Methanol Mitigation during Manufacturing of Fruit Spirits with Special Consideration of Novel Coffee Cherry Spirits

Methanol is a natural ingredient with major occurrence in fruit spirits, such as apple, pear, plum or cherry spirits, but also in spirits made from coffee pulp. The compound is formed during fermentation and the following mash storage by enzymatic hydrolysis of naturally present pectins. Methanol is toxic above certain threshold levels and legal limits have been set in most jurisdictions. Therefore, the methanol content needs to be mitigated and its level must be controlled. This article will review the several factors that influence the methanol content including the pH value of the mash, the addition of various yeast and enzyme preparations, fermentation temperature, mash storage, and most importantly the raw material quality and hygiene. From all these mitigation possibilities, lowering the pH value and the use of cultured yeasts when mashing fruit substances is already common as best practice today. Also a controlled yeast fermentation at acidic pH facilitates not only reduced methanol formation, but ultimately also leads to quality benefits of the distillate. Special care has to be observed in the case of spirits made from coffee by-products which are prone to spoilage with very high methanol contents reported in past studies

Parameters for sustainable and demand-oriented biogas production

Biogas production offers a possibility to generate electricity and heat by renewable sources. It has a great potential in terms of availability. Unlike solar radiation and wind energy, biogas production does not underlie natural fluctuations. Biogas substrates can be stored and are therefore available at all times. In an ideal scenario, biogas production should meet certain criteria. It should be (1) sustainable, (2) provide the highest possible system effectiveness and (3) must be available in times of high electricity demand. This work concentrates on aspects of all three criteria. Biogas production in a highly sustainable way can be found on organic farms (Manuscript 1). Therefore, we analysed 13 biogas plants on organic farms and looked for improvement potentials. The survey concentrated on biogas plant specifications, fermentation characteristics and feedstock compositions. The biogas plants were tested repeatedly to investigate the impact of fermentation characteristics on volatile solids (VS) degradation, indicating the effectiveness of the anaerobic digestion process. The total feedstock composed mainly of livestock residues (61 %) and grass silage (14 %). Such substrates are considered ecologically beneficial (Danner & Kilian 2012; Insam et al. 2015), when compared to possible negative effects of maize (Svoboda et al. 2015). By comparing the feedstock composition to the biogas plant specifications we found significant correlations between the amount of livestock residues and the dimension of the combined heat and power (CHP) unit. With an increasing share of livestock residues in the feedstock the installed electrical capacity of these biogas plants became smaller. Within the fermentation characteristics, the process temperatures ranged between 17 49 °C. Low temperatures were found to be the driving factor for low VS degradation (range 11.67 - 87.70 %) in the residue storage. Significantly better VS degradations were found in biogas plants with two reactor stages compared to one reactor stage. Effluents with high VS concentrations can cause harmful greenhouse gas emissions (Ruile et al. 2015). Backfitting a cover on the residue storage is highly recommended, as most biogas plant residue storages were found to be installed without (> 84 %). To maintain a highly sustainable biogas production on organic farms, the maximisation of VS degradation is required. Therefore, the biogas plants’ VS degradation could be raised by increasing the process temperatures and/or the hydraulic retention time. Most biogas plants in Germany are, however, installed on conventional farms, accounting for > 99 % of the total installed electrical capacity. Here, maize silage is the most important substrate with the highest share on the total feedstock (ca. 40 % fresh weight) (FNR 2014). During the anaerobic digestion in biogas plants, the maximisation of carbon transformation from substrate to biogas is a desired goal of every biogas plant operator. It increases the system effectiveness of the biogas plant and therefore its economical sustainability. The carbon degradation can be quantified by common carbon degradation determination methods (C degmix) (VDLUFA 1997). With this method, it is not possible to quantify substrate-bound carbon degradation independently of the inoculum. Therefore, we tested an isotope-based method in reactor mixtures of maize silage and inoculum from agricultural biogas plants (Manuscript 2) to identify solely substrate-bound carbon degradation (C deg ƒMS). As stable isotopes underlie natural variability, the method was tested on six different maize silages of different farming systems (organic or conventional farming) and maize silages of high (MS high) and low (MS low) qualities. A total of 19 lab-scale batch reactors were analysed for digestion parameters, specific biogas (sby) and methane yields (smy), stable isotopes and carbon degradations. Reactors with MS high showed up to 23.8 % higher sby and smy. The carbon degradations in reactors with MS low were significantly lower compared to reactors containing MS high. The C deg ƒMS values were, however, significantly higher compared to C degmix, as it excludes the masking effect of the inoculum. At the start of the experiments, the stable carbon isotope values (δ13C) in the reactors were highly variable. Nonetheless, C deg ƒMS showed reliable values independently of the maize silage qualities. The effect of the farming system of the maize silages was found negligible in both carbon degradation methods. We found that the isotope-based carbon degradation determination method offers a possibility to assess the substrate-bound carbon degradation in more detail and contributes to our understanding of the total system effectiveness. In a future scenario of 100 % renewable energy in Europe, a large contribution will come from wind and photovoltaic energy generation (Steinke et al. 2013). Such resources underlie, however, strong temporal fluctuations. This creates balancing challenges in the energy systems (Hahn et al. 2014). In times of sun and wind deficits energy shortage could partly be filled in by biogas plants (SRU 2011), having relatively short start-up phases. The biogas production is, however, dependent on the microbial transformation of the substrate. Therefore, a quickly adaptable microbial community is necessary to cope with flexible substrate inputs. In our survey (Manuscript 3) we focused on structural changes of microbial communities in four lab-scale reactors. Every reactor was continuously fed with different mixtures of maize silage (MS) and sugar beet silage (SBS). The mixing ratio was 1:0 (VS ratios of MS and SBS) in continuous fermenter one (CF1), 6:1 (CF2), 3:1 (CF3) and 1:3 (CF4), respectively. The organic loading rates were equal in all reactors with 1.25 kg VS m 3 d 1. Bacterial and archaeal communities’ compositions were analysed with 454 amplicon sequencing technique on the basis of 16S rRNA genes. With increasing amounts of SBS, the bacterial and archaeal communities’ compositions shifted. Despite the shifts, biogas production rates and methane concentrations were similar in all reactors. This showed that the communities adapted to different environmental conditions induced by different substrate mixtures and ensured biogas production efficiency. In conclusion, my work contributes to parameters for optimized biogas production scenarios. For sustainable biogas production on organic farms deficits were detected, which could be reduced by increasing the VS degradations during the fermentation process. High VS degradation is a crucial factor for biogas system effectivity. In conventional biogas plants high VS degradation of maize silage is also a desired goal, which is resembled by carbon degradation. The application of a stable isotope-based method for carbon degradation determination of maize silage showed reliable and more detailed results. This increases our understanding of anaerobic degradation effectiveness in biogas reactors. For demand-oriented biogas production, quickly adaptable microbial communities are crucial prerequisites. Different substrate feeding regimes in biogas reactors resulted in compositional changes of microbial communities by simultaneously ensuring biogas production efficiency. This implicates the functional redundancy of microbial communities within biogas plant reactors. The outcome of this thesis contributes to future-oriented biogas production. Optimised biogas production can improve environmental sustainability and national energy sufficiency. This supports the creditability of biogas production and emphasises its importance among all renewable energies

Reproducibility of Fruit Spirit Distillation Processes

Fruit spirit distillations processes are based on physical principles of heat and mass transfer. These principles are decisive for the separation of desired and undesired aroma compounds, which affect the quality of the distilled product. It is mandatory to control heat and mass transfer parameters to be able to perform fruit spirit distillation processes in a reproducible manner and to achieve equal products with similar volatile compound compositions repeatedly. Up to now, only limited information is available on the magnitude of reproducibility errors since fruit spirit distillation columns are typically not equipped with a suitable control or monitoring technique. We upgraded a batch distillation column with digitized instrumentation and a control technique to be able to control crucial parameters such as thermal energy inputs and reflux rates. This study aimed to identify whether control over two distillation parameters has the potential to enable us to perform distillation processes repeatedly. This study analyzed the magnitude of reproducibility errors for (i) six monitored distillation process parameters and (ii) 13 quantified volatile compounds in the product between duplicated distillation runs performed with equal setups. A total of eight different distillations were performed in duplicate (n = 16), while the six distillation parameters were monitored and logged every ten seconds. The produced distillates were equally subsampled into 20 fractions and each fraction analyzed for 13 volatile compound concentrations. Based on a dataset of 28,600 monitored duplicate distillation process data points, this study showed that process parameters can indeed be replicated with a median relative standard deviation (RSD) of <0.1% to 7% when two crucial process parameters are controlled. The comparison of 1540 volatile compound concentrations in the product fractions showed a reproducibility error with an average median RSD of 9 ± 8%. This illustrated that by gaining control over thermal energy input and reflux rates, the reproducibility of fruit spirit distillation processes and their associated products can largely be met. It is advisable to equip distillation columns with a suitable control technique to be able to reproduce the performance of fruit spirit distillations

Reproducibility of Fruit Spirit Distillation Processes

Fruit spirit distillations processes are based on physical principles of heat and mass transfer. These principles are decisive for the separation of desired and undesired aroma compounds, which affect the quality of the distilled product. It is mandatory to control heat and mass transfer parameters to be able to perform fruit spirit distillation processes in a reproducible manner and to achieve equal products with similar volatile compound compositions repeatedly. Up to now, only limited information is available on the magnitude of reproducibility errors since fruit spirit distillation columns are typically not equipped with a suitable control or monitoring technique. We upgraded a batch distillation column with digitized instrumentation and a control technique to be able to control crucial parameters such as thermal energy inputs and reflux rates. This study aimed to identify whether control over two distillation parameters has the potential to enable us to perform distillation processes repeatedly. This study analyzed the magnitude of reproducibility errors for (i) six monitored distillation process parameters and (ii) 13 quantified volatile compounds in the product between duplicated distillation runs performed with equal setups. A total of eight different distillations were performed in duplicate (n = 16), while the six distillation parameters were monitored and logged every ten seconds. The produced distillates were equally subsampled into 20 fractions and each fraction analyzed for 13 volatile compound concentrations. Based on a dataset of 28,600 monitored duplicate distillation process data points, this study showed that process parameters can indeed be replicated with a median relative standard deviation (RSD) of &lt;0.1% to 7% when two crucial process parameters are controlled. The comparison of 1540 volatile compound concentrations in the product fractions showed a reproducibility error with an average median RSD of 9 &plusmn; 8%. This illustrated that by gaining control over thermal energy input and reflux rates, the reproducibility of fruit spirit distillation processes and their associated products can largely be met. It is advisable to equip distillation columns with a suitable control technique to be able to reproduce the performance of fruit spirit distillations

Stable carbon isotope analyses offer insights into net carbon degradation of maize silages in anaerobic batch fermentations

Abstract Carbon degradation indicates the efficiency of anaerobic digestion processes. Common carbon degradation determination methods define gross carbon degradation (C deggross) of substrate and inoculum inseparably. The aim of this study was to test an isotope-based method defining solely substrate-based net carbon degradation (C degnet) on maize silage. As the natural abundance of stable isotopes in agricultural substrates vary, the method’s applicability was tested on (i) different maize silages sampled from agricultural farms, (ii) maize silage in fresh (MSfresh) and impaired storage (MSimpaired) conditions.Experiments included six maize silages digested in a total of 19 lab-scale batch reactors, analyzed for digestion parameters, stable isotopes, gross and net carbon degradation. MSimpaired showed significantly different stable carbon isotope composition at the start of the experiments, compared to MSfresh. Both methods indicated quality losses in MSimpaired. Results showed significantly higher C degnet values, ranging from 58.4% to 86.5%, compared to deggross values, ranging from 23.1% to 48.7%. This indicated the applicability of an isotope-based method C degnet to assess net carbon degradation of maize silages more detailed by excluding the masking effect of the inoculum. The isotope-based net carbon degradation method was found applicable on maize silages from (i) different farms and (ii) in different storage qualities

Introducing a Simple Method to Investigate Relative Volatilities of Flavour Compounds in Fruit Brandies

Determination of relative volatilities of flavour compounds in hydro-alcoholic solutions is usually performed by time-consuming and costly standard methods such as the Gillespie dynamic recirculation method. This study tested a quick and easy method with a simple distillation apparatus for measuring empirical relative volatilities. A total of 17 representative flavour compounds of apple and pear brandies were selected and investigated for their relative volatilities. This also included so far unpublished relative volatilities of one alcohol (2-butanol), one aldehyde (hexanal), one ketone (β-damascenone), and three esters (ethyl butyrate, butyl acetate, ethyl 2-methylbutyrate). Relative volatilities of three alcohols (methanol, isobutanol, and isoamyl alcohol), one ester (ethyl acetate), one aldehyde (acetaldehyde), and one acetal (1,1-diethoxyethane) were compared to reference data. All compounds, except for 1,1-diethoxyethane, showed a high correlation with the reference data. Vapour–liquid equilibrium for an ethanol–water mixture showed a high correlation with the reference data (r > 0.9). In accordance with the high correlation levels, we suggest that the simple distillation apparatus is a fast and simple alternative to the standard method

Co-Digestion of Sugar Beet Silage Increases Biogas Yield from Fibrous Substrates

This study tested the hypothesis that the easily degradable carbohydrates of the sugar beet silage (S) will improve the anaerobic digestion of grass silage (G) more profoundly compared to co-digestion of sugar beet silage with maize silage (M). M : S and G : S mixtures were tested in two continuous laboratory-scale AD experiments at volatile solid ratios of 1 : 0, 6 : 1, 3 : 1, and 1 : 3 at organic loading rates of 1.5 kgVS m−3 day−1. While the sugar beet effects in mixtures with maize silage were negligible, co-digestion with grass silage showed a beneficial performance. There, the specific methane production rate was 0.27 lN kg−1VS h−1 at G : S ratio of 6 : 1 compared to G : S 1 : 0 with 0.14 lN kg−1VS h−1. In comparison to G : S 1 : 0, about 44% and 62% higher biogas yields were obtained at G : S 6 : 1 and 3 : 1, respectively. Also, the highest methane concentration was found in G : S at ratio of 1 : 3. Synergistic increase of methane yield was found in co-digestion in both experiments, but higher effect was realized in G : S, independently of the amount of sugar beet silage. The findings of this study emphasize the improvement of AD of grass silage by even low addition of sugar beet silage

Absorbance Spectroscopy of Heads, Hearts and Tails Fractions in Fruit Spirits

There is a large economic interest to characterize heads, hearts and tails fractions during fruit spirit distillation by simple, fast, low-volume and low-cost analytical methods. This study evaluated the potential of ultraviolet (UV)-visible-infrared spectroscopy (230–1000 nm) to characterize and differentiate these distillate fractions. Heads, hearts and tails fractions of 10 different fruit spirits were separated by sensory evaluation and investigated by absorbance spectroscopy. Principal component analysis indicated that UV spectroscopy at a wavelength range from 230 to 310 nm had the highest potential to differentiate all three distillate fractions. While all tails fractions showed significantly different UV spectra, a clear differentiation between heads and hearts fractions was limited. However, an additional UV spectroscopy of 100 mL subfractions sampled during the shift from heads to hearts in three additional distillations did reveal significant differences. The calculated integrals of the according best-fit trendline functions of the spectra indicated a trend towards reduced area-under-the-curve and zero-point values during the shift. This could be a new lead to implement an analytical method for in-line process control during fruit spirit production

Different response of bacteria, archaea and fungi to process parameters in nine full‐scale anaerobic digesters

Biogas production is a biotechnological process realized by complex bacterial, archaeal and likely fungal communities. Their composition was assessed in nine full-scale biogas plants with distinctly differing feedstock input and process parameters. This study investigated the actually active microbial community members by using a comprehensive sequencing approach based on ribosomal 16S and 28S rRNA fragments. The prevailing taxonomical units of each respective community were subsequently linked to process parameters. Ribosomal rRNA of bacteria, archaea and fungi, respectively, showed different compositions with respect to process parameters and supplied feedstocks: (i) bacterial communities were affected by the key factors temperature and ammonium concentration; (ii) composition of archaea was mainly related to process temperature; and (iii) relative abundance of fungi was linked to feedstocks supplied to the digesters. Anaerobic digesters with a high methane yield showed remarkably similar bacterial communities regarding identified taxonomic families. Although archaeal communities differed strongly on genus level from each other, the respective digesters still showed high methane yields. Functional redundancy of the archaeal communities may explain this effect. 28S rRNA sequences of fungi in all nine full-scale anaerobic digesters were primarily classified as facultative anaerobic Ascomycota and Basidiomycota. Since the presence of ribosomal 28S rRNA indicates that fungi may be active in the biogas digesters, further research should be carried out to examine to which extent they are important players in anaerobic digestion processes