Hazelnut is 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 such as oil, chocolate, confectionery, etc. 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, pharmaceuticals and cosmetics has stimulated interest among scientists, producers, and consumers. This review provides an overview of current scientific knowledge about the main and most valuable hazelnut by-products and their actual valorisation, with a focus on their chemical composition to inspire new applications of these valuable resources and fully exploit their potential.

In this study, production and characterization of probiotic beverages using water kefir grains with apple, orange, dragon and kiwifruit juices were aimed. The total phenolic and antioxidant capacity, phenolic profiles and physicochemical properties of the beverages were investigated to determine the impact of fermentation. The results showed that fermentation period significantly affects the lactic acid bacteria and yeast counts, organic acid content and phenolic profile of the produced beverages. The yeast, mesophilic lactic cocci and Lactobacillus count in beverages were changed within the range 5.59–8.57 log CFU/mL, 4.20–7.49 log CFU/mL and 4.36–7.37 log CFU/mL, respectively. The highest Lactobacillus spp. and mesophilic lactic cocci counts were determined in dragon juice (7.78 log CFU/mL) and kiwifruit juice (7.23 log CFU/mL) (48 h), respectively. Up to 21 phenolic compounds were identified in the juice samples. Chlorogenic acid and rutin were the major phenolic compounds detected in all fruit juices samples. Lactic acid level in all beverages increased during fermentation up to 9.03 g/L in fermented dragon juice. Furthermore, the ethyl alcohol concentration in beverages is within the regulatory limitations for non-alcoholic beverages. According to the finding, dragon, apple, orange and kiwifruit juices can be fermented with water kefir grains to produce non-dairy, functional, and probiotic beverages. Graphical Abstract

Consumers' interest in functional foods has significantly increased in the past few years. Hazelnut meal, the main valuable byproduct of the hazelnut oil industry, is a rich source of proteins and bioactive peptides and thus has great potential to become a valuable functional ingredient. In this study, hazelnut protein hydrolysates obtained by a single or combined hydrolysis by Alcalase and Neutrase were mainly characterized for their physicochemical properties (SDS-PAGE, particle size distribution, Fourier-transform infrared (FTIR) spectroscopy, molecular weight distribution, etc.) and potential antiobesity effect (Free fatty acid (FFA) release inhibition), antioxidant activity (DPPH and ABTS methods), and emulsifying properties. The impact of a microfluidization pretreatment was also investigated. The combination of Alcalase with Neutrase permitted the highest degree of hydrolysis (DH; 15.57 ± 0.0%) 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 DH and the inhibition of FFA release by pancreatic lipase (PL), 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 ± 1.43 (NH) and 88.04 ± 2.22 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 ± 1.06% DPPH and 98.31 ± 0.46% 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 this study, we investigated the formation of protein–phenolic complexes from dephenolized hazelnut meal protein isolates (dHPI) and hazelnut skin phenolic extracts (HSE) and their effects on the bioaccessibility of both hazelnut proteins and phenolics. The dHPI–HSE complexes were of considerable size and were dependent on HSE concentration due to 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 concentration; 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 catechin and phenolic concentration altered the regular structure. Skin extracts did not alter the digestibility of dephenolized proteins, but dephenolization reduced the degree of hydrolysis by pancreatin. The formation of the protein–phenolic complex had a beneficial effect on the bioaccessibility of hazelnut skin phenols, predominantly those on the galloylated form of the catechins, such as gallocatechin gallate and epigallocatechin gallate. Thus, the bioaccessibility and antioxidant activity analysis results showed that protein–phenolic complexes obtained from hazelnut meal and skin may promote the transition of phenolic compounds from the gastrointestinal tract without degradation.