated kiwifruit (8.4° Brix and 21.4% DM). Furthermore, the DM estimations correlated with predicted values. These values were also higher in treated fruits (22.7%) than untreated fruits (22% DM).

Titrable acidity was not significantly different, but treated fruits (12.8 g) weighed less than untreated kiwis (14.9 g). Treated fruits were also firmer (21.9N) than untreated fruits (21.1N)

Ca-chitosan in the current experiment slowed ripening and therefore resulted in greater SSC, firmness, and DM, and can be used as a preharvest treatment. However, the scientists think more studies are needed to understand how Ca-chitosan influences the kiwifruit’s internal physiology.

Quality Change During Storage

Kiwifruits had greater SSC after being stored for 14 days than 7 days, being in a more advanced stage of ripening.

SSC of kiwifruit stored for 7 and 14 days, was also associated with DM content at maturity. Fruits with higher DM content at harvest had greater SSC post-storage, and the correlation was R2=0.65.

• DM content of 20%, 21%, 22%, and 23%, resulted in SSC of 12.3°, 14.2°, 14.5°, and 14.9° Brix, respectively after 7 days of storage.
• DM of 21% and 22% is associated with 15.5° and 17.2°Brix, respectively, after 14 days of storage. Over 40% higher than SSC at harvest.

Loss of firmness was negatively associated with DM content at harvest, see Figure 4. DM content of 20%, 21%, 22%, and 23% produced kiwifruit with firmness values of 18.5N, 15.5N, 14.6N, and 14.1N, respectively, after seven days of storage.

NIRS Assessment and Ca-chitosan Fruit Coating are Reccomended

Based on their findings, the scientists recommend that NIR spectroscopy-determined DM content can be used effectively as a maturity index and inform the storability of baby kiwifruit. The researchers suggest harvesting baby kiwifruit at over 21% DM content to produce fruits with higher SSC and eating quality after storage.

The Ca-chitosan pre-treatment was also given the nod as it was found to improve postharvest quality and shelf life. These recommendations provide vital information for kiwifruit farmers to improve their yield and ROI, while increasing sustainability by decreasing food loss in the supply chain.