## 1 Introduction  
In food production, the presence of unwanted microorganisms is a major risk for the quality and safety of the food products. Microorganisms in the food production environment can contaminate foodstuffs and cause spoilage or foodborne illnesses when ingested. Bacteria such as _Listeria monocytogenes_, _Salmonella enterica_ subesp. _ enterica_ serovar Typhimurium, _Campylobacter_ spp. or _Escherichia coli_ OH157:H7 are major causes of contamination outbreaks and foodborne illnesses [1; 2]. A total of 5079 foodborne outbreaks were reported in the European Union in 2017 [3]. Hygiene is essential in preventing microbial contamination of food, working surfaces and equipment. Cleaning and disinfection is thus an important part of the operation of food industries. Improper hygiene can lead to remaining food residues on work surfaces and equipment [4]. This has two detrimental effects: (i) partial inactivation of disinfectants used for decontamination of surfaces, resulting in limited decontamination and potential exposure of microorganisms to sublethal concentrations of biocide [5; 6]; and (ii) presence of anchor points for the attachment of bacteria, favoring the growth of a biofilm. In both cases, the result is the occurrence of persistent contamination [7], comprised of microorganisms that resist conventional hygiene procedures, either because they have developed increased tolerance to the biocides employed or because they are protected by a biofilm [8; 9; 10]. Persistent microorganisms can cause long-term contamination of the food production plant, potential outbreaks of food pathogens and ultimately, even lead to closure of the factory [11]. Hygiene procedures are usually aimed at removing physical, chemical and microbial contamination from the production environment [12] and should be based on a risk assessment program where the critical contamination points, sources of contamination and target levels to ensure food safety should be established. Validation of the hygiene process is therefore of great importance to check that cleaning and disinfection has been done correctly and that food contact surfaces and production equipment are free of contamination before production. Several tools and methodologies are currently available for this purpose, with different scopes for detection of contamination and variable features in terms of cost, ease of use, speed, and specificity [13]. The most commonly used techniques are summarized in Table 1.  
The detection of microbial contamination is usually limited by the lack of specificity of the tools available. For example, measurement of ATP luminescence is often employed for this purpose due to the presence of ATP molecules in every microorganism, but this is also the case of other cells present in food residues [14]. Additionally, microorganisms surviving a disinfection process can be stressed and have their ATP levels depleted [15], leading to underestimation of microorganism numbers when using this technique. Specific methods for detection of microorganisms, such as contact plates or swabing provide information on the numbers of microorganisms recovered from the surfaces and, if selective media are used, they will also discriminate among different classes of microorganisms. On the other hand, these methods require incubation of samples, sometimes preceded by an enrichment step [16]. For this reason, selective detection of microorganisms usually requires 18-72 h for most bacteria and up to 5 days for fungi. Finally, visual inspection is an essential part of the hygiene audit, since it allows identifying bad hygiene practices, lack of equipment maintenance, improper use of equipment, and other aspects of the hygiene procedure. However, this requires experienced personnel and a systematic approach to provide useful results. Novel methods for the detection of microbial contamination are needed to improve the validation of hygiene procedures in food industries and others where the presence of microorganisms is of concern.