Profiling Volatile Compounds in Blackcurrant Fruit using Headspace Solid-Phase Microextraction Coupled to Gas Chromatography-Mass Spectrometry

Summary

A headspace solid-phase microextraction-gas-chromatography platform is described here for fast, reliable, and semi-automated volatile identification and quantification in ripe blackcurrant fruits. This technique can be used to increase knowledge about fruit aroma and to select cultivars with enhanced flavor for the purpose of breeding.

Abstract

There is an increasing interest in measuring volatile organic compounds (VOCs) emitted by ripe fruits for the purpose of breeding varieties or cultivars with enhanced organoleptic characteristics and thus, to increase consumer acceptance. High-throughput metabolomic platforms have been recently developed to quantify a wide range of metabolites in different plant tissues, including key compounds responsible for fruit taste and aroma quality (volatilomics). A method using headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) is described here for the identification and quantification of VOCs emitted by ripe blackcurrant fruits, a berry highly appreciated for its flavor and health benefits.

Ripe fruits of blackcurrant plants (Ribes nigrum) were harvested and directly frozen in liquid nitrogen. After tissue homogenization to produce a fine powder, samples were thawed and immediately mixed with sodium chloride solution. Following centrifugation, the supernatant was transferred into a headspace glass vial containing sodium chloride. VOCs were then extracted using a solid-phase microextraction (SPME) fiber and a gas chromatograph coupled to an ion trap mass spectrometer. Volatile quantification was performed on the resulting ion chromatograms by integrating peak area, using a specific m/z ion for each VOC. Correct VOC annotation was confirmed by comparing retention times and mass spectra of pure commercial standards run under the same conditions as the samples. More than 60 VOCs were identified in ripe blackcurrant fruits grown in contrasting European locations. Among the identified VOCs, key aroma compounds, such as terpenoids and C6 volatiles, can be used as biomarkers for blackcurrant fruit quality. In addition, advantages and disadvantages of the method are discussed, including prospective improvements. Furthermore, the use of controls for batch correction and minimization of drift intensity have been emphasized.

Introduction

Flavor is an essential quality trait for any fruit, impacting consumer acceptance and thus significantly affecting marketability. Flavor perception involves a combination of the taste and olfactory systems and depends chemically on the presence and concentration of a wide range of compounds that accumulate in edible plant parts, or in case of VOCs, are emitted by the ripe fruit^1,2. While traditional breeding has focused on agronomic traits such as yield and pest resistance, fruit quality trait improvement, including flavor, has long been neglected due to the genetic complexity and the difficulty to properly ph....

Protocol

1. Fruit harvesting

1. Grow between 4 to 6 plants per genotype and/or treatment to ensure sufficient fruit material and variability.
2. If possible, harvest the samples on the same date; if there is not enough fruit material, pool together samples harvested on different dates.
   NOTE: It is recommended that the harvest time (morning, noon, afternoon) remains approximately identical as VOC profiles are affected by daytime/circadian rhythm^28,29,30,31.
3. Assess fruit ripening stage by visual observation

Results

High-throughput VOC profiling in a large set of fruit crops grown under different conditions or locations or belonging to distinct genotypes is necessary for accurate aroma phenotyping. Here, a fast and semi-automated HS-SPME/GC-MS platform for relative VOC quantification in blackcurrant cultivars is presented. VOC detection and identification were based on a library that was developed to profile berry fruit species (Table 1). A typical ripe blackcurrant fruit volatile profile (total ion chromatogram) ob.......

Discussion

Breeding for fruit aroma has long been hindered by the complex genetics and biochemistry underlying the synthesis of volatile compounds and the lack of technologies for proper phenotyping. However, recent advances in metabolomic platforms, combined with genomic tools, are finally allowing the identification of the metabolites responsible for consumer preferences and to breed crops with improved flavor³. While most progress has been achieved in the model fruit, tomato^9,.......

Materials

Name	Company	Catalog Number	Comments
10 mL screw top headspace vials	Thermo Scientific	10-HSV
18 mm screw cap Silicone/PTFE	Thermo Scientific	18-MSC
5 mL Tube with HDPE screw cap	VWR	216-0153
Centrifuge	Thermo Scientific	75002415
Methanol for HPLC	Merck	34860-1L-R
N-pentadecane (D32, 98%)	Cambridge Isotope Laboratories	DLM-1283-1
Sodium chloride	Merck	S9888
SPME fiber PDMS/DVB	Merck	57345-U
Stainless grinding jars for TissueLyser	Qiagen	69985
TissueLyser II	Qiagen	85300	Can be subsituted by mortar and pestle or cryogenic mill
Trace GC gas chromatograph-ITQ900 ion trap mass spectrometer	Thermo Scientific
Triplus RSH autosampler with automated SPME device	Thermo Scientific	1R77010-0450
Water for HPLC	Merck	270733-1L
Xcalibur 4.2 SP1	Thermo Scientific		software