Production, Purification, and in Vitro Evaluation of the Prebiotic

Article pubs.acs.org/JAFC

Production, Purification, and in Vitro Evaluation of the Prebiotic Potential of Arabinoxylooligosaccharides from Brewer’s Spent Grain Belén Gómez,† Beatriz Míguez,† Adán Veiga,‡ Juan Carlos Parajó,† and José Luís Alonso*,† †

Department of Chemical Engineering, University of Vigo (Ourense Campus), Polytechnical Building, As Lagoas, 32004 Ourense, Spain ‡ Customdrinks, Polígono Industrial Os Acivros, Parcela C-1, Chantada, Lugo 27500, Spain ABSTRACT: Brewer’s spent grain (BSG) samples were subjected to a two-step aqueous processing (starch extraction and autohydrolysis) in order to assess their potential as a raw material for obtaining a mixture of arabinoxylooligosaccharides (AXOS) suitable to be use as prebiotics for elderly. After hydrothermal treatment, the liquors were refined by a sequence of purification and conditioning steps including membrane filtration, enzymatic hydrolysis, and ion exchange. The presence of both substituted (degree of polimerization (DP) = 2−10) and unsubstituted (DP = 2−16) oligosaccharides made up of xylose and arabinose (AXOS) were confirmed in purified mixtures (in which total OS content = 84% w/w) by using chromatographic techniques and matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS). Finally, AXOS were evaluated for their prebiotic activity by in vitro fermentation assays using fecal inocula from elderly people, demonstrating that AXOS were slightly better substrates than FOS, in terms of bacterial population shifts as in the production of SCFA. KEYWORDS: prebiotics, elderly, brewer’s spent grain, arabinoxylooligosaccharides, FISH

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INTRODUCTION Prebiotics are carbohydrates that cannot be absorbed or digested by humans but that can be fermented by the human gut microbiota, favoring the proliferation of some species over others and generating benefits to overall health.1 The complex role of the human intestinal microbiota is under study, and its functions are now more and more established when considering energy metabolism, nutrient digestion, vitamin synthesis, epithelial defenses, and immune responses.2 The study of functional foods with prebiotic properties may be especially suitable for elderly people because they tend to practice inappropriate eating habits, and because of the increasing longevity observed in our society, is important to follow a balanced diet.3 (Recent statistics show that in the European Union in 2008 17% of the population is 65 years old or older.) There is a correlation between aging, chronic diseases, changes in the composition of the intestinal microbiota, and the host immune system.4 Under normal conditions, the total number of anaerobic bacteria seems to remain relatively constant in older people, but the composition of the microbiota changes with age. It may change in response to various factors, including medication, gastrointestinal infections, and diet. Human studies that have examined the composition and alterations in the intestinal microbiota during aging have shown a decrease in the number of bifidobacteria and an increase in the levels of enterobacteria, lactobacilli, and some species of Clostridium.5 Woodmansey et al.6 indicated that antibiotic-treated elderly had significantly lower total counts in bifidobacteria and markedly higher clostridial diversity compared to two other groups: healthy young and healthy elderly. In addition, healthy elder patients were characterized by lower total bacteroides and lactobacilli numbers than those in healthy young. However, Claesson et al.7 reported a greater proportion of Bacteroides spp. in elderly subjects than in younger adults. © XXXX American Chemical Society

Changes in gastrointestinal bacteria due to age might result in increased putrefaction in the colon and a greater susceptibility to diseases, such as gastroenteritis or Clostridium difficile infection,6 and prebiotic consumption could be a suitable strategy to improve the intestinal microbiota in the elderly and, as a consequence, their health. Inulin, lactulose, galactooligosaccharides (GalOS), or fructooligosaccharides (FOS) are already considered prebiotics because there is enough scientific evidence, and they are commercially available. However, others such as xylooligosaccharides (XOS) are gaining importance in this field. XOS are considered nondigestible oligosaccharides by gastric or pancreatic enzymes, reaching the colon intact after oral intake but able to be used by a selected group of beneficial gut microflora for exhibiting several physiological changes.8−10 The preferential fermentation of XOS by bifidobacteria, which may affect the human gastrointestinal tract beneficially, has been demonstrated via in vitro and in vivo experiments.8,11,12 Thus, evidence for the prebiotic capacity of XOS is promising, and XOS are already commercially available although further studies are required for obtaining more scientific evidence. XOS can be obtained from plant biomass by chemical treatments, autohydrolysis, enzymatic hydrolysis, or a combination of these processes.9 In this context, Brewer’s spent grain (BSG) is a fiber-rich byproduct generated in beer factories at high volume and low cost,13−15 and it could be used as raw material for AXOS production because it contains about 42% w/w of polymeric/oligomeric material mainly composed of xylose.12 Received: June 25, 2015 Revised: September 7, 2015 Accepted: September 7, 2015

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DOI: 10.1021/acs.jafc.5b03132 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry

Figure 1. Scheme of the process after obtaining the optimal autohydrolysis conditions.

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When BSG is subjected to hydrothermal treatment under selected conditions, hemicelluloses are mainly broken down into oligosaccharides (OS) with a variable degree of polymerization.16−18 Afterward, if AXOS with a low degree of polimerization (DP) are desired, the enzymatic hydrolysis is a suitable alternative because it is possible to control the reaction, limiting the formation of undesirable products. In addition, no chemicals are used in the process, which is in line with the green chemistry perspective.11 However, in addition to AXOS production, other effects occur during the process (including extractives solubilization, monosaccharides production, sugar decomposition product generation, and lignin depolymerization). Because of this, if the AXOS are to be used for food purposes, then the autohydrolysis liquors need to be refined to remove these nondesired impurities. This goal can be achieved by a variety of technologies including liquid−liquid extraction, membrane filtration, or ion exchange.18−20 The aims of this work were (1) to evaluate the production of AXOS from BSG by hydrothermal treatment, (2) to modify and refine the AXOS obtained by a combined process of enzymatic hydrolysis, membrane filtration, and ion exchange, and (3) to evaluate the effects of the final purified product on the composition of the colonic microbiota and organic acids production (SCFA and lactate) using fecal inoculum from elderly.

MATERIALS AND METHODS

Raw Material. BSG samples were provided by Customdrinks (Galicia, Spain), dried at room temperature, homogenized in a single lot, and stored. Starch Extraction and Hydrothermal Processing. Two types of pretreatments were assayed to remove or, at least, to reduce the starch content of BSG: (i) an aqueous treatment at 100 °C for 30 min and (ii) a nonisothermal water extraction (from 60 to 130 °C in 9.3 min). In both cases, BSG was mixed with distilled water at a liquid−solid mass ratio of 8:1 g/g (oven-dry basis) and introduced into a stirred, stainless-steel reactor (Parr Instr. Co., Moline, IL, USA). Then, the reactor was heated up to achieve the desired temperature, and when the treatment was finished, the suspension was cooled and centrifuged. The solid phases from this first treatment were air-dried and stored until use. Solids resulting from starch extraction were mixed with distilled water at a liquid−solid mass ratio of 8:1 g/g and subjected to a hydrothermal treatment under nonisothermal operation up to maximum temperatures of 180, 190, 195, 200, 210, or 220 °C. Once the temperature was achieved, the reactor was cooled and opened, and the liquors recovered by centrifugation were analyzed and processed as described below. With the aim of facilitating the comparison between different reaction conditions, the severity factor (R0) of each treatment was calculated according to Overend and Chornet.21 This parameter combines the effects of time and temperature caused by isothermal or nonisothermal processing; it is defined as B


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Journal of Agricultural and Food Chemistry R0 =

∫0

t

⎡ T (t ) − 100 ⎤ exp⎢ ⎥ dt ⎣ 14.75 ⎦

Analytical Methods. Aliquots from BSG and spent solids from hydrothermal treatments were subjected to moisture and ash determinations (methods ISO 638 and ISO 776, respectively) and to quantitative acid hydrolysis (method TAPPI T13m). The HPLC analysis of liquors was carried out using a 1100 series Hewlett-Packard chromatograph fitted with a refractive index detector and a Aminex HPX- 87H column from Biorad (Hercules, CA, USA) operating at 50 °C and using as mobile phase H2SO4 0.003 M (flow rate = 0.6 mL/ min). In addition, uronic acids (UA) were determined by the method of Blumenkrantz and Asboe-Hansen25 using galacturonic acid (Fluka, parent company of Sigma-Aldrich, St. Louis, MO, USA) as a standard for quantification. Elemental nitrogen was determined with a Thermo Finnegan Flash EATM 1112 analyzer, using 130 and 100 mL/min of He and O2 and an oven temperature of 50 °C. Protein content was obtained by multiplying the elemental N content by 6.25. The starch content was quantified with an enzymatic kit (Boheringer-Mannheim, R-Biopharm, Darmstadt, Germany). All determinations were made in triplicate. In contrast, samples of liquors were filtered through 0.45 μm cellulose acetate membranes and assayed for the glucose, xylose, arabinose, acetic acid, formic acid, hydroxymethylfurfural (HMF), and furfural content by HPLC following the method described above. Moreover, an aliquot of the liquors were subjected to quantitative posthydrolysis (with 4% sulfuric acid at 121 °C for 20 min) before HPLC analysis in order to determine the concentrations of oligomers (GlcOS, XOS, and AraOS) present in the media. (The increase in the concentrations of monosaccharides caused by the posthydrolysis is a measure of the oligomers content.) In a similar way, the increase in acetic acid concentration allows us to measure the amount of acetyl groups (AcO) linked to oligosaccharides. Additionally, uronyl substituents were determined by the same method already cited for the raw material, using galacturonic acid as a standard for quantification. Nonvolatile compounds (NVC) were measured by oven-drying at 100 °C until constant weight, whereas the content of impurities (other nonvolatile compounds, ONVC) were calculated as follows: ONVC = (NVC − MS − OS)/ NVC. All the analyses were made by triplicate. The structural characterization of the refined product (stream R in Figure 1) was carried out by high-performance size-exclusion chromatography (HPSEC) using an Ultimate 3000 system (Dionex Corp., Sunnyvale, CA, USA) and by matrix-assisted laser desorption/ ionization-time-of-flight mass spectrometry (MALDI-TOF MS) using an Ultraflex workstation (Bruker Daltonics, Bremen, Germany). More details can be found in the article by Gómez et al.23 Cells were harvested by centrifugation (10 000 rpm), and the supernatants were filtered through 0.20 μm cellulose acetate membranes (Sartorius Stedim Biotech, Germany) before HPLC analysis for monosaccharides, oligosaccharides, formate, lactate, succinate, and SCFA (acetic, propionic, and butyric acids) as well as pH measurement. Cell pellets were used for counting the selected bacterial groups by FISH.20

(eq 1)

(where t and T(t) are the reaction time and the temperature, respectively) and gives a simplified, empirical interpretation of the complex chemistry of polysaccharide degradation in aqueous media.22 Because treatments with similar R0 values should provoke similar effects on the reaction products, a treatment was carried out under isothermal conditions at T = 180 °C and t = 12.2 min, a set of operational conditions that corresponds to a severity factor = 4439.8 min, a value identical to the one calculated for the experiment carried out under selected nonisothermal conditions where the Tmax was 195 °C. Processing of Autohydrolysis Liquors. Liquors from the hydrothermal treatment were subjected to membrane filtration (Amicon stirred cell, model 8400, Millipore, Darmstadt, Germany) using the experimental device described by Gómez et al.23 However, in this work, the operation was carried out only in diafiltration mode (two stages) at room temperature using a transmembrane pressure of 3 bar, as indicated in Figure 1. Retentate from the second diafiltration stage (stream D in Figure 1) was treated with a commercial endoxylanase (Shearzyme 2×) kindly provided by Novozymes-Spain (more details in Gullón et al.).24 The xylanases were added to the retentate at the desired enzyme loading (150 XU/kg of liquor), and the solution was kept at 40 °C in an agitated vessel (120 rpm). Hydrolysis was carried out for 48 h, and no buffer was added to the medium. Liquors from the enzymatic hydrolysis (Figure 1) were treated with Amberlite IRA 400 (a strong-anion-exchange resin) and Amberlite 200 (an acidic cation-exchange resin) to reduce the content of ionic impurities. Both resins were supplied by Sigma-Aldrich (St. Louis, MO, USA). In both cases, liquors and resin were contacted for 24 h with gentle agitation at room temperature using a liquor/resin mass ratio of 10:1 g/g (Amberlite IRA 400) and 15:1 g/g (Amberlite 200). At the end of treatment, the spent resins (streams H and N) were washed with distilled water using a RLS of 2 g of water per gram of resin in order to recover those compounds that could have been retained. The resulting stream (denoted as R) was freeze-dried and stored until use in fermentation experiments. Fermentation of AXOS Mixtures. Samples of AXOS were fermented using fecal inocula obtained from three healthy elder volunteers (age > 60 years). Fecal samples were collected in sterile vials, kept in an anaerobic cabinet, and used within a maximum of 2 h after the collection. The fecal inoculum (FI) was prepared by dilution in a reduced physiological salt solution (RPS; cysteine-HCl 0.5 g/L and NaCl 8.5 g/L) ratio of 100 g of feces to 1 L of RPS at pH 6.8. FOS (Sigma-Aldrich, St. Louis, MO, USA) and a medium without any carbon source were included in the study as positive and negative controls, respectively. The fermentation process was developed according to Gullón et al.20 An assay was carried out per donor for each substrate (FOS or AXOS), and the data shown correspond to an average of the results from the three different donors (n = 3). Changes in human fecal bacterial populations were assessed by FISH using the following 16S rRNA probes: Lab158 (Lactobacillus/ Enterococcus group), Bif164 (Bifidobacterium genus), Bac303 (Bacteroides/Prevotella group), and His150 (clusters I and II of Clostridium), all of which were purchased from Tib Molbiol, Berlin, Germany. For the total cell counting, samples were stained with 4′,6-diamidino-2phenylindole (DAPI) from Sigma-Aldrich (St. Louis, MO, USA). Bacterial counts were carried out at fixed fermentation times using a fluorescence microscope (Olympus BX41, Tokyo, Japan). One hybridization was made per analyzed sample (one sample was obtained per donor, substrate, and time) and a minimum of 15 fields were counted per hybridized sample. Differences between bacterial populations and organic acid concentrations were tested using one-way analysis of variance (ANOVA) and Tukey’s post hoc test. Differences were considered significant when p < 0.05.

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RESULTS AND DISCUSSION Aqueous Extraction and Autohydrolysis. Starch lacks prebiotic potential, and its presence in liquors is a drawback if a high-purity final product is desired. Therefore, a pretreatment is needed to remove it or, at least, to reduce its content in the raw material.26 Moreover, starch could be used for a variety of processes. Two alternatives (isothermal and nonisothermal aqueous treatment) were assayed, resulting in similar solid yields (∼76%) and starch removal percentages (85%), although small losses of target compounds were also found to occur. Taking this into account, an isothermal pretreatment at 100 °C for 30 min was chosen because lower energy consumption was expected. The composition of both BSG and pretreated solid is shown Table 1. As can be seen, glucan, xylan, acid insoluble residue (AIR), and protein were the major components in both untreated and pretreated solids. As a C


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2b, and maxima of XOS content and their main substituents (uronic acids and acetyl groups) were observed near Tmax= 200 °C. At 200 °C, up to 77% of the xylan was hydrolyzed into XOS, whereas the percentage of acetyl groups remaining as substituents of XOS exceeded the 60%. Regarding the total OS yield (excluding GlcOS), a maximum of 13.7 g per 100 g of dry BSG was achieved at 195 °C. This value was then selected as the optimum, and it is in agreement with the results published by Carvalheiro et al.16 Figure 3a shows a fast increase in the acetic acid, formic acid, furfural, and HMF concentrations, which starts at 195 °C and is

Table 1. Chemical Composition of BSG and the Pretreated Solid Obtained by Aqueous Extraction at 100 °C for 30 min component

BSG (%)

pretreated solid (%)

glucan xylan arabinan acetyl groups UA AIR starch protein ashes

24.91 13.94 6.68 0.80 6.45 22.82 9.42 26.77 2.98

19.73 17.43 7.99 0.88 6.13 20.77 2 24.10

result of the aqueous extraction, a solid enriched in xylan and arabinan with lower starch content was obtained. Following the process scheme (Figure 1), pretreated solids were subjected to hydrothermal processing, first under nonisothermal conditions (until reaching a temperature of 180, 190, 200, 210, or 220 °C) and second under isothermal conditions (at 180 °C), as explained above. Figure 2 shows the dependence of monosaccharide and oligosaccharide concentrations on the maximum temperature of the treatment. As can be seen, the xylose concentration increased significantly from 195 to 210 °C, as a result of hydrolysis of oligomers, whereas arabinose showed a maximum at 200 °C and decreased rapidly. The evolution of the oligosaccharides content with temperature is shown in Figure

Figure 3. (A) Dependence of the concentrations of organic acids and furans in autohydrolysis liquors. (B) Temperature dependence of the concentrations of NVC and ONVC in reaction liquors (NVC, nonvolatile compounds; ONVC, other nonvolatile compounds or impurities).

partially due to the sugar decomposition. As a consequence of the increased severity, a rapid increase in the ONVC impurities concentration can be observed in Figure 3b. Meanwhile, the NVC content shows a maximum at 200 °C (0.048 g/g of liquor) and then decreases because of the decomposition reactions that give volatile compounds as products. These results compared favorably with the ones obtained from barley husks and wheat straw by Garrote et al.27 and Carvalheiro et al.,28 respectively. Operating under isothermal conditions at the same severity (T = 180 °C and t = 12.2 min), 12.8 g of OS (OS = XOS + AraOS + AcO + UA) were obtained from 100 g of dry BSG. As indicated in Table 2, these liquors (stream A) contained 8% of MS, 61% of OS (including GlcOS), and 31% of ONVC. Most of the ONVC could correspond to products derived from the protein fraction, being necessary to develop a refining process. Purification of Liquors. Figure 1 shows the purification scheme proposed for reducing the average DP of AXOS and to increase the purity of the final product. Membrane filtration is a

Figure 2. Effect of temperature on the composition of autohydrolysis liquors: (A) monomers expressed in g/L and (B) oligomers expressed in g/L. D


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Journal of Agricultural and Food Chemistry Table 2. Chemical Composition and Material Balances of Streams in Figure 1a streams of the process A

D

F

G

L

M

R

2.17 0.045 0.008 0.013 0.002 0.053 0.500 0.136 0.018 0.040 0.066 0.747 0.187

1.79 0.054 0.010 0.015 0.000 0.052 0.570 0.154 0.021 0.047 0.079 0.844 0.077

1.63 0.055 0.010 0.015 0.000 0.053 0.562 0.153 0.021 0.048 0.080 0.837 0.083

b

NVC glucose xylose arabinose acetic acid GlcOS XOS AraOS AcO UA MS OS ONVC recovered liquors NVC glucose xylose arabinose acetic acid GlcOS XOS AraOS AcO UA MS OS ONVC

4.20 0.008 0.017 0.055 0.010 0.104 0.345 0.106 0.018 0.033 0.080 0.606 0.314 100.00 4.20 0.03 0.07 0.23 0.04 0.44 1.45 0.45 0.08 0.14 0.33 2.56 1.31

3.12 0.004 0.006 0.012 0.002 0.117 0.392 0.109 0.020 0.038 0.022 0.676 0.302 99.19 3.09 0.01 0.02 0.04 0.01 0.36 1.21 0.34 0.06 0.12 0.07 2.09 0.93

Chemical Composition 3.05 2.51 0.046 0.046 0.008 0.008 0.011 0.012 0.001 0.002 0.044 0.053 0.416 0.500 0.115 0.138 0.020 0.018 0.038 0.041 0.065 0.066 0.633 0.750 0.302 0.184 Material Balancesc 98.84 92.00 3.01 2.31 0.14 0.11 0.02 0.02 0.03 0.03 0.00 0.01 0.13 0.12 1.25 1.16 0.35 0.32 0.06 0.04 0.12 0.10 0.20 0.15 1.91 1.73 0.91 0.43

110.85 2.41 0.11 0.02 0.03 0.01 0.13 1.20 0.33 0.04 0.10 0.16 1.80 0.46

103.82 1.85 0.10 0.02 0.03 0.00 0.10 1.06 0.29 0.04 0.09 0.15 1.57 0.14

117.97 1.92 0.11 0.02 0.03 0.00 0.10 1.08 0.29 0.04 0.09 0.15 1.61 0.16

a

MS, monosaccharides; OS, total oligosaccharides including acetyl groups and uronic substituents. bExpressed as mass fractions (kg NVC/kg stream), except for NVC, which is expressed as kg NVC/100 kg stream. cOn the basis of 100 kg of autohydrolysis liquors obtained under optimal conditions: 180 °C and 12.2 min.

Enzymatic Hydrolysis. Because the DP distribution of AXOS is an important factor affecting their biological properties, the aim of the second step was to reduce the average DP of the oligomers by partial enzymatic hydrolysis using endoxylanases. As can be seen in Table 2, the content of the main part of the components remained almost constant. Some variations were observed for GlcOS and glucose concentrations, demonstrating the presence of cellulase activity in the enzymatic preparation. Ion Exchange. As observed in previous studies, significant removal of impurities can be reached when XOS-containing liquors are treated by ion-exchange resins.26,29,32 Taking this into account, an aliquot of stream F (liquors from the enzymatic hydrolysis) was subjected to ion exchange using Amberlite IRA400 (an anion exchange resin) under the conditions mentioned above. As a result of the treatment, a decrease of the ONVC by >50% and losses below 10% of total OS were achieved. The OS losses could be reduced to a value of 6% if the washing waters generated during the resin regeneration stage were recovered and added to the stream G to give stream L. To increase the purity of the final product, stream L was treated with a cationic exchange resin generating stream M, which was joined with stream O to give the final refined product (stream R, Table 2). The percentages of product recovery in stream R were 80.3% for GlcOS, 89.9% for XOS,

simple technology that has many advantages: it is energetically efficient, not harmful solvents are needed, it is easily scaled-up, and it was even employed successfully for the purification, fractionation, and/or concentration of oligosaccharides.18,23 In contrast, ion exchange allows change of the ionic composition of a certain solution without introducing undesirable substances. Even though these resins are expensive, they can be reutilized after a simple regeneration with NaOH or HCl and distilled water. The scaling-up of this technique is not difficult, and it is used in several industrial applications. Moreover, ion exchange was also successfully applied to the purification of autohydrolysis liquors.19,26 Table 2 lists compositional data of the streams involved in the process as well as the mass balances. Diafiltration. First, autohydrolysis liquors were diluted with distilled water and concentrated to their original volume using a 1 kDa molecular weight cutoff membrane. This sequence was repeated twice. As a result of this treatment, the concentrations of unwanted compounds (monosaccharides and ONVC) decreased to 79.4 and 28.5%, respectively. In contrast, high recoveries were achieved for oligomers and substituents (83.6% for XOS, 75.7% for AraOS, 81.6% for AcO, and 84.2% for UA). These results confirm the suitability of this technology for refining autohydrolysis liquors as mentioned in the literature.29−31 Nevertheless, a higher degree of purity is desired because the ONVC content of the stream D is still 30%. E


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carried out using fecal samples obtained from three volunteers over 60 years old. SCFA production and pH Evolution in the Fecal Cultures. As is well-known, a variety of the beneficial effects of prebiotics are related to SCFA production and pH reduction in the gut. As shown in Table 3, almost no differences were found between the pH values observed with FOS and AXOS. The decrease of pH was more pronounced during the first 11 h of fermentation, in concordance with the increases in the concentrations of SCFA and with the largest percentage of total OS consumption (Table 4), especially for GlcOS that were consumed up to 90% of their initial amount. In contrast, longer fermentation times resulted in slow decreases in the concentration of substrates, suggesting that some products could have low fermentability and, as a consequence, could reach distal parts of the colon. In fact, some oligomers (10.7 and 17.3% of the initial XOS and AraOS, respectively) remained in the media at the end of fermentation time assayed (45 h). In agreement with our results, a recent study on the prebiotic activity of refined AXOS26 reported that some XOS and AraOS were not depleted at the end of fermentations (33 h). The incomplete consumption of these compounds could be attributed to the presence of high-DP linear chains because the acetyl groups were metabolized after 7 h, and according to Reis et al.,36 smaller and highly ramified arabinoxilooligosaccharides are preferably consumed to linear fragments. However, this was not confirmed in this work, and it also should be taken into account that the fermentation pattern adopted by gut microbiota not only depends on the substrate structure but also on the composition of the initial microbiota. Furthermore, Table 3 also details the generation profiles of SCFA (acetate, propionate, and butyrate), lactate, and formate. As can be seen, the evolution of the total SCFA content in media containing AXOS was similar to that observed with FOS, and their production mainly took place along the period of 0− 11 h (as a result of consumption of the OS). At 29 h, the total organic acids production observed for AXOS was basically acetic acid ≫ butyric acid > propionic acid. In a previous work using XOS derived from rice husks, fermentation times longer than 9 h also resulted in increased SCFA production and a similar product profile.29 By contrast, a different profile was observed for commercial FOS because lower amounts of acetic acid and significant amounts of lactate (11.6 mM) and formate (19.1 mM) were achieved in the medium. Lactate was usually detected at low concentrations in cultures containing OS, even disappearing in a second fermentation stage (from 11 h), but it is often found at higher concentrations in FOS cultures.23,37,38 Specifically, the maximum significant amount of lactic acid quantified in this study was 13.96 mM using FOS at 11 h. It has been described as behaving as an intermediary product rapidly converted into SCFAs by the cross-feeding of other bacteria,36,39,40 which is the likely cause of its decreasing concentration. In addition to the differences in the organic acids profile, a significant difference (125.04 vs 104.32 mM) was found in total acid concentrations between both substrates at the last time assayed. This fact is in agreement with previous studies; for instance, Reis et al.36 obtained SCFA average values of 108.4 and 80.6 mM when using arabinoxylan from BSG and FOS, respectively. In the same way, Gullón et al.26 determined after 33 h of fermentation an average total content of SCFA of 95.0 and 75.6 mM when employing as substrates AXOS from wheat bran and FOS, respectively.

89.9% for AraOS, 93.0% for AcO, and 95.8% for UA, but were just 32.6% for ONVC. As can be seen in Table 2, a final product containing 84% of substituted oligosaccharides (XOS, 56.2%; AraOS, 15.3%) and 8% of nonvolatile impurities was obtained by applying this processing scheme. These results are in agreement with other previous related studies.29,32 Structural and Chemical Characterization of the Final Product (Stream R). As a result of the process reflected in Figure 1, AXOS-enriched samples were obtained and freezedried in order to obtain a solid product that was evaluated for their prebiotic potential. However, in order to obtain more information about the composition of the product, samples of stream R were further characterized by HPSEC and MALDITOF MS. It must be taken into account that the structural features of OS affect their prebiotic potential as confirmed in the literature. Sarbini et al.33 indicated that low-molecular-mass OS are often more selectively fermented by bifidobacteria and lactobacilli than are their parent high-molecular-weight carbohydrates. According to Sanchez et al.,34 who used a human colon model system, the chain length of the AXOS is critical in determining the site of fermentation because shorterchain-length molecules are fermented in the proximal colon, whereas longer molecules (average DP = 29) reach the distal colon and increase SCFA concentrations in all the compartments. HPSEC analysis (Figure 4) proved the presence of large amounts of OS, including those with DP > 40 (molecular

Figure 4. HPSEC chromatogram (including pullulan standards) of stream R.

weight >5900 Da), but the highest area was the one observed in the range 180−738 Da (using pullulan standards), namely, DP ≈ 2−6. This is interesting data taking into account that according to Moura et al.35 a decrease in XOS consumption by species such as Bifidobacterium adolescentis was observed when DP was increased. Moreover, L. brevis showed preference for the commercial preparation, which basically constituted of xylobiose. Likewise, the MALDI-TOF MS analysis data (Figure 5) confirmed the presence of a wide range of OS made up of both unsubstituted (DP = 3−16) and acetyl-substituted (DP = 3− 10) pentoses. In accordance with the compositional information shown in Table 2 (XOS/AraOS = 3.67), the oligomers present in the refined product correspond not only to XOS but also to AXOS. In Vitro Fermentability of the Refined Final Product. To assess the effects of AXOS consumption on the stimulation of four bacterial groups, an in vitro fermentation study was F


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Figure 5. MALDI-TOF mass spectrum of stream R.

number of bifidobacteria, increases in this genus were higher for AXOS, in comparison with commercial FOS, so it can be stated that they have a similar or even greater bifidogenic effect than FOS. On the contrary, the increases in the number of lactobacilli were significantly higher than the ones observed in the control assay but lower than the ones achieved with FOS. This is in agreement with the lactic acid generation in the experiments assayed with FOS, as well as with other studies that demonstrated the potential of FOS to promote the growth of lactobacilli.23,47,48 Moreover, the increases in clostridia numbers (often considered harmful bacteria because of their capacity to produce branched-chain fatty acids and a variety of nitrogenand sulfur-containing compounds) were, in general, lower than those observed in the other bacterial groups, and when AXOS are used as substrate, these increases were slightly lower than those achieved with FOS and similar to the ones observed in the control assays. This means that the growth of clostridia numbers observed in these assays could partially be due to the components of the basal medium. Gullón et al.20 observed a similar effect on the clostridia population with fermented oligosaccharide mixtures derived from apple pomace using the same nutrient medium. Meanwhile, the rise in the bacteroides counts was similar in both AXOS and FOS assays after 20 h of fermentation, but almost the same increases were also observed in control experiments because this group of bacteria has saccharolytic and peptolytic activity. Rycroft et al.48 obtained similar results for Bacteroides populations when FOS and XOS were used as substrates, and Jaskari et al.49 reported that oat xylooligomers were not selectively used by bifidobacteria because Bacteroides sp. or C. difficile also showed moderate growth on these substrates. By contrast, Yamada et al.50 reported that wheat arabinoxylan hydrolysates (arabinosylxylobiose, arabinosylxylotriose, arabinosylxylotetraose, and diarabinosylxylotetraose) were utilized by B. adolescentis and Bifidobacterium longum but not by Bacteroides sp.

Acetate was the main acid in all the experiments, reaching the highest values when AXOS were used as substrate. In fact, observing the data obtained for this acid, significant differences were found between the three substrates assayed from 11 h to the end of fermentation. The acetate proportions (with respect to total acids) and its concentrations obtained at 45 h with AXOS, FOS, and the negative control were 70.7, 42.8, and 70.5% and 88.4, 44.7, 30.9 mM, respectively. This acetogenic potential of XOS was previously observed by Gullón et al.29 using rice husks as raw material or by Gullón et al.26 with AXOS from wheat bran. Similarly, the highest propionate concentrations were found in media containing AXOS, which is in agreement with the results reported by Hughes et al.41 and Gullón et al.,26 ranging from 6.6 mM in the control assay until to 14.8 mM with AXOS at 45 h. Propionate generation has been associated with the presence of side chains in XOS,42 and it is utilized primarily by the liver. Its role as a potential modulator of cholesterol synthesis has been proposed.43 By comparison, butyrate levels were found in similar amounts in both AXOS and FOS cultures (Table 3), being slightly higher in experiments with FOS. These results were similar to those reported by Gullón et al.26 with hemicellulosederived soluble AXOS or by Rivas et al.44 employing saccharides derived from wood mannan. The butyrate accumulation leads to potent effects on a variety of colonic mucosal functions and may promote satiety.45 In the case of the negative control, the SCFA generation is associated with the protein degradation, which consequently also lead to increases of the ammonia and phenols (generated in putrefactive processes) levels in the gut. Dynamics of the Bacterial Populations. Nowadays, potential prebiotic consumers expect from these products the promotion of the growth of specific groups able to enhance their health. Many investigations have focused only on the stimulation of Bifidobacterium and Lactobacillus species.46 However, in addition to these two bacterial groups, Bacteroides and Clostridium also are of great interest and should be included in all the studies. Following the Figure 6 and regarding the G


Article

Journal of Agricultural and Food Chemistry Table 3. Concentrations of Lactic Acid, Formic Acid, and Short-Chain Fatty Acids (mM) Produced during Fermentation Using AXOS, FOS, and Control (without Carbohydrate Added) and pH Values*

Table 4. Oligosaccharide Consumption during Fermentationa

carbon source organic acid (mM)

lactic acid

formic acid

acetic acid

propionic acid

butyric acid

total

pH

time (h)

AXOS

FOS

control

4 7 11 20 29 45 4 7 11 20 29 45 4 7 11 20 29 45 4 7 11 20 29 45 4 7 11 20 29 45 4 7 11 20 29 45 0 4 7 11 20 29 45

1.65 (0.49)a 4.33 (1.42)a 3.69 (2.10)a,b 0.28 (0.15)a 0.05 (0.05)a 0.03 (0.03)a 1.28 (0.21)a,b 5.48 (0.97)a 5.11 (0.70)b 2.92 (0.10)a 2.04 (0.06)b 1.41 (0.56)a 10.31 (2.74)a 52.50 (8.42)b 78.80 (2.96)c 82.32 (2.74)c 84.19 (3.68)c 88.41 (2.50)c 3.12 (0.19)a 5.55 (1.39)a 8.81 (2.57)a 11.55 (2.21)a 13.43 (1.03)b 14.77 (0.84)b 1.76 (0.33)a 7.14 (1.26)b 13.20 (0.66)b 18.04 (1.86)b 19.20 (2.53)b 20.43 (2.49)b 18.12 (3.6)a 74.99 (6.64)b 109.63 (2.19)b 115.11 (1.96)b 118.91 (2.53)b 125.04 (1.55)c 6.76 (0.09) 6.62 (0.08) 5.90 (0.27) 5.13 (0.05) 5.12 (0.07) 5.15 (0.07) 5.15 (0.06)

1.84 (0.64)a 10.61 (4.77)a 13.96 (4.54)b 11.80 (6.17)a 11.56 (5.84)a 4.59 (4.55)a 1.72 (0.43)b 13.44 (1.81)b 18.62 (0.89)c 18.64 (1.72)b 19.08 (0.42)c 18.19 (1.25)b 7.73 (1.97)a 31.03 (4.78)a,b 43.58 (1.33)b 41.15 (2.29)b 45.11 (1.49)b 44.70 (2.96)b 2.50 (0.20)a 4.72 (1.94)a 6.69 (2.46)a 7.78 (2.12)a 9.69 (1.31)a,b 10.76 (1.20)a,b 1.70 (0.43)a 8.21 (1.64)b 15.68 (0.40)b 18.88 (3.08)b 21.46 (2.05)b 26.08 (3.32)b 15.49 (3.24)a 68.00 (11.66)b 98.53 (3.69)b 98.25 (7.55)b 106.91 (3.76)b 104.32 (4.35)b 6.85 (0.04) 6.67 (0.06) 5.69 (0.46) 5.06 (0.18) 5.04 (0.19) 5.12 (0.22) 5.28 (0.16)

0.04 (0.04)a 0.00 (0.00)a 0.00 (0.00)a 0.00 (0.00)a 0.00 (0.00)a 0.00 (0.00)a 0.16 (0.10)a 0.85 (0.48)a 0.46 (0.23)a 0.00 (0.00)a 0.00 (0.00)a 0.00 (0.00)a 3.27 (0.41)a 9.16 (1.08)a 15.27 (1.43)a 20.47 (1.64)a 24.18 (1.57)a 30.89 (1.87)a 2.25 (0.33)a 2.87 (0.52)a 3.37 (0.52)a 4.72 (0.91)a 5.63 (1.06)a 6.62 (1.28)a 0.84 (0.13)a 1.68 (0.47)a 3.16 (0.65)a 5.06 (0.67)a 5.94 (0.60)a 6.33 (0.36)a 6.56 (0.36)a 14.56 (0.73)a 22.26 (1.38)a 30.24 (2.67)a 35.75 (2.58)a 43.84 (2.12)a 6.87 (0.02) 6.89 (0.02) 6.92 (0.11) 6.97 (0.14) 6.84 (0.06) 6.84 (0.14) 6.72 (0.09)

a

time (h)

GlcOS

XOS

AraOS

AcO

4 7 11 20 29 45

69.7 88.5 90.1 90.6 90.7 100.0

13.8 69.5 79.5 85.2 87.1 89.3

29.8 46.2 63.4 73.8 76.8 82.7

20.2 100.0 100.0 100.0 100.0 100.0

Average values expressed as percentage of their initial amounts.

Figure 6. Increase of bacterial populations (expressed as log cells/mL) in fecal batch cultures using three different human inoculums and various substrates: (a) 7 h and (b) 20 h. Error bars indicate standard error (n = 3). Asterisks indicate significant differences (Tukey’s test, p < 0.05) between different carbon sources. DAPI: 4′,6-diamidino-2phenylindole; Bif164: Bifidobacterium; Lab158: Lactobacillus/Enterococcus; Bac303: Bacteroides/Prevotella; and His150: Clostridium histolyticum clusters I and II.

foodstuff is considered to be a mild anti-inflammatory through increases in colonic butyrate concentration and the number of Bifidobacterium sp., or with Jaskari et al.,49 who proved that most of the intestinal bacteria tested (Bacteroides thetaiotaomicron, Bacteroides vulgatus, Bacteroides fragilis, and C. diffcile) were able to degrade at least some xylooligomers from oat spelt xylan. In addition, the data obtained by Falck et al.11 from batch fermentation experiments showed that XOS hydrolysates from cereal xylan (rye flour) and hardwood (birchwood) were fermented by both B. adolescentis and L. brevis, whereas nontreated polymeric xylans were not fermented by any of the strains. However, Finegold et al.1 reported that XOS increases the counts of Bifidodobacterium without increasing the counts of Lactobacillus in healthy adults. Another study assessing the effects of a 4 g per day of XOS on the intestinal microbiota, gastrointestinal function, and nutritional

*

Standard error in parentheses (n = 3). Different letters indicate significant differences (p < 0.05) among carbon source for each acid.

This influence of AXOS on the growth of the four bacterial groups (Figure 6) presented a pattern in close correspondence with the substrate consumption because the largest increase in total cell counts took place within the first 7 h of fermentation, coinciding with the largest decrease in the concentration of OS and with the pH decreasing. Also, the highest production of lactic or acetic acids correlated with changes in the numbers of bifidobacteria and lactobacilli. These results are in agreement with Mussatto et al.,13 who indicated that germinated barley H


Article


H.; Lambrey, G.; Abdelnour, A. M.; Pouillart, P. R. Xylooligosaccharide (XOS) in combination with inulin modulates both the intestinal environment and immune status in healthy subjects, while XOS alone only shows prebiotic properties. Br. J. Nutr. 2012, 108, 1847−1858. (3) Woodmansey, E. J. Intestinal bacteria and ageing. J. Appl. Microbiol. 2007, 102, 1178−1186. (4) Rowland, I.; Gill, C. Prebiotics and nutrition in the elderly: The concept of healthy aging. In Handbook of Prebiotics; Gibson, G. R., Roberfroid, M. B., Eds.; CRC Press: New York, 2008, 405−419. (5) Scheid, M. M. A.; Moreno, Y. M. F.; Maróstica Junior, M. R.; Pastore, G. M. Effect of prebiotics on the health of the elderly. Food Res. Int. 2013, 53, 426−432. (6) Woodmansey, E. J.; McMurdo, M. E. T.; Macfarlane, G. T.; Macfarlane, S. Comparison of compositions and metabolic activities of fecal microbiotas in young adults and in antibiotic-treated and nonantibiotic-treated elderly subjects. Appl. Environ. Microbiol. 2004, 70, 6113−6122. (7) Claesson, M. J.; Cusack, S.; O'Sullivan, O.; Greene-Diniz, R.; De Weerd, H.; Flannery, E.; Marchesi, J. R.; Falush, D.; Dinan, T.; Fitzgerald, G.; Stanton, C.; Van Sinderen, D.; O'Connor, M.; Harnedy, N.; O'Connor, K.; Henry, C.; O'Mahony, D.; Fitzgerald, A. P.; Shanahan, F.; Twomey, C.; Hill, C.; Ross, R. P.; O'Toole, P. W. Composition, variability, and temporal stability of the intestinal microbiota of the elderly. Proc. Natl. Acad. Sci. U. S. A. 2011, 108 (Suppl. 1), 4586−4591. (8) Childs, C. E.; Röytiö, H.; Alhoniemi, E.; Fekete, A. A.; Forssten, S. D.; Hudjec, N.; Lim, Y. N.; Steger, C. J.; Yaqoob, P.; Tuohy, K. M.; Rastall, R. A.; Ouwehand, A. C.; Gibson, G. R. Xylo-oligosaccharides alone or in synbiotic combination with Bifidobacterium animalis subsp. lactis induce bifidogenesis and modulate markers of immune function in healthy adults: A double-blind, placebo-controlled, randomised, factorial cross-over study. Br. J. Nutr. 2014, 111, 1945−1956. (9) Samanta, A. K.; Senani, S.; Kolte, A. P.; Sridhar, M.; Sampath, K. T.; Jayapal, N.; Devi, A. Production and in vitro evaluation of xylooligosaccharides generated from corn cobs. Food Bioprod. Process. 2012, 90, 466−474. (10) Carvalheiro, F.; Garrote, G.; Parajó, J. C.; Pereira, H.; Gírio, F. M. Kinetic modeling of brewery’s spent grain autohydrolysis. Biotechnol. Prog. 2005, 21, 233−243. (11) Falck, P.; Precha-Atsawanan, S.; Grey, C.; Immerzeel, P.; Stålbrand, H.; Adlercreutz, P.; Nordberg Karlsson, E. Xylooligosaccharides from hardwood and cereal xylans produced by a thermostable xylanase as carbon sources for lactobacillus brevis and bifidobacterium adolescentis. J. Agric. Food Chem. 2013, 61, 7333−7340. (12) Kabel, M. A.; Schols, H. A.; Voragen, A. G. J. Complex xylooligosaccharides identified from hydrothermally treated Eucalyptus wood and brewery’s spent grain. Carbohydr. Polym. 2002, 50, 191− 200. (13) Mussatto, S. I.; Dragone, G.; Roberto, I. C. Brewers’ spent grain: generation, characteristics and potential applications (Review). J. Cereal Sci. 2006, 43, 1−14. (14) Treimo, J.; Westereng, B.; Horn, S. J.; Forssell, P.; Robertson, J. A.; Faulds, C. B.; Waldron, K. W.; Buchert, J.; Eijsink, V. G. H. Enzymatic solubilization of brewers’ spent grain by combined action of carbohydrases and peptidases. J. Agric. Food Chem. 2009, 57, 3316− 3324. (15) Robertson, J. A.; I'Anson, K. J. A.; Treimo, J.; Faulds, C. B.; Brocklehurst, T. F.; Eijsink, V. G. H.; Waldron, K. W. Profiling brewers’ spent grain for composition and microbial ecology at the site of production. LWT - Food Sci. Technol. 2010, 43, 890−896. (16) Carvalheiro, F.; Esteves, M. P.; Parajó, J. C.; Pereira, H.; Gírio, F. M. Production of oligosaccharides by autohydrolysis of brewery’s spent grain. Bioresour. Technol. 2004, 91, 93−100. (17) Conde, E.; Gullón, P.; Moure, A.; Domínguez, H.; Parajó, J. C. Fractionation of industrial solids containing barley husks in aqueous media. Food Bioprod. Process. 2009, 87, 208−214.

parameters in elderly patients over a 21 day period concluded that XOS supplementation promoted intestinal health and showed no adverse effects on the elderly study population.51 This is an important point because a requirement for an effective prebiotic would be the absence of gastrointestinal side effects.

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CONCLUSIONS Brewer’s spent grain (BSG) was evaluated as a raw material for obtaining a mixture of arabinoxylooligosaccharides (AXOS) suitable to be use as prebiotics for elderly. After hydrothermal treatment of BSG, AXOS were refined by membrane filtration and ion exchange and modified by enzymatic hydrolysis. The presence of both substituted (DP = 2−10) and unsubstituted (DP = 2−16) oligosaccharides made up of xylose and arabinose were confirmed in purified mixtures (OS content = 84% w/w) by MALDI-TOF MS. Finally, AXOS were evaluated for their prebiotic activity by in vitro fermentation assays using fecal inocula from elderly people. Experimental results showed that AXOS fermentation resulted in increased bifidobacteria and lactobacilli populations with respect to the negative control cultures as well as the accumulation of SCFA, confirming a stimulatory effect. Moreover, taking into account the variations in the bacterial populations and the production of organic acids, it can be concluded that the results obtained with AXOS are slightly better than the ones achieved with FOS. Therefore, the intake of this kind of AXOS could enable the improvement of intestinal microbiota and their functions and, as a consequence, the health of the elderly.

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AUTHOR INFORMATION

Funding

We are grateful to Xunta de Galicia (project ref GRC2014/018 and “INBIOMED” Project) for the financial support of this work. Both projects were partially funded by the FEDER Program of the European Union (“Unha maneira de facer Europa”). B.G. thanks the Spanish Ministry of Education, Culture, and Sports for her FPU research grant. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS USED AcO, Acetyl substituents in oligomers; AIR, Acid insoluble residue chromatography; AraOS, Arabinosyl substituents; AXOS, Arabinoxylooligosaccharides; BSG, Brewer’s spent grain; DAPI, 4′,6-diamidino-2-phenylindole; DP, Degree of polimerization; FI, Fecal inocula; FISH, Fluorescent in situ hybridization; FOS, Fructooligosaccharides; GalOS, Galactooligosaccharides; GlcOS, Glucooligosaccharides; HMF, Hydroxymethylfurfural; HPLC, High-performance liquid chromatography; HPSEC, High-performance size exclusion; MALDI-TOF MS, Matrix-assisted laser desorption/ionization-time-of-flight; MS, Monosaccharides; NVC, Nonvolatile content; ONVC, Other nonvolatile content; OS, Oligosaccharides; SCFA, Shortchain fatty acids; UA, Uronic acids; XOS, Xylooligosaccharides

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