A Study of Structural Change During In Vitro Digestion of Heated Soy Protein Isolates

The structure-function relationships and techno-functions of β-conglycinin

β-conglycinin (β-CG) is a prominent storage protein belonging to the globulin family in soybean (Glycine max) seeds. Along with other soybean proteins, it serves as an important source of essential amino acids and high-quality nutrition. However, the digestibility and nutritional value of β-CG are key factors affecting the nutritional profile of soy-based foods. The heterotrimeric, secondary, and quaternary structures of β-CG, particularly the spatial arrangement of its α, α', and β subunits, influence its functional properties. Considering these aspects, β-CG emerges as a significant protein with diverse applications in the food and health sectors. Therefore, this review explores β-CG's composition, structure, function, health implications, and industrial uses. Salient discussions are presented on its molecular structure, nutrition, digestibility, allergenicity, and techno-functions including emulsification, solubility, gelling, and structure-function complexities. Overall, the multifaceted potential of β-CG in the healthcare sector and the food industry is evident.

High-intensity ultrasound-assisted alkaline extraction of soy protein: Optimization, modeling, physicochemical and functional properties

This study examined the effect of high-intensity ultrasound-assisted alkaline extraction (HUAE) on the extraction yield and the physicochemical and functional properties of soy protein (SP) using the two-pot multivariate method for the first time. Plackett-Burman Design (PBD) coupled with Response Surface Methodology (RSM) was systematically utilized to select and subsequently optimize the HUAE parameters. Based on PBD results, the significant extraction factors were liquid to solid ratio (LSR), temperature, ultrasonic amplitude, and extraction time. The optimum conditions for the maximal extraction yield and minimal energy consumption were 50:1 mL/g LSR, 50 °C, 48 % ultrasonic amplitude, and 10 min extraction time. At optimum conditions, the extraction yield (35.28 %) was significantly improved compared to traditional extraction (26.39 %). Besides, HUAE resulted in modification of the protein secondary and tertiary structures due to the unfolding of protein molecules and the exposure of hydrophobic groups or regions as shown by FTIR spectroscopy, free sulfhydryl analysis, and scanning electron microscopy. These structural changes led to decreased solubility and emulsifying activity but improved emulsion stabilization and antioxidant properties. With future development, HUAE could potentially produce soy protein for targeted applications, broadening its utilization and meeting the need for more sustainable alternative processing.

Plant-based flaxseed oil microcapsules fabricated from coacervation of gluten at oil droplet surface: Microstructure, oxidation stability, and oil digestion control

This study aimed to develop microcapsules with wheat gluten-coated oil droplets to enhance the oxidation stability and control the digestibility of flaxseed oil. The microcapsules were fabricated using a three-step procedure: (i) flaxseed oil was homogenized with an alkaline gluten solution to form oil-in-water emulsions containing small gluten-coated oil droplets (320-400 nm); (ii) the pH of these emulsions was then neutralized to facilitate the deposition of gluten around the oil droplets, thereby forming a thick layer; (iii) a flaxseed oil microcapsule powder was then prepared by spray drying. During the microcapsule formation, intermolecular interactions, including hydrophobic interactions and hydrogen bonds, were involved in the coacervation of gluten at the emulsion surface. The resultant microcapsules with a multiple-core structure had external diameters of 4-26 µm and encapsulation efficiencies of 90%-94%. The microencapsulated oil powders contained a relatively high flaxseed oil content (60%-80%). Among them, the sample with 60% oil content demonstrated the best stability in resisting oil droplet coalescence; thus, it exhibited a higher lipolysis rate and extent during simulated gastrointestinal digestion. A 30-day accelerated storage study showed that encapsulation of the flaxseed oil improved its resistance to oxidation. These findings suggest that the pH-deposition method can successfully produce microencapsulated polyunsaturated lipids using all plant-derived ingredients, which may facilitate their use in new plant-based foods through a green and sustainable approach.

Assessment of transglutaminase catalyzed cross-linking on the potential allergenicity and conformation of heterologous protein polymers

Transglutaminase (TGase)-mediated cross-linking has gained significant attention due to its potential to reduce the allergenicity of food proteins. This study investigates the effects of TGase cross-linking on allergenicity and conformational modifications in a dual-protein system comprising soy protein isolate (SPI) and β-lactoglobulin (β-LG). The results showed that TGase cross-linking effectively decreased the allergenic potential of both SPI and β-LG, with a more pronounced reduction observed in the allergenicity of soy protein in the dual-protein system. SDS-PAGE analysis revealed that the 7S and 11S subunits of soy protein were more easily cross-linked than β-LG. Secondary structure analysis indicated that TGase treatment disrupted β-sheet structures, increased the content of random coils, and enhanced protein flexibility. Ultraviolet absorption and intrinsic fluorescence analyses confirmed these structural alterations, with TGase treatment exposing additional aromatic amino acids. A reduction in free sulfhydryl groups and altered intermolecular forces further corroborated the occurrence of cross-linking. These findings suggest that TGase-mediated cross-linking effectively reduced the allergenicity of SPI and β-LG by modifying their conformations, offering potential strategies for the development of hypoallergenic dual-protein food products. PRACTICAL APPLICATION: This study has practical applications in the food industry to develop hypoallergenic food products, particularly those that combine soy and dairy proteins. By using TGase to cross-link these proteins, the allergenicity can be reduced, resulting in products that are safer for consumers with food allergies.

Effects of preheat treatment and syringic acid binding on the physicochemical, antioxidant, and antibacterial properties of black soybean protein isolate before and after in vitro digestion

This study investigated the effects of preheat treatment (70-100 °C) and syringic acid (SA) grafting on the antioxidant, antibacterial, and physicochemical properties of black soybean protein isolate (BSPI) before and after in vitro digestion. The results revealed that both preheat treatment and SA grafting increased the digestibility and the absolute zeta potential value of BSPI. However, as the preheating temperature increased, the antioxidant ability of BSPI decreased, which was improved by SA grafting. During in vitro digestion, the absolute zeta potential and antioxidant activities of preheated BSPI and preheated BSPI-SA complex followed the order: intestine > gastric > before digestion. Compared with before digestion, preheated BSPI with its SA complex after in vitro digestion exhibited excellent antibacterial activities. Importantly, the preheated BSPI-SA complex enhanced the SA recovery rate during digestion and SA stability, with the highest recovery rate observed for the SA-grafted BSPI with preheat treatment at 100°C (BSPI100-SA). The principal component analysis sufficiently distinguished preheated BSPI and preheated BSPI-SA complexes. There were partitions between BSPI and BSPI-SA treated at different preheating temperatures. This study contributes to expanding the potential applications of BSPI with its SA complex in food products and offers guidance for designing SA delivery systems. PRACTICAL APPLICATION: Preheated BSPI-SA complexes could serve as functional ingredients in food or health products. Besides, preheated BSPI has application potential as a carrier for SA delivery.

The Effect of Dihydromyricetin (DMY) on the Mechanism of Soy Protein Isolate/Inulin/Dihydromyricetin Interaction: Structural, Interfacial, and Functional Properties

The combination of proteins with polysaccharides and polyphenols is expected to improve their physicochemical and functional properties. In this study, a novel plant-based antioxidant emulsifier was formed by soybean protein isolate (SPI), inulin (INU), and dihydromyricetin (DMY). Based on the binary system of SPI/INU, we focused on exploring the effect of the DMY concentration (0.5 mg/mL~2.5 mg/mL) on the formation and properties of the ternary complex. The structure, interaction mechanism, and interfacial and functional properties of the ternary complex were investigated. The results indicate that compared to the SPI/INU binary complex, the SPI/INU/DMY ternary complex had a significant decrease in particle size (~100 nm) and a slight decrease in absolute zeta potential. The SPI/INU binary complex with DMY mainly interacted by hydrogen bonding and hydrophobic interactions. Due to the incorporation of DMY, the structure of SI was denser and more flexible. The ternary complex exhibited an ideal three-phase contact angle and demonstrated better foaming and antioxidant ability. Additionally, compared to SPI/INU, the ternary complex had a significant improvement in EAI. These results provide a strategy for polyphenols to modify the structure, interfacial properties, and functions of protein/polysaccharide complexes. This provides a potential reference for the preparation of more ternary complexes with excellent emulsifying and antioxidant properties for application in emulsions.

Effect of preheating-induced denaturation of proteins and oleosomes on the structure of protein and soymilk properties

In this paper, effects of preheating-induced denaturation of proteins and oleosomes on protein structure and soymilk quality were studied. The protein in soybeans baked at 55 °C (B-55) and 85 °C (B-85) showed an increase of β-sheet content by 3.2 % and a decrease of α-helix content by 3.3 %, indicating that proteins were gradually unfolded while oleosomes remained intact. The protein resisted thermal denaturation during secondary heating, and soymilks were stable as reflected by a small d3,2 (0.4 μm). However, raw soymilk from soybeans baked at 115 °C (B-115), steamed for 1 min (ST-1) and 5 min (ST-5) presented oleosomes destruction and lipids aggregates. The proteins were coated around the oil aggregates. The β-turn content from soybeans steamed for 10 min (ST-10) increased by 9.5 %, with a dense network where the OBs were tightly wrapped, indicating the serious protein denaturation. As a result, the soymilks B-115 or steamed ones were unstable as evidenced by the serious protein aggregation and larger d3,2 (5.65-12.48 μm). Furthermore, the soymilks were graininess and the protein digestion was delayed due to the formation of insoluble protein aggregates. The flavor and early-stage lipid digestion of soymilk from steamed soybeans was improved owing to lipid release.

In vitro digestibility and solubility of phosphorus of three plant-based meat analogues

Interest in plant-based meat analogues has increased and can be expected to be applied to pet foods, which necessitates the understanding of the nutrient supply in those foods. Our primary aim was to advance our understanding of the digestive properties of sterilized plant-based meat analogues. The impact of the preparatory processing steps on the solubility of meat analogues was studied. Meat analogues were made by mixing water, salt, and wheat gluten with soy protein isolate, pea protein isolate, or faba bean concentrate. Mixed materials were processed into model meat analogues using shear cell technology. Products were canned in water or gravy and sterilized. An animal-based canned pet food was made as a reference. Products sampled at the processing steps (mixing, shearing, sterilization) were digested in vitro. Samples of digestate were taken at the gastric phase (0 and 120 min) and small intestinal phase (120, 200, 280, and 360 min) for analysis of protein hydrolysis. The extent digestion of nitrogen and dry matter was determined at the end of incubation. Total phosphorus, soluble phosphorus after acid treatment, and after acid and enzymatic treatment were determined. The degree of hydrolysis after gastric digestion was low but increased immediately in the small intestinal phase; products based on pea had the highest values (56%). Nitrogen digestibility was above 90% for all materials at each processing step, indicating that bioactive compounds were absent or inactivated in the protein isolates and concentrate. Phytate seemed to play a minor role in meat analogues, but phosphorus solubility was influenced by processing. Shearing decreased soluble phosphorus, but this effect was partly reversed by sterilization. Nutrient digestibility as well as phosphorus solubility in plant-based products was higher than or comparable with the reference pet food. These findings show that the digestive properties of the tested plant-based meat analogues do not limit the supply of amino acids and phosphorus.

Assessment of In Vitro Digestive Behavior of Lactic-Acid-Bacteria Fermented Soy Proteins: A Study Comparing Colloidal Solutions and Curds

This study investigated the effect of lactic-acid-bacteria fermentation on the microstructure and gastrointestinal digestibility of soy proteins using a digestomics approach. Fermented soy protein isolates (FSPIs) under varied fermentation-terminal pH demonstrated a colloidal solution (FSPI-7.0/6.0) or yogurt-like curd (FSPI-5.0/4.0) state. Cryo-electron microscopy figures demonstrated the loosely stacked layer of FSPI-7.0/6.0 samples, whereas a denser gel network was observed for FSPI-5.0/4.0 samples. Molecular interactions shifted from dominant ionic bonds to hydrophobic forces and disulfide bonds. The gastric/intestinal digestion demonstrated that the curd samples afforded a significantly low particle size and high-soluble protein and peptide contents in the medium and late digestive phases. A peptidomics study showed that the FSPI-6.0 digestate at early intestinal digestion had a high peptidome abundance, whereas FSPI curd digestates (FSPI-5.0/4.0) elicited a postponed but more extensive promotion during medium and late digestion. Glycinin G2/G4 and β-conglycinin α/α' subunits were the major subunits promoted by FSPI-curds. The spatial structures of glycinin G2 and β-conglycinin α subunits demonstrated variations located in seven regions. Glycinin G2 region 6 (A349-K356) and β-conglycinin α subunit region 7 (E556-E575), which were located at the interior of the 3D structure, were the key regions contributing to discrepancies at the late stage.

Understanding the Performance of Plant Protein Concentrates as Partial Meat Substitutes in Hybrid Meat Emulsions

Hybrid meat products are an excellent strategy to incorporate plant proteins into traditional meat formulations considering recent market trends focusing on the partial reduction in red meat content. In this work, we evaluated the effects of different concentrated plant proteins (soy, pea, fava bean, rice, and sunflower) in partially replacing meat in meat emulsion model systems. Soy, pea, and sunflower proteins showed great compatibility with the meat matrix, giving excellent emulsion stability and a cohesive protein network with good fat distribution. Otherwise, adding rice and fava bean proteins resulted in poor emulsion stability. Color parameters were affected by the intrinsic color of plant proteins and due to the reduction in myoglobin content. Both viscoelastic moduli, G′ and G″ decreased with the incorporation of plant proteins, especially for rice and fava bean. The temperature sweep showed that myosin denaturation was the dominant effect on the G′ increase. The water mobility was affected by plant proteins and the proportion between immobilized and intermyofibrillar water was quite different among treatments, especially those with fava bean and rice proteins. In vitro protein digestibility was lower for hybrid meat emulsion elaborated with rice protein. It is concluded that soy, pea, and mainly sunflower proteins have suitable compatibility with the meat matrix in emulsified products.

Heat treatment of quinoa (Chenopodium quinoa Willd.) albumin: Effect on structural, functional, and in vitro digestion properties

Quinoa seeds are rich in protein, polyphenols, phytosterols, and flavonoid substances, and excellent amino acid balance that has been revisited recently as a new food material showing potential applied in fitness and disease prevention. Heat treatment is one of the most effective strategies for improving the physiochemical characteristics of a protein. However, research examining the effects of temperature on quinoa albumin (QA) properties is limited. In this study, QA was subjected to thermal treatment (50, 60, 70, 80, 90, 100, and 121°C). SDS−PAGE revealed that QA is composed of several polypeptides in the 10−40 kDa range. Amino acid (AA) analysis showed that the branched-chain amino acids (BCAAs), negatively charged amino acid residues (NCAAs), and positively charged amino acids (PCAAs) contents of QA were more than double that of the FAO/WHO reference standard. Additionally, heating induced structural changes, including sulfhydryl-disulfide interchange and the exposure of hydrophobic groups. Scanning electron microscopy demonstrated that the albumin underwent denaturation, dissociation, and aggregation during heating. Moreover, moderate heat treatment (60, 70, and 80°C) remarkably improved the functional properties of QA, enhancing its solubility, water (oil) holding capacity, and emulsification and foaming characteristics. However, heating also reduced the in vitro digestibility of QA. Together, these results indicate that heat treatment can improve the structural and functional properties of QA. This information has important implications for optimizing quinoa protein production, and various products related to quinoa protein could be developed. which provides the gist of commercial applications of quinoa seeds for spreading out in the marketplace.

Effect of gastrointestinal alterations mimicking elderly conditions on in vitro digestion of meat and soy proteins

Among the physiological functions declining with aging, decreased secretion of gastric fluid, achlorhydria, is commonly observed. We evaluated the digestion of meat (chicken, beef, and pork) and soy proteins using in vitro conditions mimicking gastrointestinal (GI) digestion in adults (control, C) and elderly individuals with achlorhydria (EA). Changes in degrees of hydrolysis (DH), SDS-PAGE profiles, peptide concentrations, and proteomic profiles during digestion were investigated. Digestion under the EA conditions markedly decreased DH, especially for soy proteins. SDS-PAGE profiling and proteomics showed that myofibrillar/sarcoplasmic proteins from meat and glycinin/beta-conglycinin from soy were most affected by digestion conditions. Our results indicated that differences in the digestibility of meat protein between EA and control conditions gradually narrowed from the gastric to the intestinal phase for meat protein, while a pronounced difference persisted in the intestinal phase for soy protein. Our work provides new insight of value for future dietary recommendations for elderly individuals.

Effects of heat treatment on protein molecular structure and in vitro digestion in whole soybeans with different moisture content

The effects of heat treatment on protein structure and in vitro digestibility in whole soybeans with different moisture content (10.68%, 29.70%, 46.29%, and 62.05% wet basis) were investigated. Scanning electronic microscopy presented that thermal treatment destroyed the subcellular structure of soybean seeds and resulted in formation of protein aggregates. When β-conglycinin (7S) was heat-denatured, the protein aggregates were maintained mainly by hydrogen bonds and hydrophobic interactions (non-covalent) for each moisture content. Also, the decrease of the protein solubility and increase of in vitro digestibility were observed. However, when glycinin (11S) was denatured in soybeans with 10.68% and 29.70% moisture content, the insoluble and indigestible protein aggregates with protein oxidation-induced crosslinking and high content of β-sheet were presented; in contrast, for 46.29% and 62.05% moisture content, mild protein oxidation, low content of β-sheet, non-covalent interactions and increased protein digestibility were shown. Non-covalent interactions were shown a positive correlation with gastrointestinal digestibility (r = 0.59, p < 0.05). Meanwhile, protein oxidation or β-sheet content was significantly negatively correlated with in vitro protein digestibility (r = -0.69 and -0.61, respectively, p < 0.05). Protein structure rather than solubility contributed to difference of in vitro digestibility. The optimum thermal conditions to obtain high-quality digestible protein in whole soybeans are 160 °C for 10.68%, 145 °C for 29.70%, 160 °C for 46.29% and 115 °C/140 °C for 62.05% moisture content.

In vitro gastro-small intestinal digestion of conventional and mildly processed pea protein ingredients

We report on the effect of processing, particularly heating, on the digestion dynamics of pea proteins using the standardised semi-dynamic in vitro digestion method. Fractions with native proteins were obtained by mild aqueous fractionation of pea flour. A commercial pea protein isolate was chosen as a benchmark. Heating dispersions of pea flour and mild protein fractions reduced the trypsin inhibitory activity to levels similar to that of the protein isolate. Protein-rich and non-soluble protein fractions were up to 18% better hydrolysed after being thermally denatured, particularly for proteins emptied later in the gastric phase. The degree of hydrolysis throughout the digestion was similar for these heated fractions and the conventional isolate. Further heating of the protein isolate reduced its digestibility as much as 9%. Protein solubility enhances the digestibility of native proteins, while heating aggregates the proteins, which ultimately reduces the achieved extent of hydrolysis from gastro-small intestinal enzymes.

Effect of Fractionation and Processing Conditions on the Digestibility of Plant Proteins as Food Ingredients

Plant protein concentrates and isolates are used to produce alternatives to meat, dairy and eggs. Fractionation of ingredients and subsequent processing into food products modify the techno-functional and nutritional properties of proteins. The differences in composition and structure of plant proteins, in addition to the wide range of processing steps and conditions, can have ambivalent effects on protein digestibility. The objective of this review is to assess the current knowledge on the effect of processing of plant protein-rich ingredients on their digestibility. We obtained data on various fractionation conditions and processing after fractionation, including enzymatic hydrolysis, alkaline treatment, heating, high pressure, fermentation, complexation, extrusion, gelation, as well as oxidation and interactions with starch or fibre. We provide an overview of the effect of some processing steps for protein-rich ingredients from different crops, such as soybean, yellow pea, and lentil, among others. Some studies explored the effect of processing on the presence of antinutritional factors. A certain degree, and type, of processing can improve protein digestibility, while more extensive processing can be detrimental. We argue that processing, protein bioavailability and the digestibility of plant-based foods must be addressed in combination to truly improve the sustainability of the current food system.

Ultrasonic Assisted Extraction of Quinoa (Chenopodium quinoa Willd.) Protein and Effect of Heat Treatment on Its In Vitro Digestion Characteristics

To extract and utilise the protein in quinoa efficiently, we investigated the effect of rate of quinoa protein isolate (QPI) extraction by ultrasound-assisted alkaline extraction and traditional alkaline extraction methods using single-factor experiments and Box-Behnken design. The effect of different heat treatment temperature and time on QPI functional properties and in vitro digestion characteristics were also investigated. The results showed that the optimal conditions of ultrasound- assisted alkaline extraction process were: ultrasonic time 99 min, solid-liquid ratio 1:20 w:v, ultrasonic temperature 47 °C, and pH 10, and its extraction rate and purity were 74.67 ± 1.08% and 87.17 ± 0.58%, respectively. It was 10.18% and 5.49% higher than that of the alkali-soluble acid precipitation method, respectively. The isoelectric point (pI) of QPI obtained by this method was 4.5. The flexibility and turbidity of QPI had maximum values at 90 °C, 30 min, and 121 °C, 30 min, which were 0.42 and 0.94, respectively. In addition, heat treatment changed the 1.77–2.79 ppm protein characteristic region in QPI’s nuclear magnetic resonance hydrogen spectroscopy (¹H NMR). After heating at 90 °C and 121 °C for 30 min, the hydrolysis degree and total amino acid content at the end of digestion (121 °C, 30 min) were significantly lower than those of untreated QPI by 20.64% and 27.85%. Our study provides basic data for the efficient extraction and utilisation of QPI.

Gastric lipase can significantly increase lipolysis and carotenoid bioaccessibility from plant food matrices in the harmonized INFOGEST static in vitro digestion model

Gastrointestinal digestion of carotenoids has received much attention, as these lipophilic compounds have been related to several health benefits. Most commonly, static digestion models such as the consensus INFOGEST model are employed to study their bioaccessibility from test matrices. However, an aspect that has been much neglected is the use of gastric lipase. Its inclusion to gastro-intestinal (GI) digestion is expected to foster emulsification of lipophilic constituents prior to their incorporation into mixed micelles. In this study, we compared the effect of various lipases from R. niveus, R. oryzae, and rabbit gastric extracts (RGE), at different concentrations (0, 30, and 60 U mL^-1), on carotenoid bioaccessibility from several food matrices (tomato juice, spinach, and carrot juice). We also investigated whether co-digestion of pure proteins (whey and soy protein isolates) at 0, 25, and 50% of the equivalent recommended dietary allowance, would interact with carotenoid bioaccessibility in presence or absence of RGE. Lipolysis was also studied. Considering all matrices combined, lipases significantly improved the bioaccessibility of carotenoids (p < 0.001). Compared to other lipases, RGE consistently increased carotenoid bioaccessibility in all tested matrices, by up to 182% (p < 0.001), this effect was partly maintained in the presence of co-digested proteins. Unexpectedly, all 3 lipases improved gastric lipolysis in all matrices, by an average of 10-fold (p < 0.001). In conclusion, only RGE contributed significantly to improving both lipolysis extent and carotenoid bioaccessibility in all tested matrices, while the presence of proteins mitigated the positive effect of lipases on carotenoid bioaccessibility.

Effects of high-temperature pressure cooking on cold-grind and blanched soymilk: Physico-chemical properties, in vitro digestibility and sensory quality

The present work investigated effects of high-temperature pressure cooking (HP) on physico-chemical properties and in vitro protein digestibility of cold-grind soymilk (CS) and blanched soymilk (BS). Confocal laser scanning microscopy presented that proteins and lipids in BS were obviously coalesced compared with CS, while HP treatment contributed to homogeneous dispersion of protein and lipid. Particle size of the BS and BSHP were larger than that of CS and CSHP. Tertiary structure of protein suggested that hydrophobic interactions and disulfide bonds in CS and CSHP were main force among protein molecules. Meanwhile hydrophobic interactions were account for 70% of the total bonds in BS and BSHP. The maximum fluorescence excitation wavelength (λ_max) of BSHP (349.0 nm) was shown a red shift compared with CS (346.5 nm). For secondary structure of protein, β-sheet was 30.61% in CS, while CSHP, BS and BSHP decreased by 2.65%, 5.73% and 7.77%, respectively. Thus, the protein in BS and BSHP were evidently aggregated, loose and disorder; while the CS and CSHP protein were dense and orderly. Also, HP treatment led to loose and unordered protein structure. Moreover, the in vitro digestibility was BSHP > BS > CSHP > CS. The difference in digestibility of four soymilk was determined by the protein structure. Sensory evaluation showed that blanching treatment effectively reduced beany flavor and HP decreased the diameter of oil bodies of BS, which led to smooth mouth feel and thus the highest global impression of BSHP.

Influence of soy and whey protein, gelatin and sodium caseinate on carotenoid bioaccessibility

Proteins could alter carotenoid bioaccessibility through altering their fate during digestion, due to emulsifying properties of resulting peptides, or influencing access of digestion enzymes to lipid droplets. In this investigation, we studied whether whey protein isolate (WPI), soy protein isolate (SPI), sodium caseinate (SC) and gelatin (GEL), added at various concentrations (expressed as percentage of recommended dietary allowance (RDA): 0, 10, 25 and 50%) would influence the bioaccessibility of lycopene, β-carotene or lutein, added as pure carotenoids solubilized in oil, during simulated gastro-intestinal (GI) digestion. Protein and lipid digestion as well as selected physico-chemical parameters including surface tension, ζ-potential and micelle size were evaluated. Adding proteins influenced positively the bioaccessibility of β-carotene, by up to 189% (p < 0.001), but it resulted in generally decreased bioaccessibility of lutein, by up to 50% (p < 0.001), while for lycopene, the presence of proteins did not influence its bioaccessibility, except for a slight increase with WPI, by up to 135% (p < 0.001). However, the effect depended significantly on the type of protein (p < 0.001) and its concentration (p < 0.001). While β-carotene bioaccessibility was greatly enhanced in the presence of SC, compared to WPI and GEL, the presence of SPI strongly decreased carotenoid bioaccessibility. Neglecting individual carotenoids, higher protein concentration correlated positively with carotenoid bioaccessibility (R = 0.57, p < 0.01), smaller micelle size (R = -0.83, p < 0.01), decreased repulsive forces (ζ-potential, R = -0.72, p < 0.01), and higher surface tension (R = 0.44, p < 0.01). In conclusion, proteins differentially affected carotenoid bioaccessibility during digestion depending on carotenoid and protein species, with both positive and negative interactions occurring.