## 2 An Outline of Proteomics  
In recent years, the study of proteomes has been applied in food technology in order to correlate the quality and safety of foods with health issues and welfare of the public. Microbial metabolism and responses to stress can be described more precisely by using proteomics [2]. Proteomics is an important method because multiple species detection and unknown target screening can be performed by using mass spectrometry technologies, gas chromatography-mass spectrometry (GC-MS) and ultra-high-performance liquid chromatography-MS (UHPLC-MS). For example, UHPLCMS-MS and liquid extraction surface analysis mass spectrometry (LESA-MS) methods can identify species-specific markers for meat adulteration detection and heat-stable peptide markers in processed meat. In addition, two-dimensional gel electrophoresis mass spectrometry (2 DE-MS) and OFFGEL-MS technologies successfully determined undeclared species in commercial processed meat products [4] and tandem mass spectrometry (LC-MS/MS) determined the relationship between proteomes and beef exudate with the color and oxidative quality [18]. Additionally, two-dimensional gel electrophoresis together with MS and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) technologies were used not only to identify specific protein but also to understand proteome modifications as a consequence of growth, development, postmortem metabolism and tenderness [5; 19]. Last, but not least, LC-high-resolution mass spectrometry (HRMS) and LC-MS/MS multiple reaction monitoring acquisition mode (MRM) methods are considered complementary tools for the detection, identification and confirmation of species-specific heat-stable peptides in processed meat products [20]. In this line, several MS approaches have been developed recently, and along with chromatographic and nuclear magnetic resonance (NMR) techniques they have become some of the most commonly applied approaches for metabolomic fingerprinting [20]. Conventionally, MS methods are joined with chromatographic separation techniques, such as LC-MS [20].  
Briefly, proteomics has been classified as targeted or untargeted, and targeted methods can be further classified into four categories (Figure 2). The first category performs targeted data acquisition for the molecule of interest, the second category is based on targeted data analysis acquiring data for all molecular species, the third category is based on peptide ion data and relies on the mass of the entire molecule, while the last category is based on peptide fragments and relies on the fragments of a molecule [21].  
Regarding quantitation using the mass spectrometry, proteomics can be classified into two broad categories: the label-based method and label-free method. In the label-based method, samples are first differentially labeled and pooled before being subjected to LC-MS/MS analysis. As a consequence of using the label-based method, the expected disparities are minimized. On the other hand, in the label-free quantitative proteomics method the samples are labeled, processed and analyzed independently by LC-MS/MS. The quantification is performed by the measurement of the peak area and/or consideration of the number of MS/MS spectra from each peptide using various software due to the large amount of data [22].