Sensory characteristics and volatile composition of a cereal beverage fermented with Bifidobacterium breve NCIMB 702257

Abstract

Most of the commercialized lactic acid fermented products are dairy-based. Hence, the development of non-dairy fermented products with probiotic properties draws significant attention within the functional foods industry. The microorganisms used in such products have complex enzyme systems through which they generate diverse metabolites (volatile and non-volatile) that provide significant flavour attributes of importance for fermented foods. The correlation of the volatile flavour compounds of a malt beverage fermented with a Bifidobacterium breve strain with its unique sensory characteristics was performed. The volatile composition analysis exposed the presence of 12 components. Eight of these flavour volatiles were produced through the metabolic activity of the bifidobacteria strain. Notably acetic acid, of reported sour flavour characteristics, exhibited the greatest intensity. Four components of considerable organoleptic characteristics were identified as Maillard-derived products, namely maltol, pyranone, 2 (5H)-furanmethanol and 3-furanmethanol. The sensory evaluation exhibited that the fermented cereal beverage had a sour flavour with mild sweet and malty notes. These results indicate that the volatile compounds identified can be appointed as significant flavour markers of the novel fermented cereal beverage.

Keywords

Fermented malt beverage flavour probiotic volatile compounds

Introduction

The creation of food products with health promoting properties has received significant attention in the past two decades. Awareness of foods that promote gut health has increased and a wide range of probiotic and prebiotic containing products are now available. Probiotic microorganisms belong mainly to the genus Bifidobacterium and Lactobacillus, although Enterococcus, Pediococcus or Saccharomyces strains have also been considered (Rozada-Sánchez et al., 2008). Bifidobacteria extensively colonize the gastrointestinal tract of humans and animals and are commonly related with health promoting effects. The beneficial properties of these bacteria on the intestinal flora, as well as other physiological effects, are well documented and have been thoroughly reviewed (Yonezawa et al., 2010). Specifically, gut health promotion and beneficial effects such as the reduction of abnormal abdominal signs in infants, the promotion of the colonization of beneficial bacteria through the formation of normal intestinal flora in low weight infants, the production of fatty acid-conjugated linoleic acid from free linoleic acid and enhancement of the antigen IgA–antibody activity (Coakley et al., 2003; Kitajima et al., 1997; Li et al., 2004; Picard et al., 2005).

Bifidobacteria strains have been incorporated into a wide variety of food products including fermented dairy products, baby foods, livestock feed supplements and pharmaceutical preparations (Gaggìa et al., 2010; Muto et al., 2010; Sanders and Marco, 2010; Saulnier et al., 2009). Thus, there is great interest in the development of novel probiotic foods containing these lactic acid strains as an alternative to the traditional dairy-based formulation cereals can be used for the production of such products. These raw materials are a good source of carbohydrates, proteins and vitamins and also they support the growth of probiotic bacteria (Charalampopoulos and Pandiella, 2010; Kedia et al., 2008a, 2008b; Rozada et al., 2009).

Consumer acceptance of new food products is strongly determined by the flavour characteristics and is one of the main challenges encountered during the development of novel products (Breslin, 2001). Bifidobacteria have enzyme systems that induce the formation of flavour compounds during the metabolism of substrates. Therefore, cereal-based probiotic products with unique characteristic and flavours can be created through the use of these bacteria. Consumer sensory evaluation is used to asses weather a consumer likes a product, prefers it over another product or finds the product acceptable based on its sensorial characteristics (Earle et al., 2001).

In an earlier study, it was found that Bifidobacterium breve NCIMB 702257 had the ability to grow to probiotic levels (10⁹ viable cells/ml) during the fermentation of a malt-based beverage (Rozada-Sánchez et al., 2008). Furthermore, during the evaluation of the survivability of this bifidobacteria strain in a similar fermented malt beverage stored at different temperatures, it was observed that B. breve maintained its viability at probiotic levels (10⁹–10⁷ viable cells/ml) through a 6-day storage period at 4 ℃ (Rozada-Sánchez et al., 2009). However, the volatile determination and flavour characterization have not been yet reported in cereal-based probiotic beverages fermented with bifidobacteria. The objective of this study was to analyse the volatile compounds and flavour characteristics of a fermented beverage formulated with malt and a Bifidobacterium spp. The flavour attributes of the volatile compounds detected in this beverage were appointed and a sensorial evaluation of the fermented drink was performed in order to identify the relationship between these results and their volatile compounds profile.

Materials and methods

Materials

Microorganism

A strain of Bifidobacterium breve NCIMB 702257 of human origin (infant intestine) was used to perform the fermentation. This was obtained from the National Collection of Industrial and Marine Bacteria (Aberdeen, Scotland).

Preparation of the inoculum

A frozen ampoule containing the strain of bifidobacteria was added directly to sterilized Reinforced Clostridial Medium (RCM) purchased from Oxoid and incubated anaerobically for 20 h at 37 ℃. Stocks were prepared routinely in RCM medium. Before use, the bifidobacteria culture was propagated in sterile malt medium at 37 ℃ for 20 h.

Malt medium

Spray dried malt (Muntons PLC) and yeast extract (Oxoid) were dissolved in distilled water to concentrations of 134 and 10 g/l, respectively. The pH was adjusted to 6.8 ± 0.2 with NaOH (Fisher). The medium was autoclaved at 121 ℃ and 1 bar for 15 min, and aseptically centrifuged at 4500 rpm for 30 min.

Methods

Fermentation process

The bifidobacteria strain was inoculated in the malt media at 1% (v/v). The fermentation was carried out at 37 ℃ for 24 h. Anaerobic conditions in the reactor were obtained by introducing sterile oxygen-free of nitrogen in to the malt medium until the dissolved oxygen reading was close to zero.

Solid phase extraction

Solid phase extraction (SPE) was carried out in order to prepare the sample prior to the gas chromatography–mass spectrometry (GC-MS) analysis. This was done using 1 ml Bakerbond SPE extraction SDB-1 columns (J.T. Baker, Deventer, The Netherlands) packed with 200 mg of SDB. This column was conditioned with 2 ml of methanol and with 2 ml of deionized water. Afterwards, a 2-ml sample of the fermented malt beverage was passed through the cartridge under vacuum. The volatile compounds were eluted with 2 ml of methyl acetate and analysed by GC.

Volatile analysis

Separation of volatiles was performed on a gas chromatograph (6890N series, Agilent Technologies, Santa Clara, CA, USA), coupled with a mass selective detector (5973, Agilent Technologies) fitted with a DB-Wax capillary column (30 m × 0.25 mm internal diameter; 0.25-µm film thickness). Helium was used as a carrier gas, and 2 µl of samples were injected in a splitless mode under the following temperature programme: 7 min at 40 ℃, increased to 200 ℃ at 10 ℃/min and constant at 200 ℃ for 5 min. The compounds were identified by probability-based matching with mass spectra in the MSD Chem Station Library (Data Analysis Application 2003, Agilent Technologies).

Sensory evaluation

The flavour sensations evoked by the fermented cereal beverage were assessed by performing two quantitative descriptive analyses (QDAs) (Lawless and Heymann, 1999). First, the intensity of 12 flavour attributes perceived in the aroma, taste and aftertaste was evaluated by giving a score for these ranging from 0 to 10. A panel of 10 trained judges aged 20–45 years carried out this assay. Each of the samples (35 ml) was placed in 260-ml disposable plastic cups and presented to the panellist. All samples were evaluated once and results were registered on a descriptive test form. Water was served to rinse the palate between samples and the testing sessions were conducted under normal light conditions. In the second QDA test, 21 terms (Table 1) were used to illustrate the flavour properties of the lactic-fermented cereal beverage. The magnitudes of these were scored on an 8-point scale. This test was executed by the sensorial panel previously described, which ranked the samples (35 ml) presented in 260-ml disposable plastic cups. An overall flavour profile of the beverage was assembled throughout the grading of the 21 flavour characteristics leading to the construction of its unique flavour wheel.

Table 1.

Sensory attributes^a selected for the flavour profile determination of the malt beverage fermented with Bifidobacterium breve NCIMB 702257 at 37 ℃ for 24 h

Attribute	Characteristic
Alcohol	Sprint, warming
Malty	Taste associated with malt
Worty	Root, herb-like
Caramel	Toffee, horlicks, biscuit, flavour of caramel
Roast	Burnt toast
Bitter	Taste associated with tonic water, caffeine
Astringent	Harsh and mouth drying
Hoppy	Floral, grassy, citrus
Fruity	Delicate, associated with peach
Apple	Taste associated with fresh apple
Diacetyl	Butter scotch, buttery
Vegetable	Cooked vegetable
Spicy	Burning or stinging sensation
Sulphur	Drain
Meat	Flavour of roasted meat
Medicinal	disinfectant
Sour	Taste associated with citric acid
Metallic	Taste associated with metallic nails
Sweet	Taste associated with sugar or sweetener
Fullness	Thick with body

^aSource: ©Brewlab, University of Sunderland, UK.

Statistical analysis

Data were expressed as mean ± SD and statistically analysed using the one-way ANOVA procedure of Minitab software (Version 15.0). The differences among means were detected by Tukey's honestly significant difference (HSD) test, p < 0.05.

Results and discussion

After fermenting the malt medium for 24 h with the Bifidobacterium breve strain, a turbid deep brown beverage was obtained. From this drink, volatile compound analysis and sensory tests were performed. The aim of the experiments was to correlate the flavour attributes of the volatile compounds detected in the fermented beverage with the flavour profile of the product given by a panel of judges in order to better understand the effect of the volatile compounds detected on the beverage sensory characteristics.

Identification of flavour compounds

The carbohydrate metabolic ability of B. breve allows it to yield acetate and lactate as the main organic acids during fermentation. The presence of these primary flavour metabolites during the fermentation of a similar malt substrate with B. breve after 24 h exhibited final concentrations of 2.9 g/l of lactic acid and 1.9 g/l of acetic acid (Rozada-Sánchez et al., 2008). The presence of secondary flavour metabolites has not been reported in a great manner in non-dairy fermented beverages with bifidobacterial strains. In this study, a total of 12 volatile components were identified in the malt-fermented beverage (Figure 1), which were correlated with their flavour attributes depicted in Table 2. Four of them are related with a sour or pungent flavour, three of them impart sweet- or caramel-like notes, the rest of the compounds interact with the previous components to give the final overall product flavour (Figure 2). The origins of these compounds that play an important role on the flavour quality of the fermented malt beverage have two pathways: the first is the enzymatic activity derived from the bacterial metabolism. The second path is the non-enzymatic reactions produced during the heating process known as the Maillard and Strecker reactions. In this sense, of the compounds identified in the malt beverage, we have previously described, in the malt medium before fermentation, the presence of butanoic acid (synonym: butyric acid) and acetic acid increased after fermentation. Also in such medium, we reported the presence of the Maillard-derived compounds maltol and pyranone (synonyms: DDMP or 4H-pyran-4-one, 2,3-dihydro-3,5-dihydroxy-6-methyl), which decreased after fermentation. Similarly, 2-furanmethanol an isomer of 3-furanmethanol was detected. After fermentation of the malt medium, we described the occurrence of 2-butanone 3-hydroxy also called acetoin (Salmeron et al., 2009). In such work, we did not detect the presence of the alcohols ethanol 2-butoxy and 1,2-ethanediol, nevertheless, the alcohols detected in the malt beverage were formed after fermentation. Therefore, we can state that ethanol 2-butoxy and 1,2-ethanediol were formed during the metabolic activity of B. breve.

Table 2.

Volatile compounds identified in the fermented malt beverage inoculated with Bifidobacterium breve NCIMB 702257 and their respective organoleptic properties

Peak No.	Compound	Organoleptic property	Reference
1	2-Butanone 3-hydroxy (acetoin)	Flowery, wet	Fisher and Scott (1997) and Romano and Suzzi (1993)
2	2-Pentanone 4-hydroxy-4-methyl	Butter, caramel	Fisher and Scott (1997); Howard et al. (2005); Tsakiris et al. (2004) and Ugliano and Moio (2005)
3	Ethanol 2-butoxy	Alcohol, sweet, fruity, sharp	Barron et al. (2005) and Garruti et al. (2006)
4	Acetic acid	Sour, pungent, vinegar	Xu et al. (2007)
5	Propanoic acid	Slightly sour	Dieguez et al. (2002) and Urbach (1995)
6	Butanoic acid (butyric acid)	Sour, rancid, cheesy	Rychlik et al. (2006); Kishimoto et al. (2006); U et al. (2007) and Zea et al. (2001)
7	1,2-Ethanediol	Coffee, burnt	Mancilla-Margalli and Lopez (2002); Petersen et al. (1999) and Siebert (2005)
8	3-Furanmethanol	Roasty, popcorn-like	Alasalvar et al. (2003); Chen et al. (2004) and Mancilla-Margalli and Lopez (2002)
9	2(5H)-Furanmethanol	Sweet, nutty, meaty, burnt	Alasalvar et al. (2003); Chen et al. (2004) and Mancilla-Margalli and Lopez (2002)
10	Maltol	Sweet, burnt sugar, caramel-like	Acree and Teranishi (1993); Lee et al. (2000); Mancilla-Margalli and Lopez (2002) and Savchuk and Kolesov (2005)
11	Pyranone (DDMP)	Toasted	Natali et al. (2006)
12	3,5-Dimethoxyphenol	Astringent, grape-like	Vernin and Vernin (1982) and Dufour and Bayonove (1999)

Among the acids detected in the analysed sample, acetic acid exhibited the highest peak in the chromatogram (see Figure 1). It has been previously described that this metabolite is produced by bifidobacteria during the hexose catabolism (Berdagué et al., 1993; Mantel et al., 1998; Stahnke, 1995). Such metabolic route used by these bacteria to dissipate carbohydrates is known as the Bifidobacterium shunt or the fructose-6-phosphate pathway. The key enzyme is fructose-6-phosphate phosphoketolase (Amaretti et al., 2007; Fandi et al., 2001). This metabolic passage yields acetic acid and lactic acid as major metabolites in a theoretically molar ratio of 3:2, respectively (Caspi et al., 2006; de Vries et al., 1967; Hou et al., 2000; Schramm et al., 1958). From a sensorial point of view, acetic acid imparts an unpleasant ‘vinegar’ flavour while lactic acid is preferred due to its mild acidic flavour (Xu et al., 2007). Previous studies have shown that the deviation of the acetic to lactic acid ratio varies according to different factors such as the bifidobacteria strain employed, the carbon source for bacterial growth or the fermentation conditions (Hou et al., 2000; Ruas-Madiedo et al., 2005; Scalabrini et al., 1998; Tsangalis and Shah, 2004; Van der Meulen et al., 2004). This variation has been proved to be the result of the production of other metabolites such as formic acid and ethanol which limit the generation of lactic acid resulting in an increase of the theoretical ratio of acetic acid to lactic acid (Van der Meulen et al., 2006). Therefore, the production of acetic acid could potentially be modulated to achieve a concentration that will impart a more pleasurable taste to the fermented product through the variation of substrates available for the bifidobacteria strain to metabolize and by testing different Bifidobacterium strains.

The compounds butanoic acid, propanoic acid, 3,5-dimethoxyphenol, 2-butanone 3-hydroxy (acetoin), ethanol 2-butoxy, 1,2-ethanediol and 2-pentanone 4-hydroxy-4-methyl exhibited chromatographic peaks of less intensity, however, they contribute in the same manner to the flavour of the fermented malt beverage.

Butanoic acid is a metabolite of controversial sensory properties since it has been described to play an important role in the typical flavour of fermented milk products such as yoghurt where it imparts a favourable ‘sour’ note (Rychlik et al., 2006). However, in beverages such as cider, beer or Spanish sherry-type wines, it has been described as an undesirable compound and has been defined as posing a ‘rancid’ and ‘cheesy’ aroma (Kishimoto et al., 2006; U et al., 2007; Zea et al., 2001).

Propanoic acid has been reported to contribute to the acidic and glue-type odours of a grape distillate spirit. In this case, the generation of this acid was throughout the oxidative deamination of free amino acids (Dieguez et al., 2002; Urbach, 1995).

The component 3,5-dimethoxyphenol belongs to the chemical group of phenols, substances that are the main contributors to the bitter taste and odour in wines (Vernin and Vernin, 1982). These compounds have attracted much attention in wine processing because of their ability to interact with proteins to form astringent flavours, which provide a sensation of puckering inside the mouth determinant to the sensory quality of this product (Dufour and Bayonove, 1999; Fisher and Scott, 1997).

The alcohols 2-butanone 3-hydroxy (acetoin), ethanol 2-butoxy and 1,2-ethanediol belong to a compound group that has been described as having arguable sensory characters. Favourable notes are ‘piney’ and ‘caramel’ and unfavourable notes include ‘bitter’ and ‘medicinal’ (Fisher and Scott, 1997). Acetoin in particular is one of the most important compounds in this group as it has been described as a significant flavour constituent in wine where its sensorial attributes have been described as ‘flowery’ and ‘wet’. The production of this metabolite could be linked to the citrate metabolism of the probiotic bacteria (Romano and Suzzi, 1993). Ethanol 2-butoxy has been identified in fermented milk and fruit products where it is described to embrace ‘alcohol’, ‘sweet’, ‘fruity’ and ‘sharp’ odour characteristics (Barron et al., 2005; Garruti et al., 2006). The volatile 1,2-ethanediol has been reported as a flavour compound detected in alcoholic beverages such as tequila and beer, and it has been described to possess a ‘coffee’ and ‘burnt’ odour characteristic (Mancilla-Margalli and Lopez, 2002; Petersen et al., 1999; Siebert, 2005).

Finally, the component 2-pentanone 4-hydroxy-4-methyl belongs to the chemical class of ketones that have been reported to encompass a ‘butter’, ‘caramel’ sensory characteristic. It has also been described as a flavour component in the complex wine aroma (Blom et al., 1999; Fisher and Scott, 1997; Howard et al., 2005; Tsakiris et al., 2004; Ugliano and Moio, 2005).

The second group of volatile components detected (depicted in Figure 1) were not produced throughout the enzymatic reactions linked to the fermentation process as they were conceived during the thermal processes involved in the malting process and beverage preparation. In this case, the breakdown of carbohydrates, proteins and lipids occurs giving way to the non-enzymatic browning mechanisms known as Maillard and Strecker reactions, which play an important role in the formation of a wide number of important volatile flavour components. Among the most important compounds originated from the thermal breakdown of sugars is the α-pyron derivate maltol (3-hydroxy-2-methyl-4H-pyran-4-one), identified in the fermented malt beverage under study. This compound has been defined as the characteristic impact compound in malt. In the same matter, it has been described as an important flavour element in several products such as caramel, coffee, roasted filberts, wine, beer, tequila, cognac spirits, scotch whiskies and fermented dairy products. This compound has a sweet, burnt sugar, caramel-like aroma and is formed by the heating of hexose reductone produced in the transformations of Amadori products (Acree and Teranishi, 1993; Lee et al., 2000; Mancilla-Margalli and Lopez, 2002; Savchuk and Kolesov, 2005; Vernin and Vernin, 1982). Pyranone (synonyms: 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one or DDMP) has been described among the substances responsible for the ‘toasted’ notes which contribute to the organoleptic characteristics of wood-aged alcoholic beverages (Natali et al., 2006). It has also been detected in Madeira wines where it is described to be responsible for the generation of potent aromas originating a combined flavour of ‘cinnamon’ and ‘dry fruits’ (Alves et al., 2005). Finally, the substances 2(5H)-furanmethanol and 3-furanmethanol belong to the compound class of furans, which posses a wide range of sensory properties and are associated with caramel-like, sweet, fruity, nutty, meaty and burnt odour impressions (Alasalvar et al., 2003; Chen et al., 2004; Lopez and Mancilla-Margalli, 2000).

Sensory evaluation

Descriptive test

The aroma, taste and aftertaste evaluation of the fermented malt beverage was done via the assessment of 12 flavour attributes performed by trained panellists. The mean values of the flavour attributes evaluated are presented in Table 3. In general, the sensory data indicated that malty and roast flavour notes were distinguishable during the three stages (aroma, taste and aftertaste) since their scores were among the highest during such tests. The sour attribute was strongly perceived during the taste and aftertaste assessment. In particular, during the aroma assay, the beverage encompassed a significantly intense malty note with milder caramel, roast and vegetable attributes. The taste trial indicated that the fermented malt beverage had a strong sour, malty and roast flavour with moderate diacetyl, caramel and vegetable flavour attributes. The aftertaste assessment showed that the most intense attributes perceived were sour and malty notes. These results can be compared with those reported during the sensory evaluation of a whey-based beverage fermented with a mix culture containing a Bifidobacterium bifidum strain, where it was clear that the main flavour descriptors of the product were acidity and sweetness (Hernandez-Mendoza et al., 2007). During the fermentation of soy milks with Bifidobacteria spp., a product containing added glucose encompassed a moderate sour taste compared with the formulation deprived of this sugar (Božanić et al., 2008). In addition, the fermentation of two different vegetable juices with a Bifidobacterium culture gave products with a reasonable sour flavour. However, the product that had a greater sugary taste was the most preferred and was described as having an overall pleasant sweet-sour taste (Moraru et al., 2007). Therefore, it is clear that during the formulation of fermented beverages including Bifidobacterium strains, products of a significant sour taste will be obtained. Nevertheless, when the sweet character is promoted higher product acceptability can be achieved.

Table 3.

Descriptive test values* of 12 flavour attributes perceived during the evaluation of aroma, taste and aftertaste of a fermented malt beverage formulated with Bifidobacterium breve NCIMB 702257

Perceived intensity
Flavour attribute	Aroma	Taste	Aftertaste
Malty	7.6 ± 1.14b	5.4 ± 0.55b	4.6 ± 1.14c
Roast	4.2 ± 1.48a,b	4.4 ± 0.84b	3.4 ± 0.55b
Caramel	2.0 ± 0.61a,b	3.2 ± 0.45a,b	2.2 ± 0.45a,b
Hoppy	0.4 ± 0.89a	0.8 ± 0.21a	0.6 ± 0.15a
Fruity	1.6 ± 0.55a	1.6 ± 0.42a	0.5 ± 0.10a
Vegetables	4.8 ± 1.64a,b	3.2 ± 1.30a,b	3.0 ± 0.71b
Diacetyl	1.4 ± 0.55a	4.0 ± 1.22a,b	0.8 ± 0.05a
Bitter	ND	2.6 ± 0.55a,b	1.8 ± 0.45a,b
Astringent	ND	2.8 ± 0.45a,b	3.0 ± 0.55b
Metallic	ND	2.8 ± 0.84a,b	1.4 ± 0.45a,b
Yeasty	1.0 ± 0.09a	1.0 ± 0.26a	1.1 ± 0.40a
Sour	1.6 ± 0.55a	5.8 ± 1.48b	4.6 ± 0.89c

*Values are expressed as mean ± SD. Means in the same column with different letters are significantly different (p < 0.05) according to Tukey’s multiple comparison method. ND = not detected.

Flavour profile

A flavour wheel of the malt beverage fermented with Bifidobacterium breve is shown in Figure 2. The flavour wheel shows that the drink had a strong sour flavour with a sweet and malty character. The perceived strong sour flavour could be an organoleptic characteristic decisive in the acceptability of the functional beverage. The organic acids acetic and lactic can be considered to play a key role in the final flavour of this product, lactic acid being preferred to acetic acid as it provides a sweeter acidic taste that is generally more favourable to the palate. An alternative to ease the concentrations of acetic acid, which promotes the sour taste, could be the use of mix cultures. Since it has been reported that during the use of mixed cultures containing bifidobacteria and lactic acid strains in yoghurt formulations, the sensory quality of these were greater when compared to a product manufactured with single cultures of bifidobacteria (Samona et al., 1996). The enhancement of the malty and sweet attributes could as well improve the flavour of the potentially probiotic malt beverage. This could be accomplished through the addition of fermenting sugars such as glucose, which has been previously reported to lower the sour flavour and improve the sugary taste preferred by most consumers (Božanić et al., 2008; Moraru et al., 2007).

Conclusions

A fermented cereal beverage of a sour flavour with sweet notes was obtained during the inoculation of a malt medium with a B. breve strain. Chemical compounds such as organic acids, alcohols, furans and ketones merged to provide its unique taste and aroma. The organoleptic attributes of the volatile compounds detected gave way to an overall flavour profile similar to the one originated during the sensory assessment. In these sense, the flavour volatiles detected in this work can be used as sensory markers during further development of the fermented malt beverage. These results suggest that the information obtained by sensorial experiments and instrumental analysis could be used to create novel functional products with acceptable organoleptic properties.

Footnotes

Acknowledgments

We thank the National Council on Science and Technology of México (CONACYT) for giving the award of the PhD studentship (Reg. No. 197235) to I.S. We also sincerely thank Marie Curie Fellowships for the research grant (FP6) given to R.R.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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