Mechanical-based and Optical-based Methods for Nondestructive Evaluation of Fruit Firmness

Fruits are important agricultural products in the current society. The demands for high-quality fruits have promoted the development of various nondestructive firmness evaluation methods in recent 20 years. The nondestructive and fast techniques used in the prediction of the firmness of various fruits can be divided into two categories: optical-based methods and mechanical-based methods. Optical-based methods mainly refer to visible/near-infrared (Vis/NIR) spectroscopy, hyperspectral or multispectral imaging (HIS/MSI), spatially resolved reflectance spectroscopy (SRRS), laser light backscatter imaging (LLBI), and others. Mechanical-based methods including several techniques such as micro-deformation, vibration measurements, acoustic impact response, falling impact, hammer impact, and ultrasonic methods. Fundamental principles, as well as measured performances of mechanical-based methods and optical-based methods for measuring the firmness of fruit, are discussed in detail in this paper. The techniques that can be applied to on-line sorting are considered emphatically. Both mechanical-based and optical-based methods have advantages and disadvantages in predicting fruit firmness. The correlations between the firmness measured by mechanical-based methods and puncture firmness are poor in some studies, and the optical methods also have not obtained satisfactory results. Therefore, the on-line measurement of fruit firmness with high accuracy is still facing some challenges.