# Internet of Food (IoF), Tailor-Made Metal Oxide Gas Sensors to Support Tea Supply Chain
## 1 Introduction  
Tea is native to the northern hills at the foot of the Himalayas where the inhabitants chewed _Camelia sinensis_ for medicinal purposes. The Chinese populations then invented preservation techniques to increase the shelf life and facilitate the transport of the product. Over time, the techniques have improved, and the cultivation areas have increased so as to arrive at the point today where many varieties are known [1]. It is one of the most consumed beverages worldwide, with an increase in the consumption of 23.4% in the last 7 years, reaching 297 billion liters by 2021. Generally, tea is an aromatic beverage prepared by pouring hot or steaming water over dried or fresh leaves of the _Camelia sinensis_ (green, white, and Oolong teas), an evergreen shrub (bush) and _Camelia assamica_ (black and Pu-Erh tea), which originate and are cultivated, respectively, in China and India [2, 3]. The most produced and consumed teas worldwide are green and black teas [4]. Tea is an infusion, but not all infusions are tea, as there are infusions or herbal teas that originate from red fruits, chamomile, mint, lavender, etc. The tea leaves undergo different treatments that determine their classification. Spices or herbs are added to the basic tea in order to spice up the flavor and taste. The first classification is based on the fermentation treatment and is also the classification used at customs; in particular, we have:  
* Fermented tea: black and Pu-Erh tea* Unfermented tea: green and white tea,
* Partially or semi-fermented tea: Oolong tea. In addition to this classification, the teas are placed in other classes based on the following specific treatments: scented tea, flavored tea, smoked tea, blends, pressed teas, and a bouquet of tea and flowers. There are many distinctive types of tea; some, such as Darjeeling, Ceylon, Oolong, etc., have cooling, slightly bitter, and astringent flavors, [5; 6; 7; 8] while others have vastly different profiles that include sweet, nutty, floral, or grassy notes. Tea leaves contain thousands of chemical compounds and release volatile compounds (VOCs), which contribute to the definition of product quality [9; 10; 11; 12; 13; 14]. The aroma is one of the determining factors in the quality of tea and is due to the volatile compounds. Volatile compounds are mainly responsible for the flavor and aroma of the infusion, and many of them are not present in the fresh leaf but develop after processing. There are more than 600 volatile compounds (VCs) in tea, resulting from the enzymatic action on odorless compounds present in the leaf that are released after rolling and fermentation [15]. The volatile aromatic compounds differ according to the types of tea, both for the \"different\" fresh material and for different production processes, without forgetting that the final consumer requires that the purchased product be recognized and standardized. Recent research has shown that the volatile aromatic components of tea are influenced by several factors: cultivar, area of cultivation, cultural practices, production methods, and conservation [16; 17; 18; 19]. Many kinds of classical analytical chemical techniques such as the gas chromatography coupled with mass spectrometry (GC-MS) have been largely used and have demonstrated their accuracy and specificity, but present important limits are normally expensive and time consuming and require appropriately trained staff to operate them. On the other hand, an innovative tailor-made gas sensor device named small sensor systems (S3) has been applied broadly in the quality control in food the field and environmental monitoring as well exhibiting remarkable results [20; 21; 22]. This innovative device, S3, is fast, remotely controllable, totally user friendly, and, once trained, does not need any special skilled staff to operate it. In particular the S3 device is totally tailor-made for the specific application.