Surface Texture Analysis of Fresh Produce using Optical Interferometry

Introduction: The methods to reduce pathogens on fresh produce are of limited effectiveness, and development of new methods are challenging given the variable surface topography of fruits and vegetables.  Evaluating the surface characteristics of produce can aid in understanding contamination and potentially lead to the development of novel strategies for decontamination.  While previous microscopy methods utilized to evaluate produce surface characteristics involve extensive sample preparation and are destructive, optical interferometry provides an alternative method that requires no sample preparation and is nondestructive.  

Purpose: The purpose of this study was to use an optical interferometer to compare the surface characteristics of Romaine lettuce, spinach, cilantro and tomatoes.

Methods: The objective lens on the optical interferometer was 20X magnification and the scan size was 0.32 x 0.24 mm.  Leafy vegetable measurements were taken of the top and bottom of leaves, while tomatoes were only tested at one location.  Five measurements per test condition were collected and the 2D and 3D surface parameter values in the roughness scale were compared using ANOVA.

Results: The 2D average roughness (R_a) values were 7.25 ± 1.67, 6.98 ± 1.73, 3.51 ± 0.91, 3.14 ± 0.99, 8.80 ± 1.07, 9.19 ± 0.46 and 1.43 ± 1.45 µm for the top of Romaine lettuce, bottom of Romaine lettuce, top of cilantro, bottom of cilantro, top of spinach, bottom of spinach and tomatoes, respectively.  There was a significant difference (P > 0.05) in R_a between the top of all the products except for Romaine lettuce and spinach.  Tomato had the smallest average peak-to-valley height (R_z) followed by bottom of cilantro, top of cilantro, top of Romaine lettuce, bottom of Romaine lettuce, bottom of spinach and top of spinach.

Significance: These data suggest that optical interferometry is a quick and effective means to evaluate the surface characteristics of fresh produce.