The increasing global population and growth in food production have led to the generation of significant amounts of byproducts and waste streams. Many of the waste products are highly nutritious and contain dietary fiber, protein, bioactive and functional compounds (Dey, Richter, Ek, Gu, & Ganjyal, 2021). Several studies have focused on the proper utilization of byproducts especially from fruits and vegetables sources due to their health promoting compounds (Kaisangsri et al., 2016; Schmid et al., 2022; Wang et al., 2017; Wang, Gu, & Ganjyal, 2019). Among these, apple byproduct is considered as sources which are advocated to have huge potential as a dietary food component (Rana, Gupta, Rana, & Bhushan, 2015). Apple is one of the most commonly consumed fruit crops, with globally production of 86 million tons in 2020 (FAOSTAT, 2021). Generally, 70 to 75% production of apples is merely utilized as a fresh fruit, whereas the remaining portion (25%) is used to make several products such as juice, wine, jams, and various other products. In juice production, 75% of apple fruit is extracted as juice, while the remaining amount is collected as apple pomace (Lyu et al., 2020). Approximately 95% of generated pomace consists of skin and pulp tissues, which are rich in skin bound phenolics (dihydrochalcones, phenolic acids and flavanols) and cell wall polysaccharides such as hemicellulose, cellulose lignin, gums and pectin (Bhushan, Kalia, Sharma, Singh, & Ahuja, 2008). Apple pomace is considered a high source of antioxidants due to the presence of phenolic components such as glycosides, quercetin, epicatechin, phloridzin, and chlorogenic acid. The phenolic rich apple pomace is also considered to have a strong anticarcinogenic property that can prevent colon cancer (Iqbal, Schulz, & Rizvi, 2021). Furthermore, fruit fibers have an advantage over the cereal fibers in terms of lower phytic acid content, better soluble: insoluble ratio, and the presence of bioactive molecules such as polyphenol, flavanols and antioxidant properties (Vergara-Valencia et al., 2007). Therefore, enriching food products with apple pomace could be an effective way both to enhance their nutritional value while reducing total waste by utilizing these valuable waste products (Iqbal et al., 2021).

Extrusion processing is a highly versatile method that can be utilized to produce a variety of enriched, directly expanded food products by incorporating fruit byproducts. Although, starch is commonly used in extruded products due to its ability to expand, but using starch as a major ingredient creates low nutritional value products (Ek, Kowalski, & Ganjyal, 2020), which are often criticized as highly processed and of lower quality. Recent approaches have been made to enhance the nutritional value of such extruded products by fortifying them with high fiber fruit byproduct (Kaisangsri et al., 2016; Mohamad Mazlan et al., 2019). During cooking extrusion processing, starch is gelatinized, protein is denatured, and extrudates are texturally restructured. The dough is exposed to a high temperature and shear during cooking extrusion and is coarsely puffed as it exits through the die, due to the rapid conversion of water into steam. However, the high temperature (130–180 °C) and low moisture (˂ 22%) in the cooking extruder will potentially destroy bioactive nutrients and heat-sensitive proteins (Alavi & Rizvi, 2009; Yoon, Singha, & Rizvi, 2021). To overcome the limitations of traditional cooking extrusion, a hybrid processing technique called supercritical fluid extrusion (SCFX) has been developed. SCFX uses supercritical CO2 (SC-CO2) at milder conditions with extrusion processing. SC-CO2 worked as both a blowing agent and plasticizer, and it decouples the role of water in extrusion (Rizvi & Mulvaney, 1992). In SCFX, the extruder barrel is equipped with several annular cooling jackets which allow the coolant to be circulated in five separate temperature-controlled zones. During SCFX process, the temperature of the dough normally lies between 60 and 85 °C for a short time (Paraman, Supriyadi, Wagner, & Rizvi, 2013), thereby, retaining the maximum heat sensitive ingredients.

Unlike starch, fiber does not undergo transition to a viscoelastic state during the extrusion processing, which hinders their ability to fully expand when included at high levels (Dey, Richter, et al., 2021; Robin, Schuchmann, & Palzer, 2012) . It is commonly assumed that fibers are inert during extrusion process and does rupture the starch or protein matrix, as well as the air cell structure of the extrudates (Ek, Gu, Saunders, Huber, & Ganjyal, 2021). Similarly, the changes in protein structure (secondary, tertiary, and quaternary) involves thermomechanical unfolding and cross linking (Guerrero, Kerry, & de la Caba, 2014). To successfully incorporate dietary fiber, it is necessary to understand the mechanism of intermolecular and intramolecular interactions among biopolymers during extrusion. Although a lot of research work has been completed to evaluate the behavior of different ingredients, a very few is known about changes in biopolymers interactions during extrusion. Fourier transform infrared (FTIR) spectroscopy has been used to study the interactions between pomace and other ingredients (Chen et al., 2020; Dey et al., 2021). To understand the inter- and intra-molecular interactions among different ingredients and their impact on the expansion characteristics, FTIR was also used to qualitatively analyze SCFX-expanded products in the current study.

The aim of this work was to explore the incorporation of a significant amount (30%) of apple pomace powder into starch and MPC81-based formulations and evaluate its impact on the physio-mechanical characteristics of the final product. Additionally, this study aimed to determine the effect of SCFX processing on the preservation of polyphenols, flavonoids, vitamin C, and antioxidant activity in the extrudates. This investigation also aimed to explore any possible alterations in the inter- and intra-molecular interactions among the various components of the formulation during the SCFX processing.