Effects of pH and salt concentration on functional properties of rambutan (Nephelium lappaceum L.) seed albumin concentrate

Conclusion RSAC showed high surface hydrophobicity which favoured its emulsifying and foaming properties. Moreover, gelation capacity was one of the outstanding properties of RSAC due to high sulfhydryl content. The adjustment of pH or the addition of sodium chloride improved various functional properties of the protein. RSAC was a potential protein ingredient for formulation of new food products. Acknowledgment This research is funded by Vietnam National University – Ho Chi Minh City (VNU-HCM) under grant number B2014-20-08

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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/296698783 Effects of pH and salt concentration on functional properties of rambutan (Nephelium lappaceum L.) seed albumin concentrate Article in International Journal of Food Science & Technology · March 2016 DOI: 10.1111/ijfs.13087 CITATIONS 0 READS 82 5 authors, including: Some of the authors of this publication are also working on these related projects: Nutrition for Vegetarian View project Milk Papy Ho Chi Minh City University of Technology (H 1 PUBLICATION 0 CITATIONS SEE PROFILE Thu Tra Thi Tran Ho Chi Minh City University of Technology (H 5 PUBLICATIONS 1 CITATION SEE PROFILE Van Viet Man Le Ho Chi Minh City University of Technology (H 39 PUBLICATIONS 221 CITATIONS SEE PROFILE All content following this page was uploaded by Van Viet Man Le on 04 March 2016. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. Original article Effects of pH and salt concentration on functional properties of rambutan (Nephelium lappaceum L.) seed albumin concentrate Huynh Thanh Hai Vuong, Ngoc Minh Chau Tran, Thi Thu Tra Tran, Nu Minh Nguyet Ton & Van Viet Man Le* Department of Food Technology, Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet street, District 10, Ho Chi Minh City, Vietnam (Received 1 December 2015; Accepted in revised form 10 February 2016) Summary Searching new protein sources is essential due to an increase in protein demand. In this study, rambutan seed albumin concentrate (RSAC) with the protein content of 80.8% was isolated from defatted rambutan seed meal. The effects of pH and sodium chloride concentration on solubility and functional properties of RSAC were investigated. RSAC had minimum solubility at pH 4. Water absorption capacity at pH 7 and oil absorption capacity of RSAC were 0.79 and 6.13 mL g1, respectively. Both foaming and emulsifying capacities achieved maximal levels at pH 12. In sodium chloride solution, foaming capacity and stability achieved maximal levels at the concentration of 0.6 mol L1, while the highest emulsifying capacity and stability were noted at the concentration of 0.2 mol L1. The least gelation concentration of RSAC was 100 g L1 and this value decreased by five times as salt concentration in the protein solution was 0.6 mol L1. RSAC was a potential functional ingredient in food processing. Keywords Albumin, functional properties, pH, rambutan seed, salt concentration. Introduction In food industry, proteins not only contribute to nutritional value but also affect physico-chemical char- acteristics and sensorial properties of food (Yada, 2004). Plant proteins have attracted great attention due to low cost and high productivity. According to Osborne classification, plant proteins consisted of four major fractions: water-soluble albumin, salt-soluble globulin, alkaline-soluble glutelin and alcohol-soluble prolamin. The ratio of protein fractions varied from plant to plant. Increase in world population increases the demand for protein. Although conventional plant protein sources including soy, wheat, sorghum, lupin and chickpeas have been widely and effectively used for human consumption, searching new protein sources is essential for protein demand in developing countries (Day, 2013). Rambutan (Nephelium lappaceum L.) is a popular crop widely cultivated in tropical countries. Rambutan fruits have been used in the production of juice, jam, jelly, marmalade and canned fruit in syrup. The percentage of seed varied from 4 to 9% fruit weight (Sirisompong et al., 2011). In some Asian countries, rambutan seeds are edible after roasting, while in others, the seeds are considered as a waste material (Solıs-Fuentes et al., 2010). The oil and protein con- tents in rambutan seeds were 37.1–38.9% and 11.9– 14.1%, respectively (Augustin & Chua, 1988). Some studies focused on rambutan seed oil, the physico- chemical and thermal characteristics of which may become interesting for specific applications in several segments of the food industry (Solıs-Fuentes et al., 2010; Sirisompong et al., 2011). After oil extraction, protein is one of the valuable components in the obtained residue and defatted rambutan seed meal (DFRM) can be considered as a nonconventional protein source. There have been so few studies on rambutan seed proteins. According to Augustin & Chua (1988), the amino acid profile of rambutan seed proteins showed that the proteins were of good quality for food use. Our preliminary study revealed that albumin was the major protein fraction in rambutan seeds (Data not shown). Albumin can be extracted by water which is an inexpensive and eco-friendly solvent. In addition, water did not modify native structure of the extracted proteins as well as their functional properties as compared with other solvents such as salt solution, alkaline or alcohol (Yada, 2004). Many *Correspondent: Fax: +84 8 38637504; e-mail: lvvman@hcmut.edu.vn International Journal of Food Science and Technology 2016 doi:10.1111/ijfs.13087 © 2016 Institute of Food Science and Technology 1 studies reported functional properties and effects of processing parameters on functional properties of water-soluble proteins from different plant sources including great northern bean (Sathe & Salunkhe, 1981), oat seed (Ma & Harwalkar, 1984), tepary bean (Idouraine et al., 1991), wheat germ (T€om€osk€ozi et al., 1998), pea seed (Lu et al., 2000; Adebiyi & Aluko, 2011), African locust bean (Lawal et al., 2005), ginkgo seed (Deng et al., 2011) and kidney bean (Mundi & Aluko, 2012). These efforts were aimed at effective application of unconventional protein sources to for- mulation of new food products. However, functional properties of rambutan seed albumin concentrate (RSAC) have never been reported. In this study, RSAC was prepared from DRSM. The effects of pH and salt concentration on solubility and functional properties of the RSAC were investi- gated for the purpose of evaluating the potentiality of this new protein source in food product formulation. The investigated functional properties included water and oil absorption capacity, emulsifying capacity, emulsion stability, foaming capacity, foam stability and gelation capacity. Materials and methods Materials Seeds of rambutan (Nephelium lappaceum L.) fruits were originated from a canned fruit processing plant in Dong Nai, Vietnam. The seeds were ground, defat- ted with hexan at 40 °C for 36 h and stored at 18 °C before use. De-ionised water was used as extraction solvent. All chemicals used in this study were of analytical grade and purchased from Sigma-Aldrich (St. Louis, MO, USA). Preparation of rambutan seed albumin concentrate For albumin extraction, 100 g defatted rambutan seed meal and 1 L de-ionised water were mixed at 200 rpm, 30 °C for 2 h. The mixture was centrifuged at 1500 g, 20 °C for 30 min for solid removal. The extract was dialysed against distilled water to remove salts and to coagulate contaminating salt-soluble protein fraction (globulin) using a membrane with molecular weight cut-off of 6 KDa (Biovision, Milpitas, CA, USA). The dialysis bag was placed in a beaker containing distilled water. The outer phase was continuously stirred using a magnetic stirrer. The dialysis was performed at the ambient temperature. After 8 h, the dialysis bag was removed and the liquid phase in the beaker was replaced by distilled water. The dialysis was repeated three times. Upon completion of dialysis, the content of dialysis bag was centrifuged at 5000 g, 20 °C for 30 min and the supernatant was adjusted to pH 4.2 using HCl solution (0.1 mol L1) for albumin precipi- tation. The solid phase was recovered by centrifuga- tion at 5000 g, 20 °C for 30 min and re-dissolved in de-ionised water. The procedure of albumin precipita- tion was repeated twice. Finally, the precipitate was freeze-dried, ground and stored at 18 °C. Proximate composition of albumin concentrate Protein content was determined by Kjeldahl method (Bradstreet, 1965). Total sugar and reducing sugar contents were evaluated by spectrophotometric method with phenol–sulphuric acid (DuBois et al., 1956) and 3,5-dinitrosalicylic acid (Miller, 1959), respectively. Polyphenol content was measured by spectrophotometric method using Folin–Ciocalteu reagent (Singleton & Rossi, 1965). The moisture, ash, lipid and total acid contents were analysed using AOAC official methods (Helrich, 1990). Surface hydrophobicity of soluble proteins was measured by fluorescence spectrometric method described by Kato & Nakai (1980) using 8-anilino-1-naphthalene sulphonate. Sulfhydryl group content was determined according to the method described by Thannhauser et al. (1987). Protein solubility Protein solubility was determined by the method of Lawal et al. (2005). The protein sample (125 mg) was dispersed in distilled water (25 mL), and the slurry was adjusted to the desired pH (2–12) using HCl solu- tion (0.5 mol L1) or NaOH solution (0.5 mol L1). The slurry was mixed at 30 °C for 1 h with a magnetic stirrer before centrifuging at 12 000 g for 20 min at 4 °C. Protein content in the supernatant was deter- mined by Kjeldahl method. The nitrogen solubility index (NSI) (%) was calculated as follows: NSI ¼ Amount of nitrogen in the supernatant (g)= Amount of nitrogen in the initial sample (g). Effects of NaCl concentration (0–1 mol L1) on protein solubility were also investigated. Water absorption capacity and oil absorption capacity Water absorption capacity (WAC) and oil absorption capacity (OAC) were measured by the method of Lawal et al. (2005). The protein sample (1 g) was mixed with distilled water (10 mL) or soybean oil (10 mL) for 30 s. The samples were then kept at 30  1 °C for 30 min before centrifuging at 5000 g for 30 min. The volume of supernatant was noted in a 10-mL graduated cylinder. WAC and OAC were © 2016 Institute of Food Science and TechnologyInternational Journal of Food Science and Technology 2016 Effects of pH and salt concentration H. T. H. Vuong et al.2 calculated as mL water or mL oil trapped by 1 g protein sample. Effects of pH (2–12) and NaCl concen- tration (0–1 mol L1) on WAC were studied. Emulsifying capacity and emulsion stability Emulsifying capacity (EC) and emulsion stability (ES) were evaluated using the method suggested by Pearce & Kinsella (1978). The protein sample (500 mg) was dissolved in Brit- ton-Robinson Universal buffer (100 mL). Then, 4 mL the protein dispersion and 4 mL soybean oil were mixed and homogenised at 2000 rpm for 1 min using a Heidolph Diax 900 homogeniser (Heidolph Instru- ments GmbH & Co., Schwabach, Germany). At 0 and 10 min after the homogenization, 0.05 mL of the obtained emulsion was pipetted from the bottom of the tube and subsequently diluted into 10 mL of the same buffer containing sodium dodecyl sulphate (1 g L1). Absorbance of the diluted sample was recorded at 500 nm using a spectrophotometer. The absorbance obtained at the initial time after being homogenised was the EC. The ES was calculated as follows: ES ¼ ðA0=DAÞ  Dt where A0 is the absorbance of the diluted emulsion immediately after homogenization, and ΔA is the reduction in absorbance at the interval time (Δt). Effects of pH (2–12) and salt concentration (0–1 mol L1) on EC and ES of the RSAC were exam- ined. Foaming capacity and foam stability Foaming capacity (FC) and foam stability (FS) were evaluated by the method described by Deng et al. (2011). The protein sample (2 g) was dispersed in dis- tilled water (100 mL), and the mixture was adjusted to the desired pH (2–12) or NaCl concentration (0– 1 mol L1). The resulting blend was vigorously stirred for 2 min by a Heidolph Diax 900 homogeniser (Hei- dolph Instruments GmbH & Co.). The blend was imme- diately transferred into a 100-mL graduated cylinder. The volume was recorded before and after stirring. In addition, foam volume change in the graduated cylinder was also recorded at 120 min of storage. FC (%) and FS (%) were calculated as follows: Gelation capacity Gelation capacity was evaluated by least gelation con- centration (LGC) using the method described by Lawal et al. (2005). Rambutan seed albumin concen- trate was added to distilled water; the solid content of sample suspensions was varied from 10 to 200 g L1 with the increment of 10 g L1. Each test tube con- tained 5 mL sample suspension. The suspension was mixed on a vortex mixer for 5 min and heated in a boiling water bath for 1 h. The mixture was cooled in a water bath at 4 °C for 2 h after which the tube was inverted. The lowest concentration at which the sample did not fall down or slip from an inverted tube was taken as the LGC. Effects of pH were evaluated by adjusting pH of the sample suspensions to various values (2–10) prior to heating. Effects of salt concentration were investigated by preparing sample suspensions in NaCl solution with different concentrations (0–1 mol L1). Statistical treatment All experiments were performed in triplicate. The experimental results were expressed as means  stan- dard deviations. Mean values were considered signifi- cantly different when P < 0.05. One-way analysis of variance was performed using the software Statgraph- ics Centurion XV. Results and discussion Proximate composition of albumin concentrate Table 1 shows that the protein level in the RSAC was similar to that in the albumin concentrate from great northern bean (81.70%) (Sathe & Salunkhe, 1981), pea seed (86.26%) (Adebiyi & Aluko, 2011) and ginkgo seed (87.70%) (Deng et al., 2011). Among the nonpr otein compounds, reducing sugars may participate in Maillard reactions while polyphenols may take part in protein–polyphenol interactions. These reactions could change both nutritional value and functional proper- ties of proteins (Belitz et al., 2009). It can be noted that the RSAC had higher total and free sulfhydryl content as well as higher surface hydrophobicity than the ginkgo seed albumin (Deng et al., 2011). Accord- ing to Belitz et al. (2009), sulfhydryl content could enhance gelation capacity, while surface FC ¼ ½Volume after stirring -Volume before stirring ðmLÞ=Volume before stirring ðmL Þ; FS ¼ ½Volume after 120 min standing -Volume before stirring ðmLÞ= ½Volume after stirring -Volume before stirring ðmLÞ: © 2016 Institute of Food Science and Technology International Journal of Food Science and Technology 2016 Effects of pH and salt concentration H. T. H. Vuong et al. 3 hydrophobicity could improve emulsifying and foaming properties of proteins. Protein solubility Solubility profile of a protein concentrate is a good index of its functional properties and potential applica- tions (Kinsella & Melachouris, 1976). The pH-solubility profile of RSAC is visualised on Fig. 1a. The lowest NSI (15.56%) was recorded at pH 4. Similar pH value was also reported for minimum solubility of water-solu- ble proteins from tepary bean (Idouraine et al., 1991) and pea seed (Adebiyi & Aluko, 2011). However, the lowest solubility of African locust albumin (Lawal et al., 2005) and kidney bean albumin (Mundi & Aluko, 2012) was observed at pH 5. Albumins from different plant sources showed various pI values probably due to difference in amino acid composition. In the isoelectric region, low electrostatic repulsive forces enhanced the formation of protein aggregates and reduced the protein solubility (Mao & Hua, 2012). On the contrary, the NSI of RSAC increased as the pH decreased from 4 to 2 or increased from 4 to 7 since the electrostatic repulsive forces between protein molecules increased protein–sol- vent interactions (Belitz et al., 2009). It can be noted that the solubility of the RSAC was nearly unchanged as the pH varied from 7 to 12. The effects of salt concentration on the NSI of the RSAC are visualised on Fig. 1b. Increase in salt con- centration from 0 to 1 mol L1 gradually reduced the NSI from 66.7% to 41.7%. It was reported that the solubility of pumpkin seed albumin in NaCl solutions (0.5 and 1.0 mol L1) was also lower than that in de- ionised water. Salt ions could remove hydrate layers around protein molecules and that led to a reduced protein solubility (Rezig et al. (2016). However, these results were different to the previous findings of Deng et al. (2011) who reported that increase in salt concen- tration up to 0.5 mol L1 improved the NSI of ginkgo seed albumin due to salting-in effect. Difference in sol- ubility of rambutan seed albumin and gingko seed albumin in a salt solution was probably due to their various conformational characteristics. Water absorption capacity Figure 2a presents that Water absorption capacity (WAC) of RSAC decreased with the increase in pH from 2 to 4 and then increased when the pH raised from 4 to 9. The lowest WAC (0.69 mL g1) was observed at pH 4 which was near the pI value of the rambutan seed albumin. Interaction between protein molecules was enhanced in the isoelectric region and that resulted in poor WAC (Deng et al., 2011). As the pH was above 9, WAC of RSAC was reduced. Alkali solution can break hydrogen, amide and disulphide bonds in protein molecules (Fabian & Ju, 2011) and that could change WAC as well as other functional properties of proteins. In distilled water, the WAC of RSAC was 0.79 mL g1 which was higher than that of ginkgo seed albu- min (0.41 mL g1) (Deng et al., 2011). Nevertheless, RSAC showed much lower WAC than great northern bean albumin (3.18 mL g1) (Sathe & Salunkhe, 1981), oat seed albumin (2.4 mL g1) (Ma & Harwalkar, 1984), African locust bean albumin (3 mL g1) (Lawal et al., 2005) and kidney bean albumin (3.4 mL g1) (Mundi & Aluko, 2012). Low WAC of protein was due Table 1 Proximate composition of rambutan seed albumin concen- trate (n = 6) Moisture (%) 4.33  0.15 Protein (%) 80.78  2.38 Lipid (%) 0.27  0.01 Total sugars (%) 5.69  0.12 Reducing sugars (%) 1.04  0.01 Ash (%) 4.43  0.21 Total acid (%) 0.78  0.01 Polyphenols (%) 0.77  0.01 Free SH content (lmol g1) 26.70  1.13 Total SH content (lmol g1) 82.70  3.51 Surface hydrophobicity (S0) 652.72  6.74 0 20 40 60 80 2 3 4 5 6 7 8 9 10 11 12 N itr og en so lu bl e in de x (% ) pH value (a) (b) 0 20 40 60 80 0 0.2 0.4 0.6 0.8 1 N itr og en so lu bl e in de x (% ) NaCl concentration (mol L–1) Figure 1 Effects of pH and NaCl concen- tration on solubility of rambutan seed albu- min concentrate (Results were average of three replicate  standard deviation). © 2016 Institute of Food Science and TechnologyInternational Journal of Food Science and Technology 2016 Effects of pH and salt concentration H. T. H. Vuong et al.4 to its high solubility in distilled water (El-Adawy, 2000). Figure 2b demonstrates that increase in salt concentration from 0 to 0.6 mol L1 enhanced WAC of RSAC while further increase in salt concentration reduced the WAC. Similar observation was noted for both ginkgo seed albumin (Deng et al., 2011) and African locust bean albumin (Lawal et al., 2005). At low salt concentration, hydrated salt ions could link to charged groups on protein molecules; increase in WAC was due to water molecules which were bound to the salt ions. However, high salt con- centration increased the interactions between water molecules and salt ions; this led to dehydration of the proteins and subsequent reduction in WAC (Lawal et al., 2005). Proteins with high WAC could contribute to the body and freshness of various vis- cous foods such as soups, dough, custards and baked products (Kinsella & Melachouris, 1976). Oil absorption capacity The Oil absorption capacity (OAC) of RSAC was 6.13 mL g1 which was much higher than that of great northern bean albumin (3.29 mL g1) (Sathe & Salu- nkhe, 1981), oat seed albumin (2.8 mL g1) (Ma & Harwalkar, 1984), African locust bean albumin (3.4 mL g1) (Lawal et al., 2005) and kidney bean albumin (2.37 mL g1) (Mundi & Aluko, 2012). The OAC of RSAC (6.13 mL g1) was lower than that of ginkgo seed albumin (9.3 mL g1) (Deng et al., 2011) although the surface hydrophobicity of RSAC (S0 = 652.7) was much higher than that of ginkgo seed albumin (S0 = 23.5). Difference in OAC was probably due to various conformational characteristics, lipophilic groups, surface hydrophobicity (Kinsella & Melachouris, 1976) and nonprotein compounds in the protein concentrates (Yada, 2004). The OAC of protein could affect its emulsifying as well as flavour binding properties (Mundi & Aluko, 2012). Protein concentrate with high oil absorption could be used in the formulation of different products such as sausages, cake batters, mayonnaise and salad dressings (Chandi & Sogi, 2007). Foaming properties Figure 3a presents that FC of RSAC decreased with the increase in pH from 2 to 4 while further increase in pH improved FC. It was reported that FC was pH dependent and protein solubility was a prerequisite for good FC (Mundi & Aluko, 2012). Proteins with high solubility could diffuse easily and rapidly to the air–wa- ter interface for air bubble encapsulation and that leads to an improved FC (Belitz et al., 2009). The lowest FC (55.2%) was observed at pH 4 which was the point of least solubility of RSAC. In the isoelectric region, low solubility of kidney bean albumin (Mundi & Aluko, 2012) also resulted in low FC. It was noteworthy that the NSI of RSAC remained unchanged in the pH range of 7–12. However, the FC of RSAC was improved with the increase in pH from 7 to 12 and achieved maximum of 232.8% at pH 12. It can be explained that as the pH raised from 7 to 12, the net charge of RSAC augmented and this phenomenon could weaken hydrophobic inter- actions and enhance protein flexibility for air bubble encapsulation (Chau et al., 1997). In addition, pH 12 enhanced protein deformability (Fabian & Ju, 2011) which could facilitate the formation of cohesive films around air bubbles (Belitz et al., 2009). Figure 3b shows that maximum FS was recorded at pH 4. This observation agreed with the findings of Lawal et al. (2005) who reported that at the isoelectric point, protein film surrounding the air bubbles was stabilised due to the lack of repulsive forces between protein molecules and that generated an improved FS. Nevertheless, our result contrasted with the data of Deng et al. (2011) who stated that protein aggregation at the isoelectric point resulted in reduced FS. It can be noted that conformational characteristics of various proteins at the isoelectric point were different. As a result, protection of air bubbles by protein film in foam system could be different. 0 0.5 1 1.5 2 2 3 4 5 6 7 8 9 10 11 12 W at er a bs or pt io n ca pa ci ty (m L g– 1 ) pH value (a) 0 0.5 1 1.5 2 0 0.2 0.4 0.6 0.8 1 W at er a bs or pt io n ca pa ci ty (m L g– 1 ) NaCl concentration (mol L–1) (b) Figure 2 Effects of pH and NaCl concen- tration on water absorption capacity of rambutan seed albumin concentrate (Results were average of three replicate  standard deviation). © 2016 Institute of Food Science and Technology International Journal of Food Science and Technology 2016 Effects of pH and salt concentration H. T. H. Vuong et al. 5 The effects of salt concentration on foaming proper- ties of RSAC are visualised on Fig. 3c,d. As the salt concentration increased from 0 to 0.6 mol L1, the FC and FS increased by 79% and 25%, respectively. It should be noted that increase in salt concentration from 0 to 0.6 mol L1 decreased the NSI of the RSA by 31%. Low protein solubility negatively affected foaming properties (Yada, 2004). However, our results showed that the surface hydrophobicity of RSAC increased by 22% when the salt concentration augmented from 0 to 0.6 mol L1. Proteins with high surface hydrophobicity could be easily adsorbed at the air–water interface via hydrophobic areas and generate cohesive film around air bubbles; high surface hydrophobicity of proteins was an important characteristic for foam formation and sta- bilization (Belitz et al., 2009). When the salt concentration was higher than 0.6 mol L1, both FC and FS of RSAC were reduced. Deng et al. (2011) also noted a reduction in FC and FS of ginkgo seed albumin as the salt concentration was higher than 0.5 mol L1. These authors explained that high salt concentration promoted protein aggrega- tion and that led to decreased foaming properties. In distilled water, the FC and FS of RSAC were comparable to those of ginkgo seed albumin (Deng et al., 2011) and African locust bean albumin (Lawal et al., 2005). Proteins with good foaming properties would be used for foam stabilization in some foods such as baked goods, sweets and desserts (Kinsella & Melachouris, 1976). Emulsifying properties The effects of pH on EC and ES of RSAC were described as U-shaped curves (Fig. 4a,b). Similar results were mentioned for African locust bean albu- min (Lawal et al., 2005). Minimum EC and ES were recorded in the isoelectric region (pH 4). Under strong acidic (pH 2) or alkaline conditions (pH 12), the emul- sifying properties were strongly improved. EC achieved maximum at pH 12, while the highest ES was noted at pH 2. Emulsifying properties of a protein depended on its hydrophilic–lipophilic balance as well as net charge, which were affected by pH value (Ragab et al., 2004). Extreme acidic or alkaline pH may promote partial denaturation of protein; this phenomenon could facili- tate mutual cohesion between oil phase and protein and result in stabilised protein film around dispersed droplets in the emulsion (Lawal et al., 2005). As the salt concentration increased from 0 to 0.2 mol L1, the EC and ES of RSAC achieved maximum while further increase in salt concentration reduced both EC and ES (Fig. 4c,d). Low salt concen- tration may facilitate formation of charged layers around oil droplets and that would result in mutual repulsion between dispersed droplets in oil-in-water 0 100 200 300 2 3 4 5 6 7 8 9 10 11 12 Fo am in g ca pa ci ty (% ) pH value (a) 0 25 50 75 100 2 3 4 5 6 7 8 9 10 11 12 Fo am in g st ab ili ty (% ) pH value (b) 0 50 100 150 0 0.2 0.4 0.6 0.8 1 Fo am in g ca pa ci ty (% ) NaCl concentration (mol L–1) (c) (d) 0 25 50 75 100 0 0.2 0.4 0.6 0.8 1 Fo am in g st ab ili ty (% ) NaCl concentration (mol L–1) Figure 3 Effects of pH and NaCl concen- tration on foaming capacity and stability of rambutan seed albumin concentrate (Results were average of three replicate  standard deviation). © 2016 Institute of Food Science and TechnologyInternational Journal of Food Science and Technology 2016 Effects of pH and salt concentration H. T. H. Vuong et al.6 emulsion. As a consequence, protein emulsifying prop- erties would be improved (Yuliana et al., 2014). In addition, low salt concentration could promote forma- tion of hydrate layers around interfacial material and that would reduce interfacial energy and retard coales- cence of the oil droplets in the emulsion (Lawal et al., 2005). Nevertheless, high salt concentration reduced the emulsifying properties due to salting-out effect (Ragab et al., 2004). Proteins with good emulsifying properties would be used in formulation of various food emulsions (Yada, 2004). Gelation capacity Table 2 reveals that the LGC of RSAC in distilled water (100 g L1) was lower than that of great northern bean albumin (180 g L1) (Sathe & Salu- nkhe, 1981) and kidney bean albumin (160 g L1) (Mundi & Aluko, 2012). Gel formation from protein solution depends on different interactions between pro- tein molecules including hydrophobic and electrostatic interactions, hydrogen bonds and disulphide bonds formed from the released thiol groups during protein gelation (Belitz et al., 2009). It can be noted that the total and free sulfhydryl contents in RSAC (Table 1) were higher than those in kidney bean albumin (4.60 and 2.20 lmol g1) and that could improve the gela- tion. Based on the LGC as index of gelation, it can be predicted that RSAC was a potential gelating protein ingredient in food industry. The lowest LGC was noted for pH 4 at which the protein solubility was minimum. Similar result was reported for African locust bean albumin (Lawal et al., 2005). In the isoelectric region, minimal electro- static repulsion promoted the formation of the neces- sary intermolecular forces between the protein molecules and resulted in better gelation. At pH 2 and 10, the LGC increased probably due to increased elec- trostatic repulsion between the protein molecules. As the salt concentration varied from 0.4 to 0.6 mol L1, the LGC of RSAC significantly decreased. The LGC in salt solution of 0.6 mol L1 was 5 times lower 0 0.2 0.4 0.6 0.8 1 2 3 4 5 6 7 8 9 10 11 12 Em ul si fy in g ab ili ty (A bs ) pH value (a) 0 10 20 30 40 2 3 4 5 6 7 8 9 10 11 12 Em ul si on st ab ili ty (m in ) pH value (b) 0 0.1 0.2 0.3 0.4 0.5 0 0.2 0.4 0.6 0.8 1 Em ul si fy in g ab ili ty (A bs ) NaCl concentration (mol L–1) (c) (d) 0 5 10 15 20 0 0.2 0.4 0.6 0.8 1 Em ul si on st ab ili ty (m in ) NaCl concentration (mol L–1) Figure 4 Effects of pH and NaCl concen- tration on emulsifying capacity and stability of rambutan seed albumin concentrate (Results were average of three replicate  standard deviation). Table 2 Effects of pH and NaCl concentration on least gelation concentrations (LGC) of rambutan seed albumin concentrate (n = 3) pH 2 4 6 7 8 10 LGC (g L1) 80  0 60  0 60  0 100  0 100  0 100  0 NaCl concentration (mol L1) 0.0 0.2 0.4 0.6 0.8 1.0 LGC (g L1) 100  0 100  0 40  0 20  0 100  0 100  0 © 2016 Institute of Food Science and Technology International Journal of Food Science and Technology 2016 Effects of pH and salt concentration H. T. H. Vuong et al. 7 than that in distilled water. That was due to moderate increase in ionic strength which enhanced interaction between charged macromolecules (Belitz et al., 2009). Lawal et al. (2005) also concluded that the LGC of African locust albumin in NaCl solution of 0.4 mol L1 was 33% lower than that in distilled water. However, at high salt concentration, the LGC augmented due to shielding effect on the protein mole- cules. This phenomenon enhanced salting-out and reduced gelation capacity of the protein. Conclusion RSAC showed high surface hydrophobicity which favoured its emulsifying and foaming properties. Moreover, gelation capacity was one of the outstand- ing properties of RSAC due to high sulfhydryl con- tent. The adjustment of pH or the addition of sodium chloride improved various functional properties of the protein. RSAC was a potential protein ingredient for formulation of new food products. Acknowledgment This research is funded by Vietnam National Univer- sity – Ho Chi Minh City (VNU-HCM) under grant number B2014-20-08. References Adebiyi, A.P. & Aluko, R.E. (2011). Functional properties of pro- tein fractions obtained from commercial yellow field pea (Pisum sativum L.) seed protein isolate. Food Chemistry, 128, 902–908. Augustin, M. & Chua, B. (1988). Composition of rambutan seeds. Pertanika, 11, 211–215. Belitz, H.D., Grosch, W. & Schieberle, P. (2009). Food Chemistry. Berlin: Springer Berlin Heidelberg. Bradstreet, R.B. (1965). The Kjeldahl Method for Organic Nitrogen. 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