# Valorization of Agri-Food Waste into PHA and Bioplastics: From Waste Selection to Transformation
## 2 Production of Valeric- and Propionic-Rich VFA Streams from Agri-Food Wastes
### Agri-Food Waste Typically Used for VFA Production
#### 2.2.1 Production of VFA, Rich in Valeric Acid and Propionic Acid, from Cheese Whey  
Cheese whey is a greenish-yellow liquid that remains in the milk after the coagulation of the milk proteins has occurred. This coagulation can be achieved using acids or rennet, as well as other plant-based coagulants [45; 46]. whey is regarded as a by-product of the cheese-making process that has a detrimental impact on the environment. It is estimated that between 2021 and 2022, 145-200 million tons of whey are produced annually [47; 48], which presents a significant environmental concern when disposed of as waste with a BOD of 40,000 mg/kg [49]. Considering the environmental concerns surrounding whey, there has been a growing interest in studying its properties, composition, nutritional value, and potential technological applications. It is important to note that whey retains a high level of nutritional value, primarily due to the presence of residual proteins. This has led to a growing interest in the valorization of whey, as evidenced in 2020 by [PERSON] et al. [50]. The liquid remaining after the coagulation of casein in rennet- or plant-coagulated cheeses, which have a pH above 6.0, is therefore classified as \"sweet whey\". In contrast, the term \"acid or acidic whey\" is used to describe the remaining liquid after the coagulation of casein in acid-coagulated cheese, which has a pH below 5.8 [51; 52; 53; 54]. The whey released from the production of ricotta cheese, for example, is referred to as second whey cheese [55; 56]. This is an acidic whey with a lower fat and protein content than the primary whey [57, 58, 59]. Second, whey is also produced from non-acidified cheese and has a higher pH. The chemical composition of whey varies depending on the type of cheese produced and the type of milk from which the main cheese is derived [60, 61, 62, 63]. To illustrate, the composition of sheep's milk whey is characterized by markedly elevated levels of total solids, lipids, and proteins in comparison to cow's whey [46, 64]. Furthermore, sheep's whey contains a greater quantity of \(\beta\)-lactoglobulin, a comparable proportion of \(\alpha\)-lactalbumin and serum albumin, and immunoglobulins in comparison to cow's whey [65]. The composition of goat whey differs from that of cow whey. It contains approximately twice the amount of \(\alpha\)-lactalbumin and lower levels of \(\beta\)-lactoglobulin, serum albumin, and immunoglobulins [66, 67]. Although it depends on the type of whey, the physicochemical characteristics of whey are usually BOD 27-60 g/L and COD 50-102 g/L, so the BOD/COD ratio is usually 0.5, indicating that it is an easily degradable substrate by using aerobic or anaerobic treatments [68]. It also contains a significant amount of carbohydrates (mainly lactose, 45-50 g/L), proteins (0.6-1.1%, 6-8 g/L), lipids (0.06-0.05%), lactic acid (0.05-0.9%), total suspended solids (1.3-22 g/L), salinity (8 mS/cm), and mineral salts [69]. In terms of whey valorization, in the present era, the principle of the circular economy suggests the utilization of whey in the production of an array of products, including whey cheeseses, whey powders, edible wraps, beverages, and food supplements. As a valuable by-product, whey is esteemed for its constituent components and functional properties, which have informed the development of food-related applications containing whey components, particularly proteins [70, 71, 72, 73]. In addition to the aforementioned types of whey recovery, there are others which should be considered. For instance, energy recovery from cheese whey by means of a bioelectrochemical system consisting of a two-stage process [74] is a notable example. Furthermore, [PERSON] et al. [75] produced, in 2021, biofertilizers from whey in sustainable agriculture, reporting environmental and productive benefits. Additionally, [PERSON] et al. [76] reported studies of biomethane production in a two-stage anaerobic digester.