## 4 Literature Review and Related Work
### Distributed Ledger Solutions (2016-Today)
[PERSON] et al. [72] propose a framework that uses a two-way factor verification blockchain solution and watermarks images and documents using the discrete wavelet transform method. [PERSON] et al. [54] present the FoodTrail platform, which uses a four-layer architecture in conjunction with smart contracts as a way to conduct transactions through the whole supply chain. In the same year, the World Wildlife Fund (WWF) creates a project called \"Bait-to-plate\" focused on the traceability of tuna in New Zealand throughout the whole supply chain. WWF's project adopts RFID technologies for fish tagging and an Ethereum-based blockchain [63]. The research interest in blockchains in the agriculture sector continues its upward trend as more and more researchers, companies and scientists expand the existing knowledge base in 2019. [PERSON] et al. propose a consortium blockchain solution where a central authority (CA) defines the stakeholders of the network as well as the certifications which allow them to participate in it. The authors present a detailed description of their proposed model and algorithms, as well as the proof of concept (PoC) method that is used to validate the smart contract transactions [87]. Following the same concept, [PERSON] et al. [29] propose a framework using a consortium, Ethereum-based blockchain and smart contracts for the food supply chain in the Sardinia region. [PERSON] et al. highlight the positive aspects of using this approach in order to promote smart tourism and protect the authenticity of Sardinian local products. Another interesting publication considering consortium blockchains and smart contracts is formulated in the publication of [PERSON] et al. [30]. More specifically, [PERSON] et al. propose a system which combines blockchain, smart contracts and electronic product code information services (EPCIS). In order to make the blockchain more efficient, they promote the idea of reducing the data stored on the blockchain, by using an on-chain/off-chain approach where the on-chain data are those required to track and trace a product, and the off-chain data are enterprise data, most of which are confidential. Through this approach, the blockchain avoids being heavy-loaded and the sensitive data of enterprises are protected. In this direction, [PERSON] et al. present a platform that stores data from crop sensors in an Ethereum-based blockchain, which facilitates smart contracts for buying or selling crops in India [43]. A state-of-the-art analytical approach regarding the blockchain and smart contracts implementation in the agri-food supply chain is described by [PERSON] et al. in their publication [31]. This paper demonstrates how blockchain and Ethereum smart contracts can efficiently trace and track, as well as enable, a fully functional integration of stakeholders' transactions in the agriculture supply chain. The authors propose a series of algorithms regarding smart contract validation and execution as part of the implementation framework. The proposed solution is applied for tracing and tracking in the soybean supply chain, but could be generalized to provide trusted and decentralized traceability to other types of crops and products in the agriculture supply chain. [PERSON] et al. study the application of blockchains in protected designation of origin (PDO) and protected geographical indication (PGI) products in the European Union. Their study makes it possible for consumers to verify the origin of the raw materials, as well as obtain information on the production process, the process controls, the packaging operations, and the distribution and certification related to the product, in order to detect possible adulteration [70]. [PERSON] and [PERSON] present an analysis about the necessity of blockchains in the agriculture sector, also presenting the three generations of blockchains (Blockchain 1.0, Blockchain 2.0 and Blockchain 3.0) and their attributes [88].