Visual inspection of surface sanitation: Defining the conditions that enhance the human threshold for detection of food residues

Abstract

Visual inspection is widely used in sanitation program monitoring. However, the degree of residual food soil on equipment surfaces that is below the limit of visual detection has not been determined. The overall goal of this study was to critically evaluate the limit of detection of visual inspections and establish evidence-based strategies to improve the implementation of this monitoring activity. Respondents (n = 101) completed a visual sensory test in which they viewed 15 stainless steel and 15 white high density polyethylene (HDPE) sheets, clean or soiled with dry wheat flour, in a randomly generated order. Respondents were asked if they could visually detect food residue on each surface and, if so, if the surface should be re-cleaned. Half of the respondents failed to visually detect up to 0.05 g and 0.02 g of flour per square foot (929 cm2) on HDPE and stainless steel surfaces, respectively. Tactile inspection improved respondent detection sensitivity to 0.001 g of flour per square foot (929 cm2) on both HDPE and stainless steel. Respondent height, visual acuity, and prior food safety experience had significant (p < 0.05) effects on visual detection sensitivity. Respondents were more likely (p < 0.001) to say that a food-soiled surface should be re-cleaned after an intervention training. The findings from this study support the use of surfaces that color-contrast food residue, tactile evaluation with a gloved hand, training, and the use of flashlights or positional changes (e.g., crouching) to improve pre-operational inspections. Nonetheless, the additional use of verification tests remains necessary given the limit of detection determined for visual evaluation. Further research is needed on other food residues with different visual properties such as oils.