Product serialization aims to allocate unique serial numbers to products in a supply chain. The security challenges to product serialization are: Valid serial numbers can be stolen and used to label fake products. Thus uniqueness of a serial number should be verifiable at any stage of its life- cycle in a supply chain. A planned change of custody of a product in distribution can be corrupted by a few intimidatory nodes. Compliance with the planned change of custody should be verifiable. The manufacturer and the consumer should be able to verify that perish- able food products with expired shelf life are discarded. In this paper, we use blockchains to develop a product serialization method that solves the above security issues in a multi-party perishable good supply chain. Blockchains can revolutionize security and transparency in supply chains by providing a secure data-sharing platform in a multi-party environment. Al- though blockchains can provide a secure data storage of change of custody events of products in a supply chain, a high volume of such events poses scalabil- ity problems for blockchains. In this paper, we solve the product serialization problem using blockchain offline channels. Our solution significantly reduces the number of transactions needed to be recorded in the blockchain. We propose a secure serialization protocol to verify the authenticity of serial numbers despite not frequently engaging with the blockchain.