Investigating the valorisation potential of hazelnut by-products: Transforming waste into functional food ingredients

Abstract

Hazelnuts are one of the most widely consumed nuts around the world. Considering the nutritional value of hazelnuts, a wide range of hazelnut-based food products are available in the market. Nevertheless, the processing of hazelnuts generates a large number of by-products and waste. The most valuable by-products of the hazelnut industry are shell, skin, and meal. These by-products are rich in bioactive compounds, protein, dietary fibre, mono- and polyunsaturated fatty acids, vitamins, minerals, phytosterols, and squalene. The current utilisation of hazelnut by-products is mostly limited to animal feed supplementation of hazelnut meal and skin and use as a low-value heat source for the shells. However, disposing of these by-products or using them as a low-value heat source or animal feed supplementation results in significant waste of a natural resource rich in nutritional components. Consequently, valorising hazelnut by-products as bioactive ingredients in diverse fields such as food, pharmaceutics and cosmetics has stimulated interest among scientists, producers, and consumers. In light of the above, a research strategy to investigate the valorisation potential of hazelnut by-products has been developed. The objectives of this Ph.D. dissertation were (i) to valorise hazelnut meal and explore the potential anti-obesity and antioxidant activities of its protein hydrolysates; (ii) to investigate the effect of the hydrolysis strategy (single or sequential hydrolysis) using Alcalase and Neutrase, as well as the application of microfluidization pretreatment on these activities and the functional properties of the protein isolates and hydrolysates; (iii) to examine the formation of the protein-polyphenol complex from dephenolised hazelnut meal protein isolates (dHPI) and hazelnut skin phenolic extracts (HSE); (iv) to monitor the bioaccessibility of hazelnut proteins and polyphenols after protein-polyphenol complexation. Two different research studies (Chapters 3-4) were conducted in line with these purposes. Firstly, hydrolysates of hazelnut proteins obtained with Alcalase and Neutrase were mainly examined for their physicochemical properties, potential anti-obesity effects, antioxidant capacities, and emulsifying properties (Chapter 3). Later on, protein-polyphenol complexes formed from dephenolised hazelnut meal protein isolates (dHPI) and hazelnut skin phenolic extracts (HSE) were investigated as well as their effects on bioaccessibility (Chapter 4). The background and objectives of this Ph.D. dissertation are introduced in the first chapter. Following that, an overview of current scientific knowledge about the main and most valuable hazelnut by-products and their actual valorisation, focusing on their chemical composition to inspire new applications of these valuable resources and fully exploit their potential, has been reviewed in the second part. In the third part, hazelnut meal protein hydrolysates obtained by a single or combined hydrolysis by Alcalase and Neutrase were mainly characterised for their physicochemical properties (SDS‒PAGE, particle size distribution, Fourier transform infrared, molecular weight distribution, etc.) and potential anti-obesity effect (FFA release inhibition), antioxidant activity (DPPH and ABTS methods), and emulsifying properties. The impact of microfluidization pretreatment was also investigated. The combination of Alcalase with Neutrase permitted the highest degree of hydrolysis (DH) (15.57%) of hazelnut protein isolate, which resulted in hydrolysates with the highest amount of low molecular weight peptides, as indicated by size exclusion chromatography (SEC) and SDS‒PAGE. There was a positive correlation between the degree of hydrolysis and the inhibition of FFA release by pancreatic lipase, with a significant positive effect of microfluidization when followed by Alcalase hydrolysis. Microfluidization enhanced the emulsifying activity index (EAI) of protein isolates and hydrolysates. Low hydrolysis by Neutrase had the best effect on the EAI (84.32 (NH) and 88.04 m2/g (MFNH)), while a negative correlation between the emulsifying stability index (ESI) and the DH was observed. Again, the combined Alcalase-Neutrase hydrolysates displayed the highest radical scavenging activities (96.63% DPPH and 98.31% ABTS). FTIR results showed that the application of microfluidization caused the unfolding of the protein structure. The individual or combined application of the Alcalase and Neutrase enzymes caused a switch from the β-sheet organization of the proteins to α-helix structures. In conclusion, hazelnut meal may be a good source of bioactive and functional peptides. The control of its enzymatic hydrolysis, together with an appropriate pretreatment such as microfluidization, may be crucial to achieve the best suitable activity. In the fourth part, the formation of the protein-polyphenol complex from dephenolised hazelnut meal protein isolates (dHPI) and hazelnut skin phenolic extracts (HSE), as well as its effect on the bioaccessibility of both hazelnut proteins and polyphenols, were investigated. The dHPI+HSE complexes were of considerable size and dependent on HSE concentration due to the occurrence of aggregation. Although catechin was the main component of HSE, it did not cause aggregation, except for a slight rise in particle size. According to fluorescence quenching, the hazelnut protein-phenolic extract complex had a linear Stern-Volmer plot expressing static quenching between 0-0.5 mM concentrations, and the interaction was mainly dependent on hydrogen bonding and van der Waals forces (ΔH<0 and ΔS<0) and the reaction was spontaneous (ΔG<0). According to Fourier Transform Infrared (FTIR) Spectroscopy results, higher phenolic extract concentration caused an increase in irregular structures in hazelnut protein, while the lowest phenolic concentration and catechin altered the regular structure. Skin extracts did not alter the digestibility of dephenolised proteins, but dephenolisation reduced the degree of hydrolysis by pancreatin. The formation of the protein-polyphenol complex had a beneficial effect on the bioaccessibility of hazelnut skin polyphenols predominantly on the gallolated form of the catechins such as gallocatechin gallate and epigallocatechin gallate. The final part presents the general discussions and conclusions, as well as future perspectives on the valorisation of hazelnut by-products, based on the findings of the previous chapters. A by-product of the hazelnut oil industry, hazelnut meal, was valorised as a source of bioactive peptides with an emphasis on their potential anti-obesity and antioxidant properties. First, hazelnut meal protein isolates were treated with microfluidization to improve their hydrolysis and functional properties. Then, sequential or individual hydrolysis by Neutrase and Alcalase was performed to prepare the protein hydrolysates. Finally, by combining Alcalase and Neutrase hydrolysis, we achieved the highest degrees of hydrolysis (DH), inhibiting FFA release by pancreatic lipase and scavenging free radicals. Regarding anti-obesity and antioxidant properties, hazelnut protein hydrolysates have the potential as functional food ingredients. Therefore, it is important to understand the process for protein processing (pretreatment, degree of hydrolysis, etc.) based on its intended application. Protein-polyphenol interactions improved the bioavailability of hazelnut skin polyphenols, particularly the gallolated form of catechins like gallocatechin gallate (GCG) and epigallocatechin gallate (EGCG). In recent years, researchers have focused on polyphenolic compounds and plant-based proteins isolated from natural sources. It is essential in vegan formulations such as foam-like products (mousse) or emulsions (mayonnaise), where the functional properties of proteins are crucial. Additionally, there is no consensus on whether protein-phenolic interactions affect polyphenol bioavailability positively or negatively. Consequently, more research needs to be conducted on this subject and reported in the literature.