Interrelation of Volatile Organic Compounds and Sensory Properties of

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Biofuels and Biomass

Interrelation of Volatile Organic Compounds and Sensory Properties of Alternative and Torrefied Wood Pellets Barbara Poellinger-Zierler, Irene Sedlmayer, Carina Reinisch, Hermann Hofbauer, Christoph Schmidl, Larissa Patricia Kolb, Elisabeth Wopienka, Erich Leitner, and Barbara Siegmund Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00335 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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Energy & Fuels

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Interrelation of Volatile Organic Compounds and

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Sensory Properties of Alternative and Torrefied

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Wood Pellets

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Barbara Pöllinger-Zierler a,%, Irene Sedlmayer b,c, Carina Reinisch a,§, Hermann Hofbauer c,

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Christoph Schmidl b, Larissa Kolb a, Elisabeth Wopienka b, Erich Leitner a,

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Barbara Siegmund a *

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a

Graz University of Technology, Institute of Analytical Chemistry and Food Chemistry, Stremayrgasse 9/II, 8010 Graz, Austria

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b

BIOENERGY 2020+ GmbH, Gewerbepark Haag 3, 3250 Wieselburg-Land, Austria

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c

Technische Universität Wien, Institute of Chemical, Environmental and Bioscience Engineering, Getreidemarkt 9/166, 1060 Vienna, Austria

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* corresponding author: Tel.: +43 316 873 32506, mail to: [email protected]

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%

current address: Oberberg 120, 8151 Hitzendorf, Austria

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§

current address: Muellegg 4, 8524 Bad Gams, Austria

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KEYWORDS

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Pellets from alternative material; torrefied wood pellets, volatile compounds, gas

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chromatography-mass spectrometry; sensory evaluation; correlation

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Energy & Fuels

TOC GRAPHICS

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ABSTRACT

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The increasing demand for wood pellets on the market, which is caused by their excellent

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combustion properties, inspires the production as well as the utilization of alternative biomass

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pellets as fuel. However, the emission of volatile organic compounds gives pellet materials a

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distinct odor or off-odor, which is directly perceived by the end user. Thus, there is an urgent

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need for knowledge about the emitted volatile organic compounds and their potential

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formation pathways as well as their contributions to odor properties of the pellets. In this

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study, pellets made of biomass energy crop (i.e. straw, miscanthus), by-products from food

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industry (i.e. rapeseed, grapevine, DDGS ˗ dried distillers grains with solubles from beer

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production), eucalyptus as well as of torrefied pinewood and torrefied sprucewood were

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investigated with respect to the emitted volatile compounds and their possible impact on the

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pellet odor. Headspace solid-phase microextraction in combination with gas chromatography–

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mass spectrometry was used to enrich, separate and identify the compounds. Techniques used

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in sensory science were applied to obtain information about the odor properties of the

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samples. A total of 59 volatile compounds (acids, aldehydes and ketones, alcohols, terpenes,

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heterocyclic and phenolic compounds) was identified with different compound ratios in the

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investigated materials. The use of multivariate statistical data analysis provided deep insight

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into product-compound interrelation. For pellets produced from bioenergy crop as well as

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from by-products from food industry, the sensory properties of the pellets reflected the odor

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properties of the raw material. With respect to the volatiles from torrefied pellets, those

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volatiles that are formed during the torrefaction procedure, dominate the odor of the torrefied

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pellets covering the genuine odor of the utilized wood. The results of this work serve as a

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substantiated basis for future production of pellets from alternative raw materials.


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Energy & Fuels

1. Introduction

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Wood pellets are widely used for residential and commercial heating purposes in Europe,

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North America and Asia1. Due to the increasing demand for pellet material, the thermal

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utilization of non-wood and pretreated biomass pellets as alternatives to wood pellets has been

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topic of several recent investigations2,3 whereby attention has been paid on the fuel properties

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and combustion behavior of pellets made of energy crop biomass4, straw as well as from

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torrefied wood5. Moreover, the thermal utilization of by-products from food industry as for

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example pellets made of dried distillers grains with solubles (DDGS) – a by-product from

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beer production – has attracted attention as an additional, sustainable raw material for heating

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purposes6.

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Even though the combustion behavior of pellet material is well understood, there is a lack of

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knowledge and understanding about the off-gassing behavior of different pellet materials. It is

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well known that wood pellets emit gases during storage. Odorless gases like carbon monoxide

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(CO), carbon dioxide (CO2), methane (CH4) and hydrogen (H2) are emitted with a

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simultaneous oxygen reduction in the storage atmosphere7–14, which is most probably caused

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by oxidation processes undergone by genuine wood extractives15,16. Besides CO, CO2, CH4

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and H2 emissions, a range of other volatile organic compounds (VOCs) such as straight-chain

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aldehydes and ketones, alcohols, short chain organic acids, as well as terpenes is emitted by

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pellets7–9,17. It has been shown recently that pellets produced from different wood species can

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be differentiated according to their wood origin on the basis of the emitted VOCs18. The

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emission of VOCs is of special importance as many of these compounds are odor-active and

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are responsible for the odor or potential off-odor perceived from these materials17.

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Furthermore, the emission of VOCs is presumed to be the reason for eye and respiratory

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irritation by people working in pelletizing companies or in pellet storage facilities9,19.


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Several papers are available describing the release of volatile compounds from unpelletised

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biomass other than wood and from torrefied material, respectively20–23. However, to the best

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of our knowledge, little is known about the release of volatile and potentially odor-active

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compounds from pellets produced from alternative biomass, by-products from food industry,

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or torrefied wood. Due to the harsh conditions during the pelletising process, chemical

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reactions such as oxidation or degradation reactions with potential impact on the VOC

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composition are most likely to occur. Thus, we investigated the release of VOCs from

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different promising pellet materials (i.e., pellets made of the food processing by-products

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DDGS, rapeseed and grapevine, the energy crop biomass straw and miscanthus, eucalyptus,

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as well as torrefied sprucewood and torrefied pinewood). To gain a comprehensive picture

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about the investigated pellets, the combination of sensory evaluation conducted by

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specifically trained panelists and gas chromatography–mass spectrometry (GC-MS) after

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enrichment of the volatile compounds by headspace solid-phase microextraction (HS-SPME)

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was applied24,25. Results from sensory evaluation were used to estimate the sensory impact of

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the identified VOCs on the perceived odor of the products. Thus, the combination of the

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results obtained from this scientific approach will contribute to a better the understanding of

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the odor properties of pellets produced from alternate raw materials.

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2. Materials and Methods

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2.1 Sample materials

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Raw material for the laboratory scale production of pellets made of rapeseed (Brassica napus;

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derived from rape seed extraction), miscanthus, grapevine (Vitis vinifera) (i.e., vine pruning

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and vine pomace in a ratio of 50:50), DDGS (dried distillers grains with solubles derived from

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beer production), torrefied pinewood (Pinus sylvestris) and torrefied sprucewood (Picea 6 ACS Paragon Plus Environment

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abies) were received from different sources (Table 1). All pellets with the exception of straw

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and eucalyptus were produced in pilot-scale using a laboratory-scale pelletizing press

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(Amandus Kahl type 14-175, Amandus Kahl GmbH & Co KG, Reinbeck, Germany). Pellets

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made of straw and eucalyptus (Eucalyptus) were kindly provided by industrial pelletizing

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companies. The moisture content of the pellets was determined according to ISO 18134-326.

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The pellets were dried at 105 ± 2ºC in a drying cabinet until a constant mass had been

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achieved. The moisture contents as well as the perceived color of the investigated pellets are

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given in Table 1. Sampling was performed according to ISO 1813527. The pellets were stored

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in 1-L glass jars in the dark at 6°C until they were further used to maintain the original pellet

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quality.

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2.2 Sample preparation for the analysis of the volatile compounds

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The pellets were ground using a laboratory mill (IKA® A11 basic Analysenmühle, IKA

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Werke GmbH & Co KG, Staufen, Germany). The pellets were milled for a total of 15 sec-

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onds, applying 5 seconds per milling cycle with a break of 10 seconds between each cycle to

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prevent the samples from being excessively warmed.

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2.3 Sensory evaluation

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2.3.1

Training of the panelists

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To perform the sensory evaluation of the pellets, a sensory panel made up of 15 well-trained

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panelists was used. All panelists were trained and selected according to EN ISO 858632. Due

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to their collaboration on previous research work, the panelists had vast experience in the

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evaluation of non-food and wood-based materials. Before the sensory evaluation of the pellets

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in this study was performed, the panelists were trained on odors that were expected to occur in 7 ACS Paragon Plus Environment

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the investigated non-wood pellets. All compounds used (i.e., -terpineol, verbenone, -

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pinene, 1,8-cineol, camphor, borneol, fenchyl alcohol, pentanal, hexanal, heptanal, 2-hep-

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tanone, octanal, nonanal, benzaldehyde, 8-mercapto menthone, phenol, 2-methoxyphenol) are

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registered in the European Union as flavoring compounds and are authorized to be used in

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flavored foods according to regulation (EU) No 872/2012. All compounds used for the train-

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ing were purchased from Sigma-Aldrich (Vienna, Austria). The compounds had purities of

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≥ 96%. For evaluation purposes, ethanolic solutions of the relevant compounds were prepared

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in adequate concentrations (i.e., 0.5–2% depending on the odor threshold of the compound

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which is the concentration of the compound that is necessary for human perception). Filter

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strips were dipped into the solutions. After evaporation of ethanol, the panelists were asked to

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sniff the filter strips and describe the perceived odor. To improve the training effect, per-

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ceived odors and corresponding descriptors were discussed among members of the panel.

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2.3.2

Sensory description of pellets

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The sensory evaluation was carried out in a sensory laboratory under standardized conditions.

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Crushed pellet samples (2 g per sample) were presented in blue tasting glasses (0.11-L ca-

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pacity) that had originally been designed for the sensory evaluation of olive oil. The cups

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were covered with lids that had to be removed by the panelists immediately before they

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sniffed the samples. To avoid biasing, all samples were blind tasted. The samples were coded

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with three-digit random numbers. To avoid effects based on the order of presentation, samples

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were presented in a random order to each panelist. The panelists were asked to open the lid,

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sniff the samples and note adequate descriptors for each sample.

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Energy & Fuels

2.4 Sampling of the volatile compounds

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To extract and enrich the volatile compounds that were released from the pellets, headspace

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solid-phase microextraction (HS-SPME) was used. 300 mg of each ground pellet sample was

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transferred into a 20-mL headspace vial. 50/30-μm DVB/Carboxen/PDMS fibers (2-cm stable

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flex, Supelco, Bellfonte, USA) were used to enrich the volatiles. Prior to fiber exposure, the

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samples were equilibrated for 5 min at 40°C while stirring thoroughly. To enrich the volatile

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compounds, the fiber was exposed to the headspace of the samples for 20 min at 40°C. After

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enrichment, the SPME fiber was transferred directly into the GC injection port, where ther-

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modesorption and the direct transfer of the analytes onto the head of the GC column were

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carried out. Each sample was extracted and analyzed in four-fold repetition.

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2.5 Analysis of the volatile compounds by gas chromatography–mass spectrometry

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Separation and identification of the volatile compounds were performed by means of gas

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chromatography–mass spectrometry (GC-MS). After thermodesorption of the volatile com-

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pounds from the SPME fiber in the liner of the injection system (injector temperature 270°C,

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splitless injection mode; glass liner geometry with a constant inner diameter of 0.75 mm), the

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gas chromatographic analyses were performed on an Agilent system (GC 7890, MS 5975c VL

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MSD, Santa Clara, CA, USA) using an analytical column of medium polarity (HP5MS,

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30 m × 0.25 mm × 1 µm, Agilent Technologies) and the following temperature program:

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10°C for 1 min with a temperature ramp of 12°C min-1 up to 280°C (3 minutes) at a constant

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flow rate of 31 cm sec-1. Low temperatures were achieved by blowing liquid nitrogen into the

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GC oven. Helium was used as carrier gas for the GC separation. The mass selective detection

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(single quadruple MS) was performed in the scan mode (35-300 amu) using electron impact

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ionization (EI at 70 eV). The detection temperature was 280°C. The identification of the com9 ACS Paragon Plus Environment

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pounds was based on the comparison of the obtained mass spectra to those from MS databases

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(i.e. NIST 14 Mass Spectral Library; Adams MS Library on Essential Oils; FFNSC-Flavor

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and Fragrance Natural and Synthetic Compounds Library). In addition, the linear temperature

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programmed retention indices were calculated according to Van den Dool & Kratz as well as

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Farkaš et al.28,29. The retention times of the n-alkanes required for the calculation of the RI

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where obtained by analyzing a solution containing the homologous series of n-alkanes (C5 to

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C24 diluted in methanol, 10 ng absolute per compound) under the same chromatographic

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conditions. The experimentally obtained retention indices were compared to those from an in-

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house RI database (RI in this database had been determined with the use of authentic

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reference compounds) as well as with RI from RI databases30,31. To survey the performance of

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the HS-SPME-GC-MS procedure, the compound mixture described by Farkaš et al.29 was

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analyzed after every 10th GC run.

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To calculate correlations of the relative amounts of the released volatiles and the investigated

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pellet materials, multivariate statistical data analysis (i.e., principal component analysis

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(PCA)) was carried out. For this purpose, the average peak areas of the volatile compounds

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from GC-MS analysis were statistically processed using the Pearson correlation method.

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Cluster analysis of the data was also carried out using the average VOC peak areas and

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applying the agglomerative hierarchical clustering (AHC) regarding the Euclidean distance

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between the samples using Ward’s procedure as an agglomeration method. The multivariate

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data analysis was performed by using XLSTAT (AddinSoft (2019) XLSTAT statistical and

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data analysis solutions. Long Island, NY, USA. https://xlstat.com).

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3. Results and Discussion

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3.1 Sensory evaluation

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The odor that is emitted from pellet material during storage is regarded to be an important

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property, as the emitted odor or off-odor has a significant impact on the product acceptance

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by the end user. In this context, not only the intensity, but also the quality of the product odor

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is highly relevant. Thus, sensory evaluation of the investigated pellets was performed to char-

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acterize the odor properties. The sensory evaluation was carried out by well-trained members

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of a sensory panel and using state-of-the art techniques applied in sensory science. Results of

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the sensory evaluation are given in terms of descriptors for each type of material (Table 2).

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The results corresponded well with our expectations. For all alternative products – with the

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exception of the torrefied pellets – the sensory properties of the pellets reflected the odor

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properties of the raw material. Pellets from miscanthus and straw were described as straw- or

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hay-like, with more green, herbal odor assigned to the straw pellets. Pellets from eucalyptus

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were described as being dominated by fresh, minty, eucalyptus/camphor-like notes that were

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perceived as medicinal by some panelists. Pellets from rapeseed were perceived as fatty and

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oily with nutty notes – sensory notes that were expected from a raw material that is rich in

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lipids. The odor of DDGS, as a side product of a brewing process, reflected the roasted, malty

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and slightly fruity odor of the materials from the proceeding workup. Interestingly, the odor

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of the torrefied sprucewood and pinewood pellets was described as highly similar to one

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another. Both torrefied materials were described by the panel as having smoky, phenolic and

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burnt notes – flavor characteristics that are associated with burning or coking processes.

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However, the burnt, coked notes were predominant for the torrefied sprucewood, whereas

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typical descriptors such as resinous and woody were given as well for the torrefied pinewood

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pellets.

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3.2 Analysis of the volatile organic compounds (VOCs)

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The analyses of volatile organic compounds as well as their potential formation pathways are

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important to understand the odor properties of the investigated alternate pellet materials. To

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investigate the volatiles, analytical techniques that are generally acknowledged for the analy-

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sis of flavor compounds were applied33. Headspace solid-phase microextraction (HS-SPME)

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was used to enrich the volatiles, which were subsequently analyzed by gas chromatography-

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mass spectrometry (GC-MS). Results obtained from this study demonstrate the suitability of

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these techniques for the analysis of VOCs from pellets.

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Table 3 shows the relative amounts of VOCs that were identified in the investigated pellet

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materials. A total of 59 VOCs was identified, which belong to the chemical classes of acids,

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esters, alcohols, aldehydes and ketones, terpenes, heterocyclic compounds and aromatic

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compounds such as phenols. For better visibility, Figure 1a and Figure 1b demonstrate the

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distributions of the different compound classes per investigated pellet material. This way of

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presentation clearly shows the large differences in the distribution of the volatile compounds

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depending on the raw material and on the torrefaction process, respectively. We took a more

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detailed look at the different compound classes to better understand the sources and formation

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pathways of the compounds and their potential influences on the perceived sensory properties

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of the products.

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The identified short-chain fatty acids belong to the homologous series ranging from C2 to C6.

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Acetic acid was identified in all investigated pellet materials in varying, but large amounts.

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The source for the high relative concentrations of acetic acid is not clear. However, these

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findings correspond well with the previously reported occurrence of acetic acid in wood

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pellets18. Significant differences were detected between the investigated pellet materials with

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respect to the other straight-chain acids. Whereas acetic acid was identified in varying

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amounts in all investigated pellet materials, significant differences were detected between the 12 ACS Paragon Plus Environment

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investigated pellet materials with respect to the other straight-chain acids which are most

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likely degradation products from cell membrane lipids and/or lipids derived from the plant

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material/seeds. Pellets produced from rape – a crop with a high fat content in the seeds –

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possessed the highest diversity of straight-chain fatty acids. In straw, however, only acetic

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acid was identified. The methyl-branched fatty acid 3-methyl butanoic acid is most likely

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derived from the precursor amino acid leucine and is, thus, found in pellets made of protein-

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rich raw material like DDGS, grapevine and miscanthus. The presence of these fatty acids in

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wood pellets has been described in the results of our own previous research17. In this previous

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study, we showed that high concentrations of free fatty acids – and especially the presence of

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3-methyl butanoic acid with a low odor threshold – were strongly correlated with off-flavor

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formation in wood pellets. Hence, attention should be paid to the formation/the presence of

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short-chain free fatty acids which might contribute to off-odor formation. However, the

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formation and/or degradation pathways, respectively, of the free short chain fatty acids are not

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completely clear and would need further investigations. Two esters were identified in this

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study: methyl acetate is most likely a reaction product of acetic acid and methanol and was

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identified in all types of pellets. Methanol emission from wood pellets during storage has been

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shown before34. Methyl pentanoate, however, was only detected in pellets made of rape,

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which correlates with the presence of the corresponding free acid that was also only detected

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in this pellet type.

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Eight different alcohols were identified which are supposed to be degradation products of (i)

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membrane lipids, (ii) fat derived from the lipids of the seeds, or (iii) amino acids, especially in

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pellets made of protein-rich raw material, with respect to the methyl-branched alcohols as

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well as phenyl alcohol. The alcohol 2,3-butanediol was identified in large relative amounts in

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pellets produced from DDGS and grapevine. The fermentative formation of 2,3-butanediol is

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well described in literature35. Both raw materials  DDGS and grapevine  are derived from 13 ACS Paragon Plus Environment

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biotechnological processes during which the 2,3-butanediol formation is most likely to occur.

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A broad range of carbonyl compounds (11 aldehydes and 6 ketones) was identified, although

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the amounts varied greatly between the investigated materials. As with the other compound

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classes, different precursors can be made responsible for the aldehyde and ketone formation;

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however, lipid oxidation is supposed to be the main reaction pathway7,36. The straight-chain

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C6 aldehyde hexanal, for example, like other straight chain aldehydes, is well-known to be a

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reaction product of lipid oxidation from either membrane lipids or lipids derived from seeds.

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The formation of straight-chain aldehydes in sawdust and pellets produced from coniferous

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wood has been shown previously 7,9,37–39. With green, soapy, fatty flavor characteristics in

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dependence on chain lengths and concentrations, we suspect that these carbonyls influence the

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odor of the investigated material, especially as many of these compounds show low odor

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thresholdsi. The presence of 2- and 3-methyl butanal, which have sweet, malty odor

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properties, again strongly correlates with the presence of other methyl-branched metabolites

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in miscanthus and grapevine40. These compounds are believed to be responsible for the malty,

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beer-like odor in the latter. 6-Methyl-5-hepten-2-one is a known degradation product of

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carotenoids41. It was only identified in the grapevine pellets. Carotenoids from wine are

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thought to be the precursor compounds for this ketone in pellets derived from grapevine.

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Thousands of different terpenoid compounds are found in nature as secondary metabolites in

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plant material, all of which have different physiological roles42. Many of these also possess

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distinct odor properties that may have a significant impact on the aroma of the respective

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plant material. As the investigated alternative pellet materials were produced from different

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plant sources, the presence of a broad range of terpenes was to be expected. Eight different

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terpenes were identified in the investigated pellet materials, reflecting their occurrence in the

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respective raw material. For example, eucalyptus is well-known for its typical smell – and this

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was reflected in the presence of 1,8-cineol (i.e., eucalyptol) that was found in high relative 14 ACS Paragon Plus Environment

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concentrations in eucalyptus pellets only, as well as other terpenes that had previously been

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identified in eucalyptus oil43 as well as in eucalyptus wood from Portuguese forests22. Wine

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flavor is described as being highly influenced by the presence of terpenes, some of which, like

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limonene and -cyclocitral, have also been identified in grapevine pellets. -Copaene is the

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only terpene that could be found in DDGS pellets. We believe that this compound was not

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derived from barley, but from hops, another essential ingredient in beer production44. The

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resins and essential oils derived from coniferous trees are well-known to contain high terpene

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concentrations. All terpenoids that we identified in torrefied sprucewood and pinewood

299

pellets had been described before from these plant sources45,46.

300

Among the heterocyclic compounds, the group of alkylated furans is the most important com-

301

pound group detected in the investigated pellets. The formation of alkylated furans in ther-

302

mally processed foods has been thoroughly described, and carbohydrates are known to act as

303

precursors for 2-furancarboxaldehyde, 2-acetylfuran and 2-methylfuran. Alkylated furans

304

with longer alkyl chains such as 2-pentylfuran result from lipid degradation, whereby -

305

unsaturated aldehydes represent precursor compounds in thermally processed foods47,48. In the

306

raw material used for the production of the investigated pellet materials, excess amounts of

307

complex carbohydrates were present; furthermore, unsaturated aldehydes were detected, re-

308

sulting from lipid oxidation. Thus, the large number of alkylated furans is explained by the

309

presence of these compounds in combination with the thermal treatment during pellet produc-

310

tion. Interestingly, 2-methylpyrazine was found in pellets made of rape, DDGS but also in

311

pellets from torrefied sprucewood. 2-Methylpyzarine is a characteristic product of the Mail-

312

lard reaction, one of the most important reactions in thermally processed foods that contain

313

reducing sugars and amino acids. Many Maillard reaction products have roasted, nutty, burnt

314

odor properties and a strong odor impact on the related food material49.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Volatile compounds with aromatic rings in the structures represent a compound class, mem-

316

bers of which were identified in the torrefied pellet materials. Chen et al.50 previously de-

317

scribed the formation of alkylated methoxyphenols in the course of the torrefaction process.

318

The sensory potency of compounds from this compound class was recently described by

319

Schranz and colleagues51. In contrast to the phenolic compounds derived from torrefaction,

320

the large amount of phenolic compounds in pellets produced from eucalyptus is mainly based

321

on the presence of cymol a compound that has been described previously as constituent of

322

Eucalyptus essential oil52. With its fresh, terpeny odor properties this compound most likely

323

also contributes to the fresh odor of eucalyptus pellets.

324 325

3.3 Correlations among results

326

To obtain a clearer insight into product-compound interrelations, multivariate statistical data

327

analysis was performed, taking all relative concentrations of the identified volatile compounds

328

into consideration. Cluster analysis as a mathematical procedure to condense heterogeneous

329

objects (i.e., the investigated pellets) with a high number of characteristics (i.e., relative

330

concentrations of the volatiles) into homogenous groups was conducted on the volatile

331

compounds. The result is presented in terms of a dendrogram (Figure 2), showing the

332

similarities of the products under investigation. Three distinct clusters were identified:

333

cluster 1 contained pellets produced from DDGS and torrefied sprucewood, cluster 2 had

334

only one member (pellets from grapevine) and cluster 3 contained pellets from torrefied

335

pinewood and eucalyptus, on the one hand, and the pellets of the herbaceous plants rape,

336

straw and miscanthus, on the other hand. To obtain deeper understanding of the formed

337

clusters, principal component analysis based on the relative concentrations of volatile

338

compounds was performed. Thereby, it was possible to further structure the data set and to

339

identify compound-pellet interrelations. Results are given in Figure 3a (observations plot) and 16 ACS Paragon Plus Environment

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Figure 3b (corresponding biplot with factor loadings), whereas the first two, most informative

341

principal components were selected for the graphic presentation. The cluster labelings in

342

Figure 3b were taken from the dendrogram (Figure 2).

343

Expressed correlations among certain compound groups are found for the torrefied spruce-

344

wood pellets as well as for pellets made of grapevine. With reference to torrefied sprucewood,

345

the importance of the alkylated methoxyphenols as well as of the alkylated furan derivatives

346

can be seen clearly. These compounds are highly correlated to each other and are located in

347

quadrant II. The sensory thresholds for furan derivatives are generally highi; consequently,

348

their sensory impact on the product flavor is considered to be negligible. However, various

349

furan derivatives had been reported as volatile compounds emitted by steam exploded and

350

torriefied wood before20. In contrast, the odor thresholds of the identified methoxyphenol

351

derivatives are in general significantly lower than those of alkylated furans51. In the sensory

352

evaluation, the torrefied sprucewood pellets were described as smoky, phenolic and woody,

353

which can be explained by the presence of the phenolic compounds. Pellets made of

354

grapevine were described as dried fruit-like, green and sweet. A high correlation was found

355

with products from amino acid degradation, the corresponding free acids, methyl acetate,

356

aldehydes and alcohols from lipid degradation, as well as the two terpenes limonene and -

357

cyclocitral, all of which are located in quadrant I. These compounds have more or less

358

pronounced sweet, fruity odor properties with different odor thresholds, which influence the

359

perceived odors of the pellets. The grapevine used was a 50:50 mixture of wine pruning and

360

pomace and, thus, contained a high proportion of grape seeds. Grape seeds are known for

361

their high amounts of mono- and polyunsaturated acids, which are precursors of the straight-

362

chain aldehydes or alcohols, as well as leucine and isoleucine, which are precursor

363

compounds for 2/3-methyl-branched aldehydes and alcohols53. Clear correlations can also be i

For a survey of odor thresholds for selected compounds, please see the database „Odor & Flavor Detection Thresholds in Water,“ www.leffingwell.com, last access Dec. 19, 2018


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Page 18 of 43

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seen between the VOCs of torrefied pinewood and eucalyptus pellets. In contrast to torrefied

365

sprucewood pellets, torrefied pinewood pellets were not correlated with methoxyphenols, but

366

with the monoterpenes camphene, cumol and 1,8-cineol. These findings are also reflected by

367

the sensory descriptors of resinous, dried wood. Burnt notes derived from methoxyphenols

368

were perceived as well, but obviously did not dominate the odor of the torrefied pinewood

369

pellets. The eucalyptus pellets were also highly correlated with the same terpenes as the

370

torrefied pinewood pellets which is as well reflected in the sensory properties. Pellets

371

produced from rape, straw and miscanthus showed no clear correlations with any specific

372

compound group that could explain the odor of the products.

373 374

4. Conclusions and Future Perspectives

375

In this study, gas chromatographic and sensory techniques, which are usually applied in flavor

376

chemistry, have been used for the investigation of the odor properties and VOCs emitted from

377

pellets produced from alternative raw materials such as energy biomass crop, eucalyptus, by-

378

products from food industry as well as from torrefied sprucewood and pinewood. To the best

379

of our knowledge, this is the first time that pellets made of miscanthus and straw on the one

380

hand and DDGS, grapevine and rapeseed on the other hand were investigated with respect to

381

the released volatiles, their sensory properties and the interrelation with possible sources and

382

formation pathways. The results of this investigation deliver new insights into the VOC

383

composition and the sensory properties of these pellet types and will serve as a basis for

384

future investigations.

385

With the general increasing demand for pellets, the odor of the products will gain increasing

386

importance as the odor is perceived directly by people working in pellet production and

387

storage sites as well as by the end users. Thus, the odor properties have to be considered an

388

important quality criterion. Future investigations are required regarding the identification of 18 ACS Paragon Plus Environment

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those compounds with the highest impact on the odor formation as well as on the impact of

390

external parameters such as storage on a possible alteration of the VOC composition and

391

possible off-odor formation.

392 393 394

Acknowledgements

395

The authors thank the members of the sensory test panel for evaluating the samples.

396

This study was financially supported by the Provincial Government´s office from Lower

397

Austria, Department for Economy, Tourism and Technology as well as by the Austrian Re-

398

search Promotion Agency (FFG) in the frame of General Programme – Collective Research

399

within the projects ‘Smell - Study on malodorous emissions from wood pellets’ (FFG project

400

No. 839962 and 847138). Financial support obtained from ProPellets Austria ppA as well as

401

industry partners (Hasslacher Norica Timber GmbH, Binderholz GmbH, DEPV e.V., German

402

Pellets GmbH, Andritz AG, Agrana Stärke GmbH) is gratefully acknowledged.

403 404

Conflict of interest disclosure

405

The authors declare no competing financial interest.


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Page 20 of 43

406

Abbreviations

407

AHC, agglomerative hierarchical clustering; amu, atomic mass units; DDGS, dried distillers

408

grains with solubles; DVB, divinyl benzene; GC, gas chromatography; HS, headspace; MS,

409

mass spectrometry; PCA, principal component analysis; PDMS, polydimethylsiloxane; RI,

410

retention index; SPME, solid phase microextraction; VOCs, volatile organic compounds


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Schranz, M.; Lorber, K.; Klos, K.; Kerschbaumer, J.; Buettner, A. Influence of the Chemical

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Structure on the Odor Qualities and Odor Thresholds of Guaiacol-Derived Odorants, Part 1:

558

Alkylated, Alkenylated and Methoxylated Derivatives. Food Chem. 2017, 232, 808–819.

559

https://doi.org/10.1016/j.foodchem.2017.04.070.

560

(52)

Page 26 of 43

Tapondjou, A. L.; Adler, C.; Fontem, D. A.; Bouda, H.; Reichmuth, C. Bioactivities of Cymol

561

and Essential Oils of Cupressus Sempervirens and Eucalyptus Saligna against Sitophilus

562

Zeamais Motschulsky and Tribolium Confusum Du Val. J. Stored Prod. Res. 2005, 41 (1), 91–

563

102. https://doi.org/10.1016/j.jspr.2004.01.004.

564

(53)

Kamel, B. S.; Dawson, H.; Kakuda, Y. Characteristics and Composition of Melon and Grape

565

Seed Oils and Cakes. J. Am. Oil Chem. Soc. 1985, 62 (5), 881–883.

566

https://doi.org/10.1007/BF02541750.


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Energy & Fuels

Table 1: Detailed information on the investigated pellet materials Pellets made of

moisture content Raw material and source

production

(color of the pellets)

[mass fraction in % ]

Miscanthus

crushed miscanthus 100 %,

(medium-brown)

obtained from a German research institute

Rapeseed

extracted rape seed 100 %,

(light-brown, tan)

obtained from a German oil producing company

Straw

straw pellets 100 %

(light brown, yellowish)

obtained from a Danish pelletizing company

DDGS

Dried Distillers Grains with Solubles

(medium-brown)

obtained from a German brewery

Eucalyptus

eucalyptus pellets 100 %

(medium brown)

obtained from a Spanish pelletizing company

laboratory pelletizing press

8.2


11.2

industrial pelletizing press

7.3


12.6

industrial pelletizing press

5.3


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 569 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Grapevine

crushed vine pruning 50% and vine pomace 50%

(light to medium brown)

obtained from a German research institute

torrefied Sprucewood

torrefied spruce sawdust 100%

(black-brown)

obtained from a Dutch pelletizing company 1

torrefied Pinewood

torrefied pine sawdust 100%

(black brown)

obtained from a Dutch pelletizing company 2

Page 28 of 43


10.1


3.2


6.4


Page 29 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

570

Energy & Fuels

Table 2: Results from the sensory evaluation – summary of the descriptors given by the sensory experts

571

Pellet type

Description

miscanthus

dried grain, straw/hay-like, ‘dusty’

rapeseed

oily, nutty, rancid, like feed for fattening animals

straw

greenish, herbage, resinous, aromatic, hay-like

eucalyptus

camphor-like, minty, refreshing, medicinal, essential

DDGS

roasted, malty, cereal-like, sweetish, mature/ripe fruits

grapevine

dried fruits, green, herbage, sweet

torrefied sprucewood

smoky, phenolic, woody, aromatic. smoked ham-like

torrefied pinewood

resinous, burnt notes, dried wood, pungent

572


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 30 of 43

Table 3: Relative amounts of VOCs determined in the investigated pellets samples (rape, miscanthus, straw, eucalyptus, DDGS, grapevine, torrefied sprucewood, torrefied pinewood) expressed as average peak areas obtained from HS-SPME-GC-MS (n = 4); n.d. not detected. Volatiles/samples

RIexp.

RIDB

torr.

torr.

[peak area counts]

[HP5]

[HP5]

Rape

Miscanthus

Straw

Eucalyptus

DDGS

Grapevine

Sprucewood

Pinewood

Acetic acid1 STD [%]

612

600f

345 824 2

1 651 432 1

36 867 1

141 125 1

889 291 1

1 113 540 4

5 027 266 2

26 925 3

Propanoic acid2 STD [%]

709

668f

38 127 3

n.d.

n.d.

n.d.

27 956 1

n.d.

2 737 641 3

30 338 5

Butanoic acid1 STD [%]

780

821p

254 254 2

n.d.

n.d.

n.d.

n.d.

98 830 16

125 532 5

28 270 5

3-Methyl butanoic acid2 STD [%]

829

877p

n.d.

68 205 2

n.d.

n.d.

40 051 2

68 205 18

n.d.

30 415 3

Pentanoic acid2 STD [%]

870

911f

114 747 10

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

Hexanoic acid2 STD [%]

962

1019f

37 112 13

87 833 2

n.d.

8 850 11

22 296 6

87 833 13

n.d.

n.d.

Acids

Esters


Page 31 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Energy & Fuels

Volatiles/samples

RIexp.

RIDB

torr.

torr.

[peak area counts]

[HP5]

[HP5]

Rape

Miscanthus

Straw

Eucalyptus

DDGS

Grapevine

Sprucewood

Pinewood

Methyl acetate STD [%]

516

515p

80 949 19

182 596 16

511 484 19

39 117 4

110 439 13

1 015 351 11

248 741 15

44 806 18

Methyl pentanoate STD [%]

810

821p

25 888 5

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

3-Methyl-1-butanol STD [%]

745

737p

n.d.

24 844 6

n.d.

n.d.

n.d.

342 898 9

n.d.

n.d.

2-Methyl-1-butanol STD [%]

749

744p

n.d.

n.d.

n.d.

n.d.

n.d.

120 114 8

n.d.

n.d.

Pentanol STD [%]

772

766f

n.d.

71 882 7

162 846 4

n.d.

n.d.

148 182 9

n.d.

29 194 4

2,3-Butanediol STD [%]

782

769p

n.d.

n.d.

n.d.

n.d.

14 171 232 1

441 090 5

n.d.

n.d.

1-Hexanol STD [%]

869

869p

n.d.

107 012 12

n.d.

n.d.

n.d.

1 178 963 14

n.d.

n.d.

2-Ethyl-1-hexanol STD [%]

1031

1032f

n.d.

n.d.

n.d.

n.d.

93 846 11

n.d.

n.d.

n.d.

1-Octanol STD [%]

1071

1070p

n.d.

21 608 1

59 815 4

n.d.

n.d.

n.d.

n.d.

n.d.

Alcohols


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 32 of 43

Volatiles/samples

RIexp.

RIDB

torr.

torr.

[peak area counts]

[HP5]

[HP5]

Rape

Miscanthus

Straw

Eucalyptus

DDGS

Grapevine

Sprucewood

Pinewood

1134

1122p

n.d.

n.d.

n.d.

n.d.

6 208 439 2

456 597 9

n.d.

n.d.

2-Butenal STD [%]

666

648f

n.d.

n.d.

n.d.

n.d.

n.d.

91 460 11

176 612 2

n.d.

3-Methylbutanal STD [%]

671

656p

n.d.

128 389 3

n.d.

n.d.

n.d.

103 737 14

n.d.

n.d.

2-Methylbutanal STD [%]

683

664p

n.d.

116 613 3

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

Pentanal STD [%]

715

699p

n.d.

101 378 1

128 478 4

76 619 5

n.d.

185 530 6

n.d.

88 400 19

3-Hydroxy-2-butanone STD [%]

727

718f

n.d.

n.d.

n.d.

n.d.

60 325 4

532 807 6

86 618 2

n.d.

Hexanal STD [%]

802

800p

377 194 2

576 816 3

2 375 951 2

643 113 2

224 695 12

1 845 498 13

386 584 10

1 239 120 5

2-Heptanone STD [%]

893

889p

50 502 5

33 169 1

276 414 5

102 013 3

n.d.

142 073 18

n.d.

n.d.

Heptanal STD [%]

905

900p

101 947 4

n.d.

279 316 1

92 557 1

n.d.

248 162 19

307 290 17

301 828 14

2-Phenylethanol STD [%] Aldehydes, ketones


Page 33 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Energy & Fuels

Volatiles/samples

RIexp.

RIDB

torr.

torr.

[peak area counts]

[HP5]

[HP5]

Rape

Miscanthus

Straw

Eucalyptus

DDGS

Grapevine

Sprucewood

Pinewood

2-Heptenal STD [%]

963

957p

n.d.

n.d.

n.d.

n.d.

n.d.

643 246 7

n.d.

76 948 9

Benzaldehyde STD [%]

979

961p

155 441 7

45 697 7

104 332 1

89 727 4

153 654 7

418 479 6

387 699 6

169 345 3

6-Methyl-5-hepten-2-one STD [%]

990

958p

n.d.

n.d.

n.d.

n.d.

n.d.

232 904 18

n.d.

n.d.

2-Octanone STD [%]

993

999p

23 356 2

n.d.

34 362 5

n.d.

n.d.

n.d.

n.d.

n.d.

Octanal STD [%]

1007

1004p

61 178 7

30 826 2

149 571 3

89 543 7

n.d.

133 501 8

n.d.

114 531 18

2,4-Heptadienal STD [%]

1018

996p

n.d.

n.d.

54 774 5

n.d.

n.d.

n.d.

n.d.

n.d.

3,5-Octadien-2-one STD [%]

1077

1068p

n.d.

n.d.

109 988 6

n.d.

n.d.

n.d.

n.d.

n.d.

Nonanone STD [%]

1095

1093f

n.d.

n.d.

32 923 9

n.d.

n.d.

n.d.

n.d.

n.d.

Nonanal STD [%]

1110

1104p

90 288 6

55 062 2

302 679 4

175 002 8

70 102 4

405 155 7

n.d.

147 213 14

Terpenes


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 34 of 43

Volatiles/samples

RIexp.

RIDB

torr.

torr.

[peak area counts]

[HP5]

[HP5]

Rape

Miscanthus

Straw

Eucalyptus

DDGS

Grapevine

Sprucewood

Pinewood

α-Pinene STD [%]

952

939p

134 919 5

n.d.

201 907 8

496 637 13

n.d.

199 104 5

n.d.

85 994 6

Camphene STD [%]

971

953p

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

131 932 10

δ-3-Carene STD [%]

1029

1011p

111 078 2

n.d.

136 608 3

65 149 7

n.d.

128 993 14

n.d.

233 448 7

Limonene STD [%]

1046

1039p

33 811 4

n.d.

138 036 2

1 126 116 9

n.d.

1 146 124 17

337 344 3

97 006 5

1,8-Cineol STD [%]

1054

1039p

n.d.

n.d.

n.d.

49 83 857 6

n.d.

n.d.

n.d.

n.d.

trans-Pinocarveol STD [%]

1172

1169p

n.d.

n.d.

68 836 8

66 777 6

n.d.

n.d.

n.d.

n.d.

Camphor STD [%]

1180

1192p

n.d.

n.d.

20 900 4

n.d.

n.d.

n.d.

n.d.

n.d.

β-Cyclocitral STD [%]

1252

1224p

n.d.

n.d.

n.d.

n.d.

n.d.

21 399 11

n.d.

n.d.

α-Copaene STD [%]

1418

1391p

n.d.

n.d.

21 346 4

26 506 15

40 442 571 18

14 390 682 5

37 637 094 4

5 597 783 9

Caryophyllene

1474

1467p

n.d.

n.d.

137 489

n.d.

n.d.

n.d.

n.d.

n.d.


Page 35 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Energy & Fuels

Volatiles/samples

RIexp.

RIDB

[peak area counts]

[HP5]

[HP5]

Rape

Miscanthus

STD [%]

Straw

torr.

torr.

Eucalyptus

DDGS

Grapevine

Sprucewood

Pinewood

4

heterocyclic compounds 2-Ethylfuran STD [%]

719

728p

n.d.

n.d.

102 432 4

n.d.

n.d.

n.d.

n.d.

n.d.

2-Methyl pyrazine STD [%]

831

827p

231 275 8

n.d.

n.d.

n.d.

65 405 4

n.d.

179 323 11

n.d.

2-furancarboxaldehyde STD [%]

839

830p

69 261 7

129 033 3

n.d.

n.d.

14 462 421 1

412 855 19

16 341 146 2

810 668 7

2-(Hydroxymethyl)furan STD [%]

858

866p

n.d.

n.d.

n.d.

n.d.

871 717 2

448 155 9

1 707 580 11

n.d.

Acetylfuran STD [%]

921

910p

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

689 713 4

25 567 8

Dihydro-2(3H)-furanone STD [%]

923

915p

166 201 6

n.d.

n.d.

n.d.

1 712 475 2

1 001 430 18

991 985 3

n.d.

2-Methyl-5-formylfuran STD [%]

928

945p

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

153 562 5

n.d.

974

978p

n.d.

n.d.

n.d.

n.d.

209 394

n.d.

1 923 005

58 152

5

12

5-methylfurancarboxaldhyde STD [%]

11


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 36 of 43

Volatiles/samples

RIexp.

RIDB

[peak area counts]

[HP5]

[HP5]

Rape

Miscanthus

Straw

Eucalyptus

DDGS

997

1001p

79 508 8

216 968 7

589 530 4

417 158 8

Ethylbenzene STD [%]

875

878p

n.d.

n.d.

n.d.

Cumol STD [%]

939

941p

n.d.

n.d.

p-Cymol STD [%]

1041

1033p

109 684 2

2-Methoxyphenol STD [%]

1107

1091p

2-Methoxy-4-methylphenol STD [%]

1213

4-Allyl-2-methoxyphenol STD [%] 2-Methoxy-4-propylphenol STD [%]

2-Pentylfuran STD [%]

torr.

torr.

Grapevine

Sprucewood

Pinewood

121 840 5

402 383 18

128 707 9

181 177 10

39 748 6

n.d.

n.d.

n.d.

34 835 5

n.d.

n.d.

n.d.

n.d.

n.d.

135 412 16

n.d.

72 029 6

118 2107 7

n.d.

257 163 13

682 208 5

1 032 529 2

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

2 545 025 5

124 118 14

1190p

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

684 074 6

66 366 15

1382

1364f

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

65 594 11

n.d.

1392

1365p

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

54 499 11

n.d.

aromatic compounds

-

Results are expressed as average values of four replicates in terms of area counts.

-

RI

retention index.


Page 37 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Energy & Fuels

-

RIexp.

RI as determined in the experiments.

-

RIDB

reference-RI obtained from databases

-

STD

relative standard deviation in %

-

1

Areas were determined from the selected ion chromatograms, m/z =60

-

2

Areas were determined from the selected ion chromatograms, m/z = 73

-

f

RI obtained from www.flavornet.org

-

p

RI obtained from http://www.pherobase.com


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 43

Figure Captions

Figure 1a. Relative distribution of the compound classes (acids, esters, alcohols, aldehydes and ketones, terpenes, heterocyclic compounds, aromatic compounds and phenols) determined in pellets from alternative biomass Figure 1b. Relative distribution of the compound classes (acids, esters, alcohols, aldehydes and ketones, terpenes, heterocyclic compounds, aromatic compounds and phenols) determined in pellets from torrefied sprucewood and torrefied pinewood Figure 2: Dendrogram resulting from cluster analysis of VOCs; three clusters were detected (cluster 1: DDGS and torrefied sprucewood; cluster 2: torrefied pinewood, eucalyptus, straw, rape and miscanthus; cluster 3: grapevine), cluster analysis was carried out using the Ward procedure Figure 3a. Correlations among the investigated pellet materials based on the average areas of VOCs analysed by HS-SPME-GC-MS. PCA based on Pearson correlation. Indicated clusters were derived from the cluster analysis conducted using the Ward procedure (Figure 2) Figure 3b: Biplot score and factor loadings obtained by PCA based on the average areas of VOCs obtained from HS-SPME-GC-MS analyses


Page 39 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Figure 1a. Relative distribution of the compound classes (acids, esters, alcohols, aldehydes and ketones, terpenes, heterocyclic compounds, aromatic compounds and phenols) determined in pellets from alternative biomass

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 40 of 43

Figure 1b. Relative distribution of the compound classes (acids, esters, alcohols, aldehydes and ketones, terpenes, heterocyclic compounds, aromatic compounds and phenols) determined in pellets from torrefied sprucewood and torrefied pinewood


Page 41 of 43

Dendrogram 2,5E+15

2E+15

1,5E+15

similarity 1E+15

Miscanthus

Rape

Straw

Eucalyptus

torr. Pine

grape-vine

0

torr. Spruce

5E+14

DDGS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Figure 2: Dendrogram resulting from cluster analysis of VOCs; three clusters were detected (cluster 1: DDGS and torrefied sprucewood; cluster 2: torrefied pinewood, eucalyptus, straw, rape and miscanthus; cluster 3: grapevine), cluster analysis was carried out using the Ward procedure

1


Energy & Fuels

Observations (Axis F1 and F2: 51,02 %) 10

grapevine

Cluster 2

8

6

F2 (20,73 %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 42 of 43

4

2

Cluster 1

torr. sprucewood

0

DDGS

Miscanthus torr.pinewood

rape

-2

Cluster 3

eucalyptus

straw

-4 -10

-8

-6

-4

-2

0

2

4

6

8

F1 (30,29 %)

Figure 3a. Correlations among the investigated pellet materials based on the average areas of VOCs analysed by HS-SPME-GC-MS. PCA based on Pearson correlation. Indicated clusters were derived from the cluster analysis conducted using the Ward procedure (Figure 2)


Page 43 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Figure 3b: Biplot score and factor loadings obtained by PCA based on the average areas of VOCs obtained from HS-SPME-GC-MS analyses 1


Interrelation of Volatile Organic Compounds and Sensory Properties of

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