## 2 Literature Review  
A systematic literature review was conducted following the PRISMA 2020 guidelines [14]. A comprehensive search was performed in two academic databases--Scopus and Web of Science--for peer-reviewed articles published between 2019 and 2024. Keywords such as \"blockchain\", \"food/agriculture traceability\", \"smart contract\", \"DID\", and \"digital twin\" were used. Records were screened for relevance (English language, focused on agri-food supply chains) and de-duplicated. A total of 144 records were initially identified: 78 from Scopus and 66 from Web of Science. After removing duplicates, 118 unique records remained. These were screened based on titles and abstracts, resulting in the exclusion of 49 records. The remaining 69 articles underwent full-text assessment for eligibility. Following the full-text review, 25 articles were excluded for not meeting the inclusion criteria, specifically, they were not related to agriculture or were not peer-reviewed. As a result, 44 studies were included in the final review. In this section, twenty of these studies will be analysed; the remaining twenty-four will be discussed in Section 3. Supply chain challenges and traditional traceability limits: Agri-food supply chains face fraud, contamination, and opacity. Globalised trade heightens risks of adulteration, fake products, and disease outbreaks [15]. Traditional traceability (barcodes, manual logs) is limited by centralised databases, single points of failure, data silos, and lack of interoperability [13]. Centralised systems often produce \"information islands\" and are vulnerable to data tampering [13]. As one review notes, legacy food chains suffer from \"inadequate monitoring\" and \"insufficient policies\" in the face of fraud, undermining product provenance [13]. In sum, existing systems cannot guarantee end-to-end accuracy or real-time access. Blockchain applications in agri-food: Blockchain's immutable ledger and decentralised trust address many gaps [15]. Major initiatives include the IBM Food Trust, a Hyperledger fabric network adopted by Walmart, Carrefour, Albertsons, and others to trace commodities (e.g., pork, lettuce) in seconds [15]. TE-FOOD is a mobile-based system (used in Vietnam) that assigns QR-coded IDs to animals (pork, poultry, eggs) at each stage, enabling consumers to scan and verify the origin. Pilot projects also include Waltonchain, XinFin, and AntGroup's AntChain for food product tracking. Blockchain consortia integrate with IoT: RFID tags and sensors, which record temperature, humidity, and location on-chain or via linked storage, enable cold-chain monitoring and real-time auditing of perishable goods. Smart contracts and other technologies: Ethereum and Hyperledger platforms use smart contracts (self-executing code) to automate workflows. A smart contract is \"a self-executable, self-enforceable...piece of source code\" on blockchain. For example, contracts can release payments when IoT data confirm delivery conditions. Decentralised Identifiers (DIDs) enable self-sovereign identity for entities or assets; DIDs are URI-based IDs decoupled from central registries, allowing any party (farmer, product) to prove control of its identity. Digital twins (virtual replicas of crops or facilities) can interface with blockchain to ensure secure, real-time data sharing. Blockchain \"facilitates transparency (through the supply chain), security, quality, plant health and data storage\" for IoT-enabled twins, enabling resilient simulation and monitoring. Interoperability and governance: Multiple blockchains and legacy systems pose interoperability challenges. Cross-chain architectures are emerging to bridge permissioned and public networks, using smart contracts as oracles or bridges. Without standards, data exchange between platforms remains limited; one review notes that \"interoperability between different blockchain networks...is essential for creating integrated supply chains\".