Aroma and bitter extracts from South African hops : characterisation and extraction process development using supercritical CO2

Abstract

Hops are cone plants whose flowers are used for their bittering and anti-microbial properties in beer. Due to hops being sensitive to oxidation, they are processed into hop pellets, hop powders and hop extracts. The advantage with hops extracts are that they can provide a more consistent bittering. They provide better hop utilization, residue free, and save on storage and transportation costs. Organic solvent extraction is most commonly used; however, supercritical CO2 extraction (scCO2) is gaining more attention industrially, as it produces a solvent-free extract. Hops are typically grown 45-50° North and South of the equator, South Africa, which lies 34°S of the equator, has successfully adapted and grown hops for the last 50 years. This suggests the possibility that the profile of the South African hop might differ from that of the hops found worldwide. As the South African hop market expands, there is a potential to supply a wider variety of hop products to the world market. Although supercritical fluid extraction (SFE) is well known, there are no commercial plants in South Africa. There is thus a desire to explore the technical feasibility of producing enriched hop extract therefore This work aims to produce South African hop extract fractions using supercritical CO2, have their characteristics evaluated. To achieve this aim, the following objectives were set. The conditions for fractionation were estimated using theoretical calculations and literature considerations. The second objective was the experimental extraction and fractionation of hops at a pilot plant scale. These extracts were analysed to compare to their profiles with those known worldwide. The final objective was to compare the simulated products of extract and natural hopping. Four hop types were used in this study. The results showed that the highest yield obtained was 13.22% at a pressure of 200 bar at 40°C for an extraction time of 2 hours. The Broken-Intact-Cell (BIC) kinetic model showed a good fit with an absolute average relative deviation (AARD%) of 3.5%. The kinetic model revealed that it was possible to maintain the extraction process in the constant rate kinetic zone. This technique can thus ensure overall maximum extraction rates throughout the extraction period spanning three different extraction pressures. The analysis of the hop extract indicated that the four hop varieties had a similar hop acid profile but differed significantly in the aroma profile. In addition, it was also shown that the South African hop extract contained high amounts of the alpha-humulene, beta-caryophyllene and delta-cadinene, compared to most international varieties.

The three fractions (150, 200 and 250 bar) differed in aroma profiles, with the monoterpene hydrocarbon components generally being more dominant in the extract obtained at the lower pressures. The solubility in scCO2 alone could not fully predict the fate of each of the but it could be speculated that the kinetics of extraction of components from a solid matrices, together with solubility, played a significant role in the fate of each component. This study for the first time, provided the components data of South African hop extract in terms of hop aroma and hop acid profiles, and the application of scCO2 for the extraction and fractionation of the hop into various fractions. The results from this study provides useful reference data that can be used for the development of a commercial scCO2 plant for hop products.