Sensitive substances refer to a type of substances that are unstable and prone to degradation in adverse environments [1], including some hydrophobic compounds (fat-soluble vitamins, chlorophyll etc.), microbial components (lactic acid bacteria, probiotics, bifidobacterium etc.) and lipid (vegetable oil, essential oil, etc.), among others. Sensitive substances have attracted much attention due to their various biological activities, including antibacterial activities, antioxidant activities, anti-cancer activities and other health effects [2]. Sensitive substances are widely used in food processing, functional food, medicine and other fields. Therefore, the products prepared by sensitive substances have high commercial development and application value in improving consumer nutrition and health. Although sensitive substances have broad application prospects for human health, they have some disadvantages such as poor stability, poor performance, easy degradation under heat and light, low bioavailability and poor targeting capacity [2]. These deficiencies prevent sensitive substances from fully expressing their functional characteristics, complicate their use in food systems and limit their application in food industries. To solve the above problems, it is necessary to develop a delivery vehicle to improve the deficiency of sensitive substances.

Some delivery vehicles have been used to improve the deficiencies of sensitive substances, including liposomes, micelles, emulsions, nanoparticles, microcapsules, and so on. Among these delivery vehicles, microcapsules can use food-grade and biodegradable materials to uniformly encapsulate sensitive substances and separate sensitive substances from the surrounding environment, which has advantages such as strong encapsulation capacity and excellent stability [3]. Interestingly, proteins from different sources are widely used in the preparation of microcapsules due to their high biocompatibility, amphiphilicity and non-toxicity [4]. In recent years, many researchers have reported that protein-based microcapsules can be used as a delivery vehicle to effectively encapsulate sensitive substances to improve their defects. Zhang et al. found that microencapsulation treatment could significantly improve the biological stability and antioxidant activities of chlorophyll [5]. Similarly, Maqsoudlou et al. also found that the maltodextrin (MD) and whey protein concentrate (WPC) mixture as the wall materials for the microencapsulation of bee pollen protein had an appropriate protective capability and protection against UV radiation [6]. Protein-based microcapsules have rich applications, mainly including meat products, bread, cookies, dairy products, etc. It is worth noting that elucidating the release pattern of sensitive substances in microcapsules is particularly important for studying the molecular mechanism of their release of active substances. However, as far as we know, there is no systematic summary and discussion on the release mechanism of protein-based microcapsules to sensitive substances.

To the best of our knowledge, there has been no effective and comprehensive review of research on protein-based microcapsules to encapsulate sensitive substances. In addition, there has been no previous description and discussion of all the possibilities and obstacles in this key research field. In this review, the research progress on the delivery of sensitive substances by protein-based microcapsules in the last 20 years is reviewed and summarized. Firstly, the classification of protein-based microcapsules is summarized and elaborated. Secondly, the preparation methods for protein-based microcapsules have been proposed. In addition, the release mechanism of protein-based microcapsules to release sensitive substances was emphasized. Moreover, the application of protein-based microcapsules was concluded and discussed. Finally, the problems and further prospects in this field are discussed and prospected.