Effects of Processing on the In Vitro Digestibility of Napin Proteins from Rapeseed (Brassica napus L.)

Research output: Book/ReportPh.D. thesis

  • Marcel Skejovic Joehnke
Rapeseed (Brassica napus L.) is the second-most abundantly cultivated oilseed crop in the world and rapeseed oil is widely used in human nutrition. Meanwhile, the residual protein-rich material known as the rapeseed press cake or meal is considered a byproduct of the rapeseed oil industry and is primarily used for animal feed. Rapeseed press cake constitutes a promising alternative protein source due to its well-balanced amino acid (AA) profile with a high content of sulfur-containing AAs. However, exploitation and application of rapeseed protein (RP) as a food ingredient has been limited by the potential presence of several co-occurring antinutritional compounds such as glucosinolates, phenolic compounds, phytic acid, dietary fibres (DFs), and protease inhibitors. The present project focused mainly on the processing and purification, characterization, and nutritional value (assessed by in vitro digestibility) of water-soluble napin protein fractions and products derived from cold-pressed rapeseed (B. napus L., cv. Express) press cake. Four different RP products (RP1, RP2, RP3 Sup, and RP3 Pellet) rich in napin proteins were prepared by large-scale aqueous acidic extraction and processing according to patent WO/2012/149941 (Andersen et al., 2012a). This method permitted a mild, selective, and sustainable isolation of proteins and other products (e.g. DFs) from rapeseed with a partial or complete removal of unwanted antinutritional compounds (e.g. phenolic compounds). The effects of processing on levels of antinutritional compounds (phenolic compounds, trypsin inhibitors, DFs) and their implications on the quality, purity, and digestibility of napin proteins were investigated using a broad range of biochemical analyses, including an in-house developed in vitro protein digestibility (IVPD) assay. Study 1 (cf. Paper I and Poster I) investigated the processing of rapeseed for preparation of the four RP products with monitoring of different process control parameters (PCPs). The PCPs allowed a rapid, simple, and cost-efficient assessment of the quality and purity of RP materials and products during processing. The PCPs were either related to composition such as dry matter (DM) content, total soluble solids (TSS) content in degrees Brix (°Bx), and protein content, or purity related especially to the napin proteins and phenolic compounds (exemplified by sinapine). The PCPs exhibited different advantages, limitations, and interrelationships affecting their applicability during processing. UV spectroscopy measurements for quantifying the molar ratio of phenolic compounds in sinapine equivalents to napin proteins (RM) enabled a fast and non-destructive estimation of napin protein purity during processing under aqueous acidic conditions (pH < 7). The PCPs investigated in this study constitute valuable tools for in-process monitoring and control of the quality and purity of RP materials, especially in regard to phenolic compounds, contributing to cost-efficient production of high value protein concentrates and isolates from rapeseed. Study 2 (cf. Poster II) dealt with biochemical characterization of the prepared RP products and intrinsic napin protein fractions, as well as their physicochemical properties. The four napin-rich protein products had protein contents of approximately 63% (RP1), 70% (RP2), 99% (RP3 Sup), and 76% (RP3 Pellet) on oil-free dry basis. The protein compositions, molecular weights (MWs), and specific RM purities were determined by size-exclusion chromatography (SEC), reversed-phase high-pressure liquid chromatography (RP-HPLC), and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The isoelectric points (pIs) of the products were examined using isoelectric focusing (IEF) and surface charge/zeta-potential (ZP). Study 3 (cf. Paper II and Abstract I) investigated the IVPD of napin-rich RP products and effects of a partial substitution of milk proteins with the RP products on IVPD. The IVPD of four protein sources consisting of RP1 and RP2 and major whey proteins (WPs) in bovine milk (α-LA, alpha-lactalbumin; β-LG, beta-lactoglobulin) was studied both individually and in protein mixtures (50:50 protein w/w). IVPD was quantified using an in-house developed method employing a transient hydrolysis by pepsin (1 h) followed by pancreatin (1 h). IVPD of the individual protein sources was found to be significantly higher for the WPs (25-30%) compared to the RP products (8-15%). Characteristics of RP products related to their differential processing and especially the trypsin inhibitor activity (TIA) levels had marked effect on IVPD. The RP1/RP2 mixture exhibited a novel IVPD response type that was shifted after treatment with dithiothreitol (DTT) reducing agent due to a significant reduction in the high TIA level of RP1 (90%). The IVPD of the RP2/β-LG protein mixture (23%) was elevated to a level similar to β-LG alone (25%). These results demonstrate that napin-rich protein products prepared by proper processing may be combined with specific milk WPs at a level of 50% w/w, leading to an improved overall protein digestibility. Study 4 (cf. Paper III) was a continuation of Paper II, where the IVPD of RP2 was investigated upon inclusion (20%, 33.3%, and 50% w/w) of total dietary fibre (TDF) fractions isolated from either rapeseed hulls (TDF-RH) or purified rapeseed kernel fibres (TDF-RF). Importantly, the TDF fractions and RP2 were derived from the same rapeseed (B. napus L., cv. Express) material. IVPD of protein within the mixed samples was affected by both the TDF level and source. TDF-RH addition resulted in a dose-dependent lowering of the IVPD of RP2 at all three levels tested (9-27%), whereas TDF-RF elicited reductions of a lower magnitude solely at the medium to highest inclusion levels (7-9%). This study show that the digestibility of napin-rich protein products is differentially influenced by the presence of specific DFs fractions from rapeseed.
Original languageEnglish
PublisherDepartment of Food Science, Faculty of Science, University of Copenhagen
Publication statusPublished - 2017

ID: 200824811