Effects of Dry-Hopping on Beer Chemistry and Sensory Properties-A Review

Dry-hopping is the addition of hops to the wort on the cold side of the brewing process. Unlike standard hop additions, its main purpose is not to produce a characteristic bitterness but to extract as much of the hop essential oils as possible, which are largely lost in the standard hopping process. When dry-hopped, it is possible to obtain a beer with an aroma that is difficult to achieve when hops are used on the hot side of the brewing process. As a result, this process has become very popular in recent years, particularly in beers that belong to the 'craft beer revolution' trend. In addition, the usefulness of this process is increasing with the development of new hop varieties with unique aromas. This article presents the main components of hops, focusing on those extracted during the process. Changes in the composition of beer bittering compounds and essential oils resulting from this process are discussed. This paper presents the current state of the knowledge on the factors affecting the degree of extraction, such as hop dosage, the time, and temperature of the process. Issues such as process-related physicochemical changes, hop creep, low flavor stability, haze formation, and green flavor are also discussed.

Potential of Microorganisms to Decrease the "Beany" Off-Flavor: A Review

Vegetable proteins are in high demand due to current issues surrounding meat consumption and changes in eating habits, but they are still not accepted by consumers due to their strong bitterness, astringent taste, and "beany" off-flavor. This review aimed to give an overview of the "beany" off-flavor and the potential of microorganisms to decrease it. Twenty-six volatile compounds were identified from the literature as contributing to the "beany" off-flavor, and their formation pathways were identified in a legume matrix, pea. Biotechnological ways to improve the flavor by reducing these volatile compounds were then looked over. As aldehydes and ketones are the main type of compounds directly linked to the "beany" off-flavor, alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) were focused on. By converting aldehyde and ketones into alcohols or carboxylic acids, these two enzymes have the potential to decrease the off-flavor. The presence of the two enzymes in a selection of microorganisms (Lactobacillus acidophilus, Limosilactobacillus fermentum, Lactiplantibacillus plantarum, Streptococcus thermophilus, Saccharomyces cerevisiae, and Gluconobacter suboxydans) was done with a catabolism and a bioinformatical study. Finally, the correlation between the presence of the enzyme and the efficacy to improve the flavor was investigated by comparison with the literature. The presence of ADH and/or ALDH in the strain metabolism seems linked to an odor improvement. Especially, a constitutive enzyme (ADH or ALDH) in the catabolism should give better results, showing that some fermentative types are more inclined to better the flavor. Obligatory fermentative strains, with a constitutive ADH, or acetic acid bacteria, with constitutive ADH and ALDH, show the best results and should be favored to reduce the amount of compounds involved in the "beany" off-flavor and diminish that off-flavor in legume proteins.

Flavour stability of sterilised chickpeas stored in pouches

The increasing need for sustainable food choices places a demand on developing palatable foods from lower impact production and with a suitable shelf-life. In this context, knowledge of the sensory properties of whole sterilised chickpeas is required to be able to make them more attractive to the consumers. The sensory quality of chickpeas is largely dependent on the aroma and flavour, which can be influenced by storage conditions. In this study, sterilised chickpeas in two different packaging materials with different oxygen permeabilities, stored up to 52 weeks at ambient temperature (20 °C) were investigated using both descriptive sensory profiling and gas chromatography – mass spectrometry – olfactometry analysis (GC-MS-O). During storage, intense, sulphury and meat-like aromas decreased in intensity, while with longer storage time hay-like, green-like and potato-like flavours and aromas became more apparent. A total of 40 odour-active volatiles were detected, of which some had properties reminiscent of the chickpea flavour profiles. However, no clear relationships could be established between these odorants and the sensory changes observed during storage by descriptive sensory analysis. It was concluded that, significant changes in the sensory attributes of sterilised chickpeas occurred during 52 weeks of storage, but that packaging material does not seem to largely influence the sensory attributes during storage.

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Significant sensory changes occur in sterilised chickpeas during 52 weeks storage.

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40 odour-active volatiles are detected in the sterilised chickpeas.

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Sulphur compounds formed during sterilisation change during storage.

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Hay and green attributes are perceived more intense with increased storage time.

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Oxygen permeability of packaging does not impact the sensory quality during storage.

Recombinant Lipoxygenases and Hydroperoxide Lyases for the Synthesis of Green Leaf Volatiles

Green leaf volatiles (GLVs) are mainly C₆- and in rare cases also C₉-aldehydes, -alcohols, and -esters, which are released by plants in response to biotic or abiotic stresses. These compounds are named for their characteristic smell reminiscent of freshly mowed grass. This review focuses on GLVs and the two major pathway enzymes responsible for their formation: lipoxygenases (LOXs) and fatty acid hydroperoxide lyases (HPLs). LOXs catalyze the peroxidation of unsaturated fatty acids, such as linoleic and α-linolenic acids. Hydroperoxy fatty acids are further converted by HPLs into aldehydes and oxo-acids. In many industrial applications, plant extracts have been used as LOX and HPL sources. However, these processes are limited by low enzyme concentration, stability, and specificity. Alternatively, recombinant enzymes can be used as biocatalysts for GLV synthesis. The increasing number of well-characterized enzymes efficiently expressed by microbial hosts will foster the development of innovative biocatalytic processes for GLV production.

Effect of Saccharomyces uvarum on lipid oxidation and carbonyl compounds in silver carp mince during cold storage

Fish lipid is highly susceptible to oxidation, resulting in accumulation of toxic substances reactive carbonyl compounds (RCCs), the reduction of nutritional value, and the production of odorous substances. In this study, the effect of yeast (Saccharomyces uvarum) on RCCs, fat acid composition, volatiles, and sensory traits in silver carp mince stored at 4°C was evaluated. Yeast eliminated malondialdehyde, 4-hydroxyl-2-hexenal, and 4-hydroxyl-2-nonenal by about 80%, 68%, and 60%, which increased by about 170%, 340%, and 300% in the control, respectively. Yeast helped retain about 80% of the polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), respectively; only about 53% and 46% of EPA and DHA, respectively, were maintained in the control. Yeast removed off-odors hexanal, nonanal, and decenal, resulting in enhanced sensory traits. These findings were economically important for improving the quality of fish products. It might present an approach to improve the flavor of fish products.

Herbivory elicits changes in green leaf volatile production via jasmonate signaling and the circadian clock

The timing of plant volatile emissions is important for a robust indirect defense response. Green leaf volatiles (GLVs) are emitted by plants upon damage but can be suppressed by herbivore-associated elicitors, and the abundance and composition of GLVs vary depending on the timing of herbivore attack. We show that the GLV biosynthetic enzyme HYDROPEROXIDE LYASE (HPL) is transcriptionally regulated by the circadian clock in Nicotiana attenuata. In accordance with transcript abundance of NaHPL, GLV aldehyde pools in intact leaves peaked at night and at subjective night under diurnal and continuous light conditions, respectively. Moreover, although the basal abundance of NaHPL transcripts is upregulated by jasmonate (JA) signaling, JA does not regulate the reduction of NaHPL transcript abundance in damaged leaves by simulated herbivore treatment. Unexpectedly, the plant circadian clock was strongly altered when Manduca sexta larvae fed on N. attenuata, and this was also independent of JA signaling. Lastly, the temporal dynamics of NaHPL transcripts and total GLV emissions were strongly altered by M. sexta larval feeding. Our data suggest that the temporal dynamics of emitted GLV blends result from a combination of damage, JA signaling, herbivore-associated elicitors, and the plant circadian clock.

Arabidopsis thaliana SKP1 interacting protein 11 (At2g02870) negatively regulates the release of green leaf volatiles

AtSKIP11, a kelch-repeat containing F-box protein from Arabidopsis thaliana, negatively regulates the HPL pathway and can serve as a potential molecular switch for the biosynthesis of green leaf volatiles.

Selected dehydrogenases in Yarrowia lipolytica JMY 861: their role in the synthesis of flavor compounds

The presence of selected dehydrogenases, including alcohol dehydrogenase (ADH-YL) and aldehyde dehydrogenase (ALDH-YL), in Yarrowia lipolytica JMY 861, and their potential role in flavor synthesis were investigated. The experimental findings showed that using reduced form of nicotinamide adenine dinucleotide (NADH) as cofactor, the ADH-YL activity in vitro was 6-fold higher than that with reduced form of nicotinamide adenine dinucleotide phosphate (NADPH); however, under the experimental conditions used in this study, an ALDH-YL activity was not detected. The in situ hexanal reduction reaction was found to be instantaneous; however, when the yeast cells suspension was diluted 150 times, the initial relative hexanal concentration was increased by 84.1%. The chromatographic analyses indicated the conversion, in situ, of linoleic acid hydroperoxides (HPODs) into volatile C₆-compounds after 60 min of HPODs addition to the yeast cells suspension.

Green leaf volatiles: biosynthesis, biological functions and their applications in biotechnology

Plants have evolved numerous constitutive and inducible defence mechanisms to cope with biotic and abiotic stresses. These stresses induce the expression of various genes to activate defence-related pathways that result in the release of defence chemicals. One of these defence mechanisms is the oxylipin pathway, which produces jasmonates, divinylethers and green leaf volatiles (GLVs) through the peroxidation of polyunsaturated fatty acids (PUFAs). GLVs have recently emerged as key players in plant defence, plant-plant interactions and plant-insect interactions. Some GLVs inhibit the growth and propagation of plant pathogens, including bacteria, viruses and fungi. In certain cases, GLVs released from plants under herbivore attack can serve as aerial messengers to neighbouring plants and to attract parasitic or parasitoid enemies of the herbivores. The plants that perceive these volatile signals are primed and can then adapt in preparation for the upcoming challenges. Due to their 'green note' odour, GLVs impart aromas and flavours to many natural foods, such as vegetables and fruits, and therefore, they can be exploited in industrial biotechnology. The aim of this study was to review the progress and recent developments in research on the oxylipin pathway, with a specific focus on the biosynthesis and biological functions of GLVs and their applications in industrial biotechnology.

Synthesis of green note aroma compounds by biotransformation of fatty acids using yeast cells coexpressing lipoxygenase and hydroperoxide lyase

Green notes are substances that characterize the aroma of freshly cut grass, cucumbers, green apples, and foliage. In plants, they are synthesized by conversion of linolenic or linoleic acid via the enzymes lipoxygenase (LOX) and hydroperoxide lyase (HPL) to short-chained aldehydes. Current processes for production of natural green notes rely on plant homogenates as enzyme sources but are limited by low enzyme concentration and low specificity. In an alternative approach, soybean LOX2 and watermelon HPL were overexpressed in Saccharomyces cerevisiae. After optimization of the expression constructs, a yeast strain coexpressing LOX and HPL was applied in whole cell biotransformation experiments. Whereas addition of linolenic acid to growing cultures of this strain yielded no products, we were able to identify high green note concentrations when resting cells were used. The primary biotransformation product was 3(Z)-hexenal, a small amount of which isomerized to 2(E)-hexenal. Furthermore, both aldehydes were reduced to the corresponding green note alcohols by endogenous yeast alcohol dehydrogenase to some extent. As the cosolvent ethanol was the source of reducing equivalents for green note alcohol formation, the hexenal/hexenol ratio could be influenced by the use of alternative cosolvents. Further investigations to identify the underlying mechanism of the rather low biocatalyst stability revealed a high toxicity of linolenic acid to yeast cells. The whole cell catalyst containing LOX and HPL enzyme activity described here can be a promising approach towards a highly efficient microbial green note synthesis process.

Effects of transporting and processing Sauvignon blanc grapes on 3-mercaptohexan-1-ol precursor concentrations

The effects of different processing treatments on thiol precursor concentrations have been investigated through studies involving transportation of machine-harvested Sauvignon blanc fruit and assessment of different applications of antioxidants, along with juice preparation and enzyme inhibition experiments. The influence these trials had on 3-S-cysteinylhexan-1-ol (Cys-3-MH) and 3-S-glutathionylhexan-1-ol (Glut-3-MH) concentrations in juices is discussed. Very interesting findings included the large increase in precursor concentrations after transportation, particularly for Cys-3-MH, and the limited formation of Glut-3-MH when grape proteins were precipitated during processing. The various results provided information about the ability to modulate precursor concentrations depending on the processing technique employed. Additionally, a conjugated aldehyde, which is the obvious missing link between the reaction of (E)-2-hexenal and glutathione in the formation of Glut-3-MH, has been tentatively identified for the first time. Deuterium-labeled 3-S-glutathionylhexanal (Glut-3-MHAl) was produced through the addition of labeled (E)-2-hexenal to grapes, followed by grape crushing, and detected in the juice by HPLC-MS/MS, along with the corresponding labeled Glut-3-MH.

Purification of hydroperoxide lyase from green bell pepper (Capsicum annuum L.) fruits for the generation of C6-aldehydes in vitro

The aim of this work was to compare the efficiency of different extracts of hydroperoxide lyase from green bell peppers in producing aldehydes: a crude extract, a chloroplastic fraction, and a purified enzyme were investigated. From a crude extract, the HPO lyase was purified by ion-exchange chromatography with a 22.3-fold increase in purification factor. Analysis by SDS-PAGE electrophoresis under denaturating conditions showed only one protein with a molecular weight of 55 kDa, whereas size-exclusion chromatography indicated a molecular weight of 170 kDa. A maximum of 7500 mg of aldehydes per g of protein was obtained with the purified enzyme within 20 min of bioconversion compared to 392 and 88 mg of aldehydes per g of protein within 50 and 60 min, respectively, for the chloroplast fraction and the crude extract.