Gas-phase volatilomic approaches for quality control of brewing hops based on simultaneous GC-MS-IMS and machine learning

Rebecca Brendel,Sascha Rohn,Sebastian Schwolow,Philipp Weller

doi:10.1007/s00216-020-02842-y

Rebecca Brendel, Sascha Rohn + Show 2 more

Open Access

https://doi.org/10.1007/s00216-020-02842-y

Copy DOI

Abstract

For the first time, a prototype HS-GC-MS-IMS dual-detection system is presented for the analysis of volatile organic compounds (VOCs) in fields of quality control of brewing hop. With a soft ionization and drift time-based ion separation in IMS and a hard ionization and m/z-based separation in MS, substance identification in the case of co-elution was improved, substantially. Machine learning tools were used for a non-targeted screening of the complex VOC profiles of 65 different hop samples for similarity search by principal component analysis (PCA) followed by hierarchical cluster analysis (HCA). Partial least square regression (PLSR) was applied to investigate the observed correlation between the volatile profile and the α-acid content of hops and resulted in a standard error of prediction of only 1.04% α-acid. This promising volatilomic approach shows clearly the potential of HS-GC-MS-IMS in combination with machine learning for the enhancement of future quality assurance of hops.Graphical abstract

Highlights

Hop (Humulus lupulus L.) has been used in the brewing process, due to its contents of secondary metabolites that give beer its typical flavor
A number of 65 different pelletized samples of hop cultivars from harvesting years 2015–2018 (ESM Table S1) were analyzed by a prototype HS-GC-MS-ion mobility spectrometry (IMS) setup
For a better direct comparison, the MS data are given in form of a 3D heatmap, while IMS data are given as retention time × drift time × intensity

Summary

Introduction

Hop (Humulus lupulus L.) has been used in the brewing process, due to its contents of secondary metabolites that give beer its typical flavor. Acids, the EBC 7.7 method, using high-performance liquid chromatography (HPLC) followed by diode array detection or mass spectrometry, is usually utilized [1] Both methods require liquid extraction of the soft resin in hops with methanol and diethyl ether prior to analysis. As HSI, the ratio of the absorption at 275 nm, which is characteristic for the oxidation products of bitter acids, and at 325 nm, which is characteristic for the fresh bitter acids, is calculated [1, 12] This method does not deliver detailed information about the hop quality itself. An indicator for flavor compound aging is the epoxide fraction, and epoxides of caryophyllene and humulene are analyzed in relation to the non-epoxidized forms This procedure again is time-consuming due to the required extraction procedure and GC analysis

Objectives

Methods

Results

Conclusion