The solid-phase micro-extraction conditions for flavour analysis.
Abstract
In the last years, the demands for natural flavours have dramatically increased. To fulfil the consumer requests, researchers are looking for new and alternative methods to obtain qualitative aroma compounds by utilising microbiological pathways. Some microorganisms like lactic acid bacteria or yeasts are capable of synthesising specific flavours corresponding to diacetyl and acetaldehyde as secondary metabolites. By supplying the culture media with flavour precursors and optimising the primary culture media, high amount of specific flavours could be obtained. Also, the biosynthesis of each specific flavour is influenced by the type of amino acids and sugars involved in the bioprocess. Thus, by changing the ratio of amino acids and sugars in the culture media, different amounts of flavour can be obtained. In this context, monitoring the compositions of the culture media and fermentation conditions is crucial in obtaining high amounts of a qualitative-specific aroma.
Keywords
- natural flavours
- microorganism
- metabolic pathway
- fermentation
1. Introduction
Generally, the first major source of flavour is the extraction from plant biomass due to consumer preference for “clean” and “organically” produced aromas and fragrances. Taken into account the significant differences between the price of synthetic and non-synthetic manufacturing, the microbial flavour production is considered [1]. Consequently, in the last years, the main focus of researchers in the field was the identification of a suitable biosynthetic pathway and the optimal culture medium design for an efficient production.
Lactic acid bacteria (LABs) are an important class of microorganism for flavour manufacturer. LABs are very important for the dairy industry, being extensively used in fermented food production. During the fermentative processes, LABs influences the sensory properties of the final products, also including the flavour development. The flavour production is very much substrate and strain dependent, and the presence of the flavour precursors and regulatory responses may influence the balance of the flavour biosynthesis from a secondary metabolite product to the main compound [2]. Strains like
At the same time, the flavour biosynthesis and the changes in metabolic pathway are linked to environmental conditions. From a technological point of view, the metabolic changes are very important for the volatile compound biosynthesis, as well as for the microorganism, in order to obtain energy and to maintain the NAD+/NADH+H+ balance [3]. It is obvious that there are major differences in flavour profiles between utilised complex and standard media.
2. The influence of the culture medium composition on flavour biosynthesis
2.1. The influence of nitrogen
The LAB strains are able to survive starvation due to their capacity to utilise another energy source rather than carbon. The starvation conditions decrease the organism ability to synthesise ATP with generation of proton motive force (PMF) and also slow down the accumulation of necessary nutrients to maintain viability over time. As an additionally carbon source, the LABs are capable to catabolise amino acids which provide building blocks, cofactor recycling and limited energy source [5]. The LAB inability to synthesise many of the amino acids required for protein synthesis needs the supplementation of the culture media with high amount of essential amino acids [6], since the amino acid catabolism is a major process for flavour formation. Proteolytic enzymes from LAB play an important role in degradation of proteins by producing free amino acids. These amino acids contribute directly to flavour formation being precursors for catabolic reactions [7, 8]. The conversion of amino acids to aroma compounds by LAB is essentially initiated by a transamination reaction, which requires α-ketoacid as the amino group acceptor, pathway demonstrated for lactococci, mesophilic lactobacilli and thermophilic LAB [9].
By amino acid catabolism, LABs are able to synthesise flavours. In the first step of the pathway, the amino acids are involved in dehydrogenation and transamination reactions with the formation of α-ketoacids, compounds which have a fundamental effect on flavour type and amount. Further, by decarboxylation reaction, α-ketoacids are transformed in aldehydes (Figure 1).
From decarboxylation reaction, a proton is consumed in the process, and the product is exported from the cell, resulting in an increase of the intracellular pH [10]. Additionally, the aldehydes are transformed into alcohols or carboxylic acids by dehydrogenation, a majority of these compounds being flavour compounds. Several enzymes, for example, α-ketoacids, can thus be considered as intermediates involved in both biosynthesis and degradation of amino acids. Since branched-chain amino acids (Val, Ile, Leu), aromatic amino acids (Trp, Tyr, Phe) and sulphur-containing amino acids (Cys, Met) are important precursors of flavour compounds, the genome of
Theoretically, there are three pathways for the formation of α-ketoglutarate by bacteria using glutamate, citrate and pyruvate [9].
2.1.1. Amino acids’ first specific degradation pathway
In the first step, the glutamate dehydrogenase pathway produces α-ketoglutarate directly from oxidative deamination of glutamate, utilising NAD+, NADP or both as cofactor. NADP-dependent activity was detected in most
α-Ketoglutarate is the best α-ketoacid acceptor for amino acid transamination by
Different amino acids have diverse amino peptidase, with characteristic activity on amino acids [6].
The proline catabolism by
2.1.2. Amino acids’ second specific degradation pathway
The second possible pathway is the citrate-oxaloacetate pathway, which leads to α-ketoglutarate production from citrate and glutamate, by successive action of citrate permease, citrate lyase and aspartate aminotransferase (AspAT). Citrate permease allows citrate uptake inside the cells with the citrate catabolism initiation by transforming citrate to oxaloacetate. Oxaloacetate can then be transformed into aspartate and α-ketoglutarate, in the presence of glutamate, by an aspartate aminotransferase. For
On the other hand, the valine catabolism by non-oxidative enzymatic decarboxylation leads to the formation of α-ketoisovalerate in
In another study, the
The biosynthesis of the diacetyl from aspartate by some
LAB protein degradation determines the formation of free amino acids that vary in their concentration over time. Leucine has been reported to be dominant amino acid in Cheddar cheese after 6 months of maturation [5]. Leucine catabolism leads to the formation of α-keto-isocaproate [10] and generates aroma like 3-methylbutanal (cheesy, chocolate, malt), 3-methylbutanoic acid (cheesy, sweaty), phenylacetaldehyde and 2-hydroxy-4-methyl pentanoic acid methyl ester [18]. On the other hand, under
2.1.3. Amino acids’ third specific degradation pathway
The third pathway is the citrate-isocitrate pathway which utilises the oxidative branch of the tricarboxylic acid cycle leading to the production of α-ketoglutarate from either pyruvate or citrate with the action of pyruvate dehydrogenase, pyruvate carboxylase, citrate synthase, aconitase and isocitrate dehydrogenase. Pyruvate dehydrogenase and pyruvate carboxylase are necessary to degrade pyruvate into acetyl-CoA and oxaloacetate, respectively, both used by citrate synthase to synthesise citrate. Citrate is then transformed by aconitase into isocitrate, which is finally oxidised to α-ketoglutarate by isocitrate dehydrogenase [9].
Cysteine is catabolised by α-ketoacid enzyme with synthesis of 3-mercaptopyruvate, which by elimination of hydrogen sulphide, lead to the obtaining of pyruvate, used as a precursor for flavour biosynthesis.
2.2. The influence of carbon sources
The carbon source is very important for the microbial growth because it is the principal resource for energy production. In the same time, for aroma biosynthesis sugars with a low-molecular weight are requested to be used, which are at the same time a flavour precursor.
The addition of sucrose in the culture media stimulates the flavour biosynthesis for yeasts and LABs [25]. Di Cagno and co-workers [26] supplemented with sucrose the tomato juice that is subjected to LAB fermentation in order to stimulate the flavour biosynthesis and to reduce the intrinsic flavour acidity of tomatoes. By inoculation of six prebiotic strains in the milk culture media, supplemented with 0.75% fructose (w/v), a desired aroma for the final product was obtained [3].
De Figueroa and co-workers [27] demonstrated that
In sourdoughs, for example, flavour compounds are produced by LAB and yeasts individually or by their interactions.
2.3. The influence of the mineral composition of the medium
The minerals are very important in culture media of the microorganism because they are used as a cofactor in enzymatic activity. All enzymes have a metal as a coordinative element, and the enzyme activity depends on it. At the same time, the salt concentration is very important because it dictates the osmotic pressure and flavour improvements [31]. Similarly, aldehydes can also be generated by chemical oxidation of α-ketoacids catalysed by bivalent cations [16]. Manganese and magnesium sulphate enhanced both biomass and aroma development of 52 different yeasts by obtaining 96.05 mg/L acetaldehyde for
Recently, two manganese transport systems of
2.4. The influence of temperature
The flavour biosynthesis by microorganism is strongly influenced by the temperature of fermentation. De Figueroa and co-workers [27] demonstrate that
On the other hand, lower incubation temperature tends to selectively promote growth rate of the
For the yeast fermentations (
2.5. The influence of aeration
The presence of oxygen strongly influences the microbial growth and the flavour biosynthesis. The aerobic microorganism’s metabolism is oxygen dependent and is mandatory for flavour biosynthesis pathway. For example,
3. Acetaldehyde biosynthesis pathway
Acetaldehyde besides being a major component of tobacco smoke is the primary metabolite of ethanol [34]. Commercially, it is obtained by Wacker process of ethylene oxidation in strong acid solutions using as catalysts PdCl2-CuCl2 of crude oil, but this method is not very sustainable. The trend demands are for obtained acetaldehyde from renewable raw materials like sugars from biomass or synthesise from lactic acid. Due to its high reactivity derived from containing two conjugated hydroxyls and one carboxylic group, lactic acid (LA) is an attractive feedstock for chemical production, being in torn biosynthesise at low costs by glucose and xylose fermentation. The acetaldehyde may be produced by decarbonylation or decarboxylation of LA in the presence of aluminium phosphates and magnesium aluminate spinels, reaction promoted by acid catalysts [35].
Acetaldehyde represents a secondary metabolite in alcoholic fermentation of yeasts, being a precursor of the ethanol production in beer and wine. It is the most important carbonyl compound produced during alcoholic fermentation in concentrations between 10 and 200 mg/L depending on technological factors, such as culture medium composition, pH, fermentation temperature, aeration and SO2 concentration and on the yeast strain used [4]. Acetaldehyde is biosynthesized from glucose by the glycolytic pathway enzyme pyruvate decarboxylase. At the beginning, two molecules of pyruvate resulted from glucose glycolysis, and by pyruvate decarboxylation, the secondary acetyl-CoA product is obtained. Furthermore, two acetaldehyde molecules are resulted under alcohol dehydrogenase action on the acetyl-CoA compound (Figure 6). The high peak value of acetaldehyde biosynthesis is reached during the early fermentation phases, being then partly re-catabolised by yeast, or is combined with polyphenols or other compounds in the wine being a very reactive compound [36].
Another species that can produce acetaldehyde is the acetic acid bacteria (AAB) characteristic from grape microorganism equipment. This class of microorganism has another biosynthesis pathway, oxidising ethanol to acetaldehyde and acetic acid in concentrations up to 250 mg/L. At ethanol concentrations higher than 10% (v/v) and under low oxygen conditions, acetaldehyde tends to accumulate, in other conditions being oxidised to acetic acid. The yeasts reported to biosynthesize acetaldehyde in high amounts are
Taken into account that lactic acid bacteria are also responsible for acetaldehyde biosynthesis, being a typical flavour component of yoghurt responsible for pungent and fruity flavour [37], this microorganism has attracted the researcher attention. By using recombinant microbial processes for biotransformation approach, Balagurunathan and co-workers [38] engineered an
4. Diacetyl biosynthesis pathway
Diacetyl (2,3 butanedione) is the typical butter flavour/aroma, commonly found in fermented dairy products, such as butter, sour cream and yoghurt, and important for cheese aroma [39]. Moreover,
Citrate metabolism is the principal generator of sensory characteristics of the milk final products. One of the LAB used globally as starter fermentation is
For LAB, citrate fermentation is involved in cheese flavour and quality. In the first step of citrate metabolism catalysed by citrate lyase, oxaloacetate is obtained. The oxaloacetate is decarboxylated via oxaloacetate decarboxylase generating pyruvate. In
The pyruvate obtained is condensed by α-acetolactate synthase with the formation of α-acetolactate which is chemically unstable [40]. The α-acetolactate formed by the action of α-acetolactate decarboxylase can be converted to acetoin or diacetyl in a non-enzymatic oxidative decarboxylation reaction, the biosynthesis being more evident at pH 4.5. This pathway could be also completed with additional enzymes such as acetoin/diacetyl reductase and butanediol dehydrogenase (Figure 7) [10, 23].
There have been many attempts to redirect the metabolism of various microorganisms for improving diacetyl formation, by classical mutagenesis or directed genetic engineering trying the improvement of by-product formation [43], but the results were not very promising [40].
Liu and co-workers [40] successfully achieved to convert the homo-lactic bacterium
5. Flavour determination and quantification
The flavour determination was made by different instrumental analytical methods. The common one is the solid-phase micro-extraction analysis, in which different substrates are used for solid phase, Table 1.
Solid-phase material | GC column | Reference |
---|---|---|
Carboxen/polydimethylsiloxane, 85 μm film thickness, 220°C work temperature | Zebron ZB-624, D-0.25 mm; 1.4 μm film thickness; composition, 94% dimethyl polysiloxane; 6% cyanopropyl-phenyl; 60 m long | [44] |
Polydimethylsiloxane with 10% embedded activated carbon phase (PDMS/AC), 50 μm film thickness, 250°C work temperature | HP-5MS capillary column, 5% phenyl methyl silicone, 320 μm × 1.0 μm, 60 m long | [45] |
Polyacrylate bonded to silica core, 85 μm film thickness, 220°C work temperature | HP-INNO-WAX polyethylene glycol capillary column, 250 μm × 0.5 μm, 60 m long | [16] |
Polydimethylsiloxane fibre, 250°C work temperature | DB5 capillary column, 0.32 μm internal diameter, 1 μm film thickness, 60 m long | [26, 46] |
StableFlex divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) coated fibre, 250°C work temperature | ZB-WAXplus polyethylene glycol capillary column, 0.25 mm internal diameter; 0.50 μm film thickness, 60 m long | [47, 48] |
Silica fibre covered by Carboxen Polydimethylsiloxane (CAR-PDMS), 75 μm film thickness, 250°C work temperature | CP-Wax 52 CB polyethylene glycol coated, 0.32 mm, 1.2 mm film thickness, 50 m long | [49, 50] |
Polydimethylsiloxane divinylbenzene (PDMS_DVB) SPME fibre, 250°C work temperature | SUPELCOWAX™ 10 capillary column, 0.1 mm, 0.1 μm film thickness, 10 m long | [51] |
Other analysis techniques of flavours are (i) by purge and trap method and (ii) GC-MS separation and identification [7, 52, 53] or (iii) by proton transfer reaction mass spectrometry using PTR-MS [54, 55]. In some cases, the flavour determination was made enzymatically [36, 56] or by derivatisation with dinitrophenylhydrazine (DNPH)-acetonitrile reagent, and then the compounds are analysed by HPLC analysis with detection at 360 nm [34, 57].
6. Applications of natural flavours
Flavour release from food during consumption in the mouth is important in flavour perception and influenced by food matrix [58]. Since food matrix changes biochemically and physically during eating, the food flavour microencapsulation results in controlled release for specific situations. Different natural and synthetic polymers were used for microcapsule fabrication, of which alginate-whey protein compounds have been found to be suitable as vehicle for diacetyl flavour delivery [59].
Nowadays, the delivering of antimicrobial volatiles from polymeric systems, in a controlled manner, gained an increasing interest. In food industry, diacetyl is used not only as an approved food additive but also for food preservation, due to its antimicrobial activities. Diacetyl has been shown to be bactericidal against
Strains of
Another direction of diacetyl utilisation is related to active packaging systems. Thus, the controlled release of different volatile antimicrobial compounds was tested for packaging obtained from two or more poly(ethylene glycol) polymers of different molecular weights and/or a mixture of poly(lactic acid) and poly(ethylene oxide) [64].
7. Conclusions
In this chapter, data regarding the conditions for flavour obtained by microbial fermentations were presented. The flavour biosynthesis is strongly influenced by growth medium and fermentation conditions and, in addition, is strain dependent. One of the most important factors is the carbon source, which in some cases is flavour precursor. The nitrogen source influences the flavour biosynthesis by the metabolites generated from catabolic degradation pathway. The impact of aeration on the flavour production is significant due to the fact that almost all microorganism strains are aerobic ones and flavour is obtained in the presence of oxygen. The temperatures modulate the amount of flavour biosynthesis, while the mineral composition influences the microbial yield. The knowing of the metabolic pathway leads to the possibility to interfere on the type and the amount of flavour biosynthesised.
The natural aromas obtained by biotechnological routes offer an alternative to the synthetic ones, which appear to be one of the most promising manufacturing techniques for the future.
Acknowledgments
This work was supported by a grant of the Romanian Ministery of Research and Innovation, CCCDI ? UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0697/13PCCDI/2018, within PNCDI III.
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