###### Abstract  
Due to the increasing global population and climate change, new sustainable food sources are being intensively sought to replace less favorable livestock production. Especially new protein sources and their food applications are being focused on. In this paper, several selected protein sources that may have potential application in future functional foods, such as fermented foods, were examined and compared. These sources include single cell protein (SCP), _Arthrospin platensis_ (Algae), _Acheta domesticus_ (edible insect), potato, and rice protein. The above sources were compared to whey proteins. The parameters studied were total nutritional value, amino acid profile, fatty acid profile, the content of some elements, and the presence of toxins. unconventional proteins; novel proteins; alternative protein sources; novel food; fermented products; functional food +
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Proteins of plant origin usually have a lower digestibility than those of animal origin. The main reason for the reduced digestibility of plant proteins is the presence of anti-nutritional factors (ANFs) such as phytates, tannins, trypsin inhibitors, and lectins. One way to increase the digestibility of plant proteins is fermentation. For this reason, combining plant proteins or other non-conventional proteins with fermented beverages is justified [12]. New sources of protein that are most commonly cited in the scientific literature and may have the potential for use in human nutrition include the following listed below. Single cell protein (SCP) is derived from single-cell microorganisms such as bacteria, fungi, and molds. SCPs can be used in future foods due to their high production efficiency and protein content, with bacteria containing the most protein (50-80%) [13; 14]. Organic waste left over from the production of other food products can be used to produce SCP, which eliminates another problem of excessive waste products in the food industry [13]. Despite the wide variety of compositions depending on the type of microorganism, there are some standard features, such as high protein content with low fat and carbohydrate content and high thiamine, riboflavin, glutathione, and folic acid content. SCPs can also be a source of phosphorus and potassium [15]. Edible insects have recently gained popularity and are also increasingly described in the scientific literature. Entomophagy in the world is not new, but insects are consumed only in some parts of the world, and in Europe, for example, they are still a novelty, and only a small part of the population of this continent has the opportunity to taste edible insects. They are now being studied for their potential use in human nutrition [16; 17]. Insects are much more environmentally friendly compared to livestock production. They require much less water, space, and electricity to produce the same amount of protein. Insect production also produces much less CO\({}_{2}\) than animal production [2; 18]. Nutritionally, edible insects, according to available sources, have a high protein content (50-76%), although the results vary widely among species. Furthermore, they show some health-promoting features, such as high content of vitamins and minerals [19]. Moreover, insects exhibit some antioxidant activity, an added benefit of human consumption in place of products from slaughtered animals [20]. Algae are another potential source of alternative protein. The most popular types of algae are Spirulina (_Arthrospina platensis_) and Chlorella (_Chlorella vulgaris_). However, Spirulina appears to be a better source because, unlike Chlorella, it does not have a cellulose cell wall that is difficult to digest and which requires additional processing to destroy [21; 22]. According to available literature, Spirulina is a rich protein source (>60%). Moreover, it has a health-promoting effect, where 3 g/day supplementation can improve cardiovascular fitness and accelerate body regeneration [23].