# Distinguishing Bovine Fecal Matter on Spinach Leaves Using Field Spectroscopy
## 1 Introduction  
The production of contaminant-free fresh produce for human consumption is needed in order to reduce the occurrence of foodborne illnesses [1]. Leafy greens have been associated with several outbreaks of _Escherichia coli_ (_E. coli_) O157:H7 [2]. Fresh fruit and vegetables are susceptible to fecal contamination from livestock, wild, or domestic animals entering the growing field, manure fertilizer, and irrigation water containing animal feces [3; 4]. _E. coli_ O157:H7 commonly resides in the intestines of animals, and when transferred to humans via food it can cause foodborne illnesses and death [5]. Foods consumed fresh, such as leafy greens, have a higher risk of causing foodborne illness because there is no lethal phase for bacteria, such as heating to kill pathogens [6]. The United States Center for Disease Control and Prevention (CDC) estimated that in 2014 alone there were 19,542 infections, 4445 hospitalizations, and 71 deaths due to foodborne diseases [7]. A device to identify fecal matter would offer the producer a powerful tool as a first step in assessing if _E. coli_ O157:H7 exists in a produce field. There is a need to detect pathogenic bacteria infected fecal matter on leafy greens in the field prior to harvesting. An imaging device could be used as the first step in this process by locating fecal matter. There are many benefits to avoiding the harvesting of contaminated produce, including: (1) cross-contamination can be avoided during harvesting and downstream processes; (2) reduction of leafy green produce waste and (3) reduction of downtime required for cleaning and sanitizing equipment. Solar radiation on a leaf is reflected, transmitted, or absorbed by the leaf's surface [8]. Solar radiation emits both ultraviolet and visible light. In the red and far red spectral regions of the solar response from green vegetation, both fluorescence and reflectance typically contribute to the response signal [9; 10]. Absorbed solar energy that is not used in the photosynthesis process is emitted as fluorescence at longer wavelengths or as heat [11]. [PERSON] et al. [12] stated that \(\sim\)1% of the solar radiation absorbed by leafy greens is emitted as chlorophyll \(a\) fluorescence. [PERSON] et al. [11] reported that on average 10%-20% of the response recorded at 685 nm on vegetation foliar samples from simulated solar radiation was due to chlorophyll \(a\) fluorescence; however, artificial light sources cannot reproduce the fine spectral profile of sunlight [13]. Fluorescence emissions from foliage occur throughout the ultraviolet and visible region and have peaks at 340, 445, 530, 685 and 740 nm [9; 10]. [PERSON] et al. [11] reported that between 680 and 725 nm vegetation has relatively low reflectance due to strong chlorophyll \(a\) light absorption. Rapid hyperspectral fluorescence imaging technology for detection of fecal contamination on fresh produce has been developed by [PERSON] et al. [1]. This technology is restricted by light conditions as directly reflected light can saturate the fluorescence emission signal of interest. [PERSON] et al. [1] reported that chlorophyll fluorescence emission peaks near 685 and 740 nm for fresh leafy greens shift towards the blue region of the spectrum as the leaf decays (over the 27-day storage period studied); these emission peak shifts are similar but less extensive than the peak shifts towards the blue region observed for the chlorophyll peaks in fecal matter. The ratio of the fluorescence peaks at 685 and 740 nm has also been shown to be related to vegetation vigor [14]. [PERSON] et al. [13] reviewed developments in the area of field spectroscopy and specifically in the establishment of field spectroscopy for calibration of airborne and satellite sensors. To be of practical use, field spectroscopy must be repeatable under varying environmental conditions.