# Battery-Free Pork Freshness Estimation Based on Colorimetric Sensors and Machine Learning
## 1 Introduction  
The expiration date is typically set at 70% to 80% of the recommended consumption time, indicating whether or not the product is safe for consumption. If this standard is too strict, there may be an increase in unnecessary food disposal, and if it is too loose, there is a risk of foodborne illnesses, such as various bacterial food poisonings. When there is no systematic process in place to detect the freshness of meat, its sale within the expiration date depends on the refrigeration system. Currently, the state of food is determined by human senses, such as vision and smell, which may be unreliable. The color of meat changes based on the remaining myoglobin, and its structure changes depending on the state of iron oxide and the degree of oxygen binding, affecting the properties of light reflection and resulting in a color change [1; 2; 3; 4]. During the first slaughtering of red meat, blood accumulates, causing a purplish-red appearance due to the iron in the blood. When oxygen is obtained, it transforms into oxy-myoglobin, resulting in a red color, similar to fresh red meat purchased at the market. If oxygenation continues, the red meat may temporarily return to hemoglobin, but this is uncommon, and the duration of the change is also temporary. Generally, it changes from myoglobin to oxy-myoglobin and met-myoglobin, and the color changes from purplish-red to light-red, and then to brown. When stored in winter or dry-ripened, it transforms from brown to darker brown, and during summer or wet ripening, it turns to gray-brown. In other words, for red meat purchased at the market, the color changes from light-red to brown and gray, and the changes in myoglobin are as follows: myoglobin\(\rightarrow\)oxy-myoglobin\(\rightarrow\)met-myoglobin\(\rightarrow\)denatured met-myoglobin. Furthermore, when red meat is spoiled, it becomes unappetizing and emits a foul odor. In the case of packaged products, if they are stored for a prolonged period andthe contents expire, the packaging may bulge, and the spoiled smell may be difficult to detect [5]. The odor is the most apparent factor in determining whether the meat has gone bad [6]. However, smelling meat closely can be harmful to one's health; hence, it is not a recommended approach [3]. According to the FDA, fresh meat (beef, veal, lamb, and pork) can be refrigated for three to five days at 4.4 \({}^{\circ}\)C. Red meat stored for more than five days is usually recommended to be discarded. However, it may be displayed or sold due to a distribution or seller error, and if the refrigerator's operation is disrupted by a power outage or a mistake in the handling, the storage period will be significantly reduced. Meat freshness changes under refrigeration, albeit at a slower rate. An automated measurement system using smart sensor tags is designed to prevent management errors. Special attention is paid to ensure the stability of pork packages exposed to room temperature during the summer months, which requires caution. Currently, researchers are exploring an approach to detecting the state of a target object, and the development of a color measurement system using NFC technology is fascinating [7; 8; 9]. Researchers test for pH reactivity [7] or use temperature, humidity, oxygen, and carbon dioxide sensors to determine fruit maturity [8]. Additionally, a nanoparticle (AgNP)-based approach for detecting pollution in aquatic life has been developed [10]. Furthermore, an analytical system for detecting heavy metal pollutants in drinking water has been proposed [11]. Previous studies have been conducted to determine the freshness of existing foods. Once-used meat is stored in a sealed container and measured with a pressure sensor, which is occasionally used to detect its condition and can be useful [9; 12]. However, this method is not suitable when measuring the freshness of a new product in circulation or in storage.