Bruise Development Measurement in Apples Using Thermal Imaging Technique

Abstract Apple is an important fruit worldwide with nutrition and a huge market value. However, due to the mechanical damage during its handling after the harvest, millions of dollars are lost in the apple industry. Bruise (subcutaneous) is a mechanical damage caused to a fruit without any tearing of the fruit-skin. It could be caused by a mechanical impact, vibration, or compression which the fruit might come across during its handling after the harvest. It results in local degradation of the flesh tissue with browning effect with time. Normally, apple bruises take place beneath the peel and are difficult to detect by either visual or automatic color sorting. Thermal imaging offers a promising, contact-free, alternative technique for bruise detection in apples. Thermal imaging technique measures infrared energy emitted from the object surface which is converted into a thermal map called a thermogram. In the literature, there has been some work done on the influence of temperature and relative humidity of the surrounding on bruise susceptibility of fruits. In this paper, the influence of relative humidity in bruise development on apple was investigated applying thermal imaging technique. To accomplish this, two batches of healthy, almost same-sized, yellow and green apples with ten apples per batch were considered. The apples were subjected to a mechanical impact by dropping them from a constant height ensuring the fruit skin is not torn by the impact and the impact is same is for all the apples. Then, they were thermal imaged with the focus on their bruises. They were kept and observed at two different temperatures and relative humidity. The first set involved the relative humidity of 40–43% with room conditions whereas the second one involved the relative humidity of 80%–82% in the freezer compartment of a domestic refrigerator. For all the trials, the same condition was maintained for the fruits throughout the three-day observation. At the end of stipulated observation period, the fruits were thermal imaged to capture the bruise development for the different relative humidity levels for two different temperatures. The observation was repeated for three times to obtain the repeatability. The results show that the bruise development is faster for the higher relative humidity than for the lower one. This could be due to the lower transpiration rate of water from the fruits at higher relative humidity. Also, the water potential under the bruised skin decreases faster for lower relative humidity condition which results in decreased bruise development.