A. Background
The human nose, a remarkable sensory organ, serves as a gateway to our olfactory experience. With the advancement of technology, Electronic Nose (eNose) is one of the electronic applications that has emerged as a versatile tool, mirroring the olfactory capabilities of its biological counterpart. Electronic nose usually recognizes smells by identifying the chemical compound through an array of sensors, which is then analyzed by pattern-recognition software. [
1].Electronic noses find application in diverse commercial sectors related to agriculture, encompassing areas such as agronomy, biochemical processing, botany, cell culture, plant cultivar selections, environmental monitoring, horticulture, pesticide detection, and plant physiology and pathology within the agricultural domain [
2].
Soybeans are one of the most popular antecedents for traditional alkaline fermentation, which may be found in a variety of cuisines such as Thai Thua-nao, Japanese Natto, Indonesian tempeh, Nepalese and Indian Kinema, Korean Chungkookjang, and Chinese Douchi. In general, it has been discovered that the emitted VOCs in alkaline fermented foods and seasoning agents have different fragrances such as caramel, flowery, smokey, malt, and the aroma of cooked sweet potato.Tofu (soybean curd) is regarded as a nutritious food, low in calories and high in protein, iron, calcium, magnesium, and B-vitamins [
3]. Because of their low cost, soybean products are an attractive option for alleviating malnutrition among impoverished people on diets based on grains [
4]. Despite its nutritive content, there are some hindrances for consuming the said product. Since in the Philippines, it's mostly found in supermarkets and wet markets, where in the supermarket, the tofu was safely packed and stored in a 4 to 10℃ refrigerator and in the wet market, it is mostly stored in an open container, soaking in water under ambient temperature. According to the conducted study, there are 11 types of bacteria found in contaminated tofu [
5]. All organisms that were shown capable of causing spoilage in tofu were present in huge numbers early in production but are no longer available discovered in samples during pressure cooking [
6]. Since most of the consumers didn’t show a concern for its spoilage and microbiological quality, food poison occurred, Shigella sonnei was linked to one outbreak [
7] and Yersinia enterocolitica to the other outbreak [
9] . The capacity of pathogenic microorganisms to thrive and create poisons in tofu has also been investigated.Different types of microorganisms, such as bacteria, yeast, and mold, have been employed to affect the chemical composition of fermented foods and drinks, resulting in variations in taste, smell, color, and nutrition [
26]. Fermentation using probiotic microorganisms such as lactic acid bacteria in products such as tofu,yogurt, kefir, and kimchi, for example, can boost nutritional value by lowering cholesterol and encouraging good digestive function [
15].
Tofu's microbiological safety has become a concern due to these previously mentioned quality evaluations.Several types of equipment and procedures have been created to assess the quality of fermented food and drinks, such as physical quality, nutrition value, microbiological quality, safety, and sensory quality, ranging from spectroscopies to sensory assessment techniques [
21].In recent years, there has been a lot of interest in non-destructive measurement, quick analysis, and on-site testing equipment and procedures with low operating costs and simplicity [
22].Furthermore, electronic nose is the most recommended device in order to assess and track the quality of the tofu due to its capability of distinguishing simple or complex scents, since it consist of an array of electronic chemical sensors with limited selectivity and a pattern-recognition algorithm [
10].In fact,electronic nose provide fast and detailed information about the sample, and it can monitor and assess the samples that would otherwise be impossible to distinguish using human sensory panels [
23].The e-nose was used for identifying minor changes in food flavor and odor. Moreover, it was an effective technique for differentiating flavor qualities, and it is commonly used in food quality assessment [
24].According to a conducted study, e-nose, e-tongue, and e-eye were used together with three chemometrics approaches to identify and estimate tea quality. The findings showed that fusion signals outperformed independent signals and could accurately assess the composition of the key chemical components [
25]. Because all of the sensors are produced on a single substrate using easily scalable self-assembly methods, consume very low power, and the data acquisition is designed on very simple elements, the e-nose is potentially much more affordable than commercial ones, which together with the presented results ensures its future competitiveness with conventional electronic noses [
27].
The electronic nose concept primarily focuses on imitating the human olfactory system. Buck and Axel of Columbia University, USA, were the first to explore the mechanics, odor detection, and limits of human olfactory perception at the molecular level in 1991 [
16].The mentioned researcher aided in the development of new methods/ideas for developing an E-nose system rather than relying on complicated human olfactory sense for multipurpose applications. The human olfactory system is divided into three parts: (I) the odor-receiving component, which includes olfactory receptor glands and scent delivery systems; and (II) the scent-delivery system [
17]. (II) the neurological system for signal transmission between the brain and the rest of the body, and (III) a decision system capable of recognizing, identifying, and acting on the brain's sense of smell. The process of scent perception is quite complicated. According to psychophysical tests, people are capable of discriminating over 1 trillion olfactory stimuli [
18]. However, sensations and age of humans have a significant effect on odor recognition and classification [
19]. E-nose is additionally used in a variety of food sector applications, including shelf life determination [
28], [
29], [
30], identity [
31], counterfeit detection [
32], evaluation [
33],[
34], freshness assessment [
35], and authenticity [
36]. Furthermore, E-Nose is used in the industrial world to identify contaminants [
37], [
38], allergies [
39], [
40], and dangerous compounds in food [
41].Furthermore, toxic agents in the sample and testing time are critical obstacles in the capacity to determine smells through the human olfactory system [
20]. In this regard, the widespread use of electronic noses (E-noses) capable of encoding high-dimension data is beneficial.The transformation of VOC patterns into a smaller-dimension pattern of sensor data has emerged as a useful technique non-invasive and effective option for detecting bacteria [
48]. As a result, several bacterial VOCs have been found, and their quantities and profiles are very dependent on culture medium, incubation duration, and bacterial species and strains utilized, as inconsistent detection/non-detection data has been published [
49],[
50],& [
51].
In addition, using lightweight residual convolutional neural network (LRCNN) combined with an electronic nose (e-nose) had a capability to evaluate the soybean quality, having a classification accuracy of the network is 98.37% and precision is 98.49 indicating that the LRCNN combined with the e-nose is capable of identifying the gas information of soybeans from various growing sites, offering an innovative manner for soybean quality traceability [
11]. Another innovative development leads to an accurate distinction wherein the combination of E-nose and E-tongue data with LDA provided an accurate distinction (with a discriminant accuracy of 97.22%) of commercially fermented soybean [
12]. Indeed, the electronic nose can evaluate the accurate microbiologic content of a soybean, in fact fresh soybean seeds present 11.43% moisture, 38.09% protein, 4.58% reduced sugar, and 18.47% fat. The results of this investigation revealed a substantial decrease (P< 0.05) in the moisture content of all treated soybean samples, with roasting at 230 °C for 30 minutes yielding the lowest value of 2.18% [
13]. Moreover, the e-nose has an ability to identify 11 volatile chemicals in Korean soybean, while the e-tongue assesses the intensity of 5 basic tastes [
42]. Through the used of the principal component analysis, the contribution rates of the first and second principal components detected by electronic nose and tongue for minced chicken meat adulterated with soy protein were 99.2% and 0.6%, respectively, and the total contribution rate was 99.8% leading to the result that the combination of electronic nose and electronic tongue sensors has the potential to distinguish and predict soy protein-based or starch-based adulteration in minced chicken meat, and it has also been demonstrated to be a useful identification method for meat contamination detection with high efficiency and accuracy [
43]. Hence, this device can be utilized in breeding as a fast screening tool programs, in the selection of soybean mutants/varieties with varying volatile profiles, as well as in the mapping of the QTLs and loci responsible for these features. This platform may also be utilized to link the beany flavor to seed volatile chemicals, eventually leading to the development of cultivars with less off-flavor taste and increased popularity among consumers [
44]. Moreover,the soybean seed volatiles were examined using a electronic nose with solid phase microextraction (SPME) technique in conjunction with gas chromatography-mass spectrometry.(GC-MS) and revealed that 30 recognized volatile compounds were recovered, as well as 19 new compounds that were either confirmed or tentatively identified [
45]. It was also discovered 20 essential aroma components with flavor dilution (FD) values of at least 64 utilizing an aroma extract dilution analysis (AEDA) of the fragrance concentrate of soy milk prepared from a prominent Japanese soybean cultivar, Fukuyutaka (FK) [
46].The E-nose generates electrical resistance signals,combined with linear discriminant analysis, the four bacteria were distinguished (90% of correct classifications for leave-one-out cross-validation). Furthermore, various linear regression models were developed, allowing the number of colony-forming units (CFU) to be quantified.(0.9428 ≤ R2 ≤ 0.9946),with a maximum root mean square error of less than 4 CFU. Overall, the E-nose proved to be a strong qualitative-quantitative instrument for preliminary bacteria analysis, with potential applications in solid food matrices [
47].