Evaluating Effects of Drying Method Using Colour Image Analysis on Quality of Macadamia Nuts

Abstract

With increased demand for macadamia nuts, more emphasis is being placed on high quality and safety standards in both postharvest processing and final quality of the processed macadamia nuts. The need for accurate, fast, objective, and cost-effective drying, and quality determination of these nuts continues to grow. In Kenya, the annual production has increased from 11,000 metric tons of Nuts-In-Shell (NIS) in 2009 to 41,164 metric tons in 2018, which accounted for 19 per cent of growth. However, it is losing approximately 30% of its harvest due to poor postharvest practices, premature harvesting, and erratic farm-gate prices influenced by brokers, among others. This has been due to farmers’ having poor storage program leading to internal respiration and other chemical changes. As a result, Kenya has lost grip of the European Union market since the buyers from EU have classified Kenyan macadamia as of inferior quality compared to those from countries such as Australia, Brazil among other. The objective of this research was to evaluate the effect of drying methods on the quality of dried macadamia nuts using colour image analysis. The study involved drying of two varieties of macadamia nuts, namely KRG-15 and MRG-20 and evaluating their quality using colour image analysis. The nuts were dried using an oven dryer at (temperatures of 50°C, 60°C and 50-60°C), solar tent dryer, solar tent-oven (at 60℃) as well as solar tent-microwave (MW). A colour index was determined by measuring colour parameter using CIELab model and RGB colour model. It was observed that there is a strong correlation (P<0.05) between normalized colours C’.b and C’.r and the total colour change. Another strong correlation was observed between normalized colours C’. b and the Browning Index (BI). This revealed that colour is important in the prediction of browning of the nuts through image analysis and this was used for grading both KRG-15 and MRG-20 nuts irrespective of the drying method. Based on the grading developed, oven drying of MRG-20 at 50oC and 60oC produces 1st and 2nd grade nuts, respectively. On the other hand, oven drying of KRG-15 nuts at 50˚C and 60oC 2nd and 3rd grade nuts, respectively. This means that it is not advisable to mix KRG-15 and MRG-20 nuts. Solar tent-MW produced the least quality nuts with a BI of 49.03 and 88.21 for KRG-15 and MRG-20, respectively. Solar tent, oven drying at 50oC and solar tent-oven (at 60oC) produced the best quality nuts with a BI of 25.52-30.82 for KRG-15 and 28.95-29.78 for MRG-20. Oven-50 and oven 50-60 drying produced the best quality MRG-20 nuts with the BI of 28.00 and 29.88 respectively. However, the same drying methods produced low quality KRG-15 nuts with BI of 35.73 and 37.06, respectively. From the above findings, it was observed that for one to achieve grade one commercial nuts, one had to dry MRG-20 without mixing KRG-15 using oven drying method at a temperature of 50˚C. However, this process takes longer. In the event of mixing KRG-15 and MRG-20, it was observed that using either solar tent dryer or solar tent-oven (60oC) drying method produced 1st grade commercial nuts. However, Techno-Economic analysis and optimization analysis is further required to evaluate the performance of these drying methods before they can be adapted for on-farm application.