Stabilisation of sodium caseinate hydrolysate foams

Stabilisation of sodium caseinate hydrolysate foamsDaniel J.Walsh,Kathrina Russell,Richard J.FitzGerald*Department of Life Sciences,University of Limerick,Limerick,IrelandReceived 11July 2007;accepted 9September 2007AbstractThe foam expansion and drainage properties of hydrolysed NaCn (with a degree of hydrolysis (DH)of 0.5%)were studied at pH 2.0,4.0and 6.0.Foaming properties were compared with foams generated with non-heat-treated and heat-treated NaCn (80°C for 20min).At pH 6.0,hydrolysed NaCn displayed higher mean foam expansion and lower mean foam drainage values than the non-heat-treated and heat-treated control samples.Studies with the inclusion of glucose,sucrose and lactose indicated that lactose enhanced the foaming properties of the hydrolysate at pH 4.0.The inclusion of xanthan,guar,arabic,karaya and locust bean gum (LBG),and a combination of LBG and guar gum had different effects on the foaming properties of the hydrolysate and control samples between pH 2.0and 6.0.The results provide new information of the effects of low (sugars)and high (gum)molecular weight agents in modifying the foaming prop-erties of casein ingredients.Ó2007Elsevier Ltd.All rights reserved.Keywords:Foam expansion;Foam drainage;Sugars;Gum polysaccharides1.IntroductionFoams are colloidal systems in which tiny air bubbles are dispersed in an aqueous continuous phase (Damoda-ran,1997).Many processed foods consumed daily are liquid or solid foams,such as the head of soft drinks and beer,whipped cream,mousses,meringue,bread and ice cream.In all of these products,proteins are the main sur-face-active agents that aid in the formation and stabilisa-tion of the dispersed gas phase.Foams may be formed in several ways including,whipping,shaking and sparging.All three methods,especially whipping rely on shear forces to form the foam.During foam formation proteins rapidly adsorb at the air–water interface,decrease surface tension,increase the viscous and elastic properties of the liquid phase and form a stabilising film around gas bubbles which promote foam formation (Zayas,1997).A number of fac-tors affect the foaming properties of proteins including,protein type,composition and concentration,pH,salt type and concentration,lipids,carbohydrate type and concen-tration,foaming method,temperature and viscosity of liquid phase.Foams are thermodynamically unstable and over time may destabilise by a number of mechanisms including,disproportionation and coalescence (Prins,1988).Foam stabilisers are ingredients that decrease this insta-bility in that they contribute to the uniformity or consis-tency of a product when subjected to an array of conditions encountered during production,storage and use.Typical foam stabilisers are thickening or gelling agents such as gums,starches,pectins and gelatin.These agents act by either increasing the viscosity of the continu-ous phase or by forming a three-dimensional network that retards the movement of components within the foam.Glucose,sucrose and lactose are low molecular weight sug-ars,which at a high enough concentration may stabilise foams by increasing the viscosity of the continuous phase.Gums are water-soluble polysaccharides of high molec-ular weight derived from a variety of sources such as bac-teria and plants.Differences in gum composition,structure0963-9969/$-see front matter Ó2007Elsevier Ltd.All rights reserved.doi:10.1016/j.foodres.2007.09.003*Corresponding author.Tel.:+35361202598;fax:+35361331490.E-mail address:dick.fitzgerald@ul.ie (R.J.FitzGerald)./locate/foodresAvailable online at Food Research International 41(2008)43–52and molecular weight confers each with unique propertiessuitable for different food applications.The choice ofgum for a particular processed food depends on its abilityto interact with other food components(Janaki&Sashi-dhar,1998).The influence of xanthan gum,guar gum,locust bean gum,gum karaya,and gum arabic on foamformation and stability were investigated in this study.Xanthan gum,isolated from Xanthomonas campestris is ab-1,4-linked glucose polymer with b-D-mannose,4-b-glucu-ric acid-1,2-a-D-mannose side chain(Katzbauer,1998).Xanthan gum has high solubility in water,high viscosityat low concentrations,high stability and solubility in acidicsystems and excellent stability to freezing and thawing(Garcı´a-Ochoa,Santos,Casas,&Go´mez,2000).Guargum is produced from the seeds of the Cyamopsis tetrago-nolobus plant.This polysaccharide,rich in galactomannanis commonly used in many food products because of its lowcost,high solubility at low temperature and high viscosityat low concentration.Due to these attributes,guar gumis frequently used in many dairy products to minimise syn-eresis and improve freeze–thaw stability(Sudhakar,Sing-hal,&Kulkarni,1996).Locust bean gum(LBG)isproduced from seeds of the Ceratonia silliquia plant.Thegum is mainly composed of galactomannan but the struc-ture differs to that of guar gum.LBG is usually used incombination with other gums in food products.As withguar gum,LBG is mainly used in dairy products such asfrozen desserts(Whistler&BeMiller,1999),however,italso provides excellent spread-ability and stability tocheese-spreads,sour cream,cream dips and yoghurt.Inice-cream LBG provides heat shock resistance,desirabletexture and chewyness(Regand&Goff,2003).Gum kar-aya is an exudate of the Sterculia urens tree.It is a highlycomplex polysaccharide composed of D-galacturonic acid, D-glucuronic acid,D-galactose and L-rhamnose.Gum kar-aya yields an opaque solution and is used as a meringuestabiliser(Whistler&BeMiller,1999).Gum arabic an exu-date of the Acacia tree is heterogenous in nature and rich inacidic arabinogalactans.Gum arabic is composed of gum(70%)and a protein–polysaccharaide complex(30%).Gum arabic is used widely as a food ingredient becauseof its high solubility and low viscosity in solution.Thegum is an useful foam stabiliser and develops a high viscos-ity after heating.Gum arabic is mostly used in confection-ary and beverage products.Caseins and caseinate are exploited in food formulationsfor their emulsifying,whipping and high water bindingproperties(Mulvihill,1992).Due to their excellent nutritivevalue,enzymatically hydrolysed caseins have been gener-ated for use in chemically defined formulas for clinicalnutrition.Casein hydrolysates are used as functional foodingredients,and for infant foods,nutritional fortification,pharmaceutical,and nutraceutical applications.Extensiveenzymatic hydrolysis of caseins results in complete destruc-tion of functional properties,whereas limited hydrolysiscan bring about an improvement in functional properties(Chobert et al.,1996;Grufferty&Fox,1988Flanagan&FitzGerald,2002).The preparation of hydrolysates with improved functional properties is dependent on the speci-ficity of the enzyme activity used in their generation.Slat-tery and FitzGerald(1998)showed that at low degree of hydrolysis,casein hydrolysates generated with Protamexä, a Bacillus proteinase complex,had improved foaming properties at high and low pH.The objective of this study was to determine the effect of different sugars and gums on the foam expansion and stability characteristics of hydroly-sed sodium caseinate.2.Materials and methods2.1.MaterialsProtamexäwas supplied by Novo Nordisc(Baegsvaerd, Denmark).Sodium caseinate was supplied by Armour Pro-teins SAS(Saint-Brice-en-Cogles,France).All gums and sugars were obtained from Sigma(Dublin,Ireland).All other reagents were analytical grade.2.2.Hydrolysis of sodium caseinate(NaCn)NaCn(3L,10%(w/w)protein)was hydrolysed with Protamex at50°C.The pH was maintained constant at 7.5by constant addition of4N NaOH.Sodium azide (0.02%(w/w))was added as an anti-microbial agent.The degree of hydrolysis(DH),defined as the percentage of peptide bonds hydrolysed,was calculated from the volume and molarity of NaOH required to maintain constant pH (Adler-Nissen,1986).The enzyme-to-substrate ratio(E:S) used,(0.04%(w/w)),was calculated on the basis of weight of protein in the sodium caseinate suspension and the weight of protein in the Protamex preparation.Protein content was determined using the macro-Kjeldahl proce-dure(IDF(International Dairy Federation),1993)and a nitrogen-to-protein conversion factor of6.38.Hydrolysis of sodium caseinate was allowed to proceed to a DH of 0.5%,at this point the reaction was terminated by heating at80°C for20min.After heating the sodium caseinate was cooled,divided into aliquots and stored atÀ18°C until required.Heat-treated sodium caseinate(3L,10%protein(w/w)) was prepared by the slow addition of sodium caseinate to distilled/deionised water with constant stirring at20°C. The pH was adjusted to7.5with1N HCl or1N NaOH at20°C.Sodium azide(0.02%(w/w))was added as an anti-microbial agent.The suspension was heated,while stirring,to80°C and maintained at this temperature for 20min.On cooling the sample was divided into aliquots and stored atÀ18°C until required.Non-heat-treated sodium caseinate(1%(w/w)protein) was prepared by the slow addition of sodium caseinate (85.35%protein(w/v))to distilled/deionised water with constant stirring for1h at20°C.The pH was adjusted to2.0,4.0or6.0with1N HCl or1N NaOH at20°C.44 D.J.Walsh et al./Food Research International41(2008)43–52Sodium azide(0.02%(w/v))was added as an anti-microbial agent.This control was prepared prior to all analyses. 2.3.Foam formationFoaming properties of NaCn samples,i.e.hydrolysates, non-heat-treated control,and heat-treated control at1% (w/v)protein were determined,in duplicate,at pH2.0, 4.0and6.0as detailed by Slattery and FitzGerald(1998). Sample pH was adjusted using1N HCl or1N NaOH at 20°C.After foam formation the percentage foam expan-sion was calculated from Eq.(1)and percentage foam sta-bility or foam drainage stability(after30min)was calculated from Eq.(2)Foam Expansionð%Þ¼Volume of cylinderÀMass of foam in cylinderMass of foam in cylinderÂ100ð1ÞFoam Stabilityð%Þ¼Mass of foam after30minMass of foam at time zeroÂ100ð2Þ2.4.Foaming in presence of sugarsThe effect of glucose,lactose and sucrose(all at1%(w/ v))on the foaming properties of NaCn hydrolysates and controls at1%(w/v)protein was determined,in duplicate. Each sugar was added to NaCn samples prior to pH adjustment to2.0,4.0and6.0with1N HCl or1N NaOH. Percentage foam expansion and foam stability(after 30min)were measured as previously described.2.5.Foaming in presence of gumsThe effect of xanthan gum,guar gum,gum arabic,gum karaya and LBG on the foaming properties of the NaCn hydrolysates and controls(1%(w/v)protein)was deter-mined,in duplicate.LBG was solubilised in distilled water (1L),at95°C,to afinal gum concentration of2%(w/v). The solution,after cooling to20°C was centrifuged(Sorv-all RC5C Plus,Unitech,Dublin)at1520g for10min.The supernatant obtained by centrifugation was freeze-dried (LABCONCO,AGB Scientific,Dublin)and was used in subsequent foaming studies.Gums(0.1%(w/v))were added to sodium caseinate samples prior to pH adjustment. Percentage foam expansion and foam stability(after 30min)were measured as previously described.2.6.Foaming properties in presence of guar gum and LBGNaCn samples at1%protein were foamed in the pres-ence of guar gum(0.1%and0.2%(w/v)),LBG(0.1%and 0.2%(w/v))and different combinations of guar gum: LBG(0.05:0.05%(w/v)and0.1:0.1%(w/v).Gums were added to NaCn hydrolysates and controls prior to pH adjustment.Percentage foam expansion and foam stability (after30min)were measured in duplicate as previously described.3.Results and discussionFoam expansion values for non-heated,heated-treated and hydrolysed(0.5%DH)NaCn were measured at pH2.0,4.0and6.0(Fig.1a).At pH2.0all samples possesseda high foam expansion with values in the range of1000%. The high foam expansion at pH2.0may be due to the increased positive charge on the casein molecules at this acidic pH resulting in greater repulsion and solubility. Hydrolysed NaCn possessed the greatest foam expansion at higher pH with mean values of1277%and1511%at pH4.0and6.0,respectively.In agreement with Slattery and Fitzgerald(1998),percentage foam expansion increased with increasing pH.In general,proteolytic hydrolysis results in a decrease in molecular weight and an increase in exposed ionisable amino and carbonyl groups leading to improved solubility as the pH moves away from the isoelectric point(Nielsen,1997).Both heated and non-heat-treated NaCn displayed diminished foam expansion at pH4.0(Fig.1a).Thisfinding is inD.J.Walsh et al./Food Research International41(2008)43–5245agreement with Slattery and Fitzgerald(1998),who reported that intact NaCn had poor foaming properties at this pH.In contrast,other studies have reported that foam volume and stability was greatest at the p I of a pro-tein(Kim&Kinsella,1985;Uraizee&Narsimhan,1996). Reduced foam expansion at this pH may be due to a reduc-tion in the overall net charge and solubility as the pH approached the p I.Jahaniaval,Kakuda,Abraham,and Marcone(2000)showed that at pH3.75–4.0,non-aggre-gated casein may exhibit greater hydrophobicity and enhanced functional properties compared to casein at alka-line pH.Clarkson,Cui,and Darton(2000)showed that bovine serum albumin,immunoglobulin G and lysozyme had diminished foam properties at the p I due to extensive aggregation of protein.Differences in the foam expansion of heat-treated and non-heat-treated NaCn were observed at pH4.0(Fig.1a).Presumably,the differences in foam expansion observed between these samples at this pH relates to the different degrees of hydration of each sample as a result of prior heat treatments and the proximity of the sample pH to the p I.Heat-treated sodium caseinate was subjected to80°C for20min before cooling and freezing to–18°C for storage.This heat treatment,although below reported temperatures known to cause thermally induced modifications of casein(Guo,Fox,Flynn,&Mohammad, 1989;Law,Horne,Banks,&Leaver,1994),however,could increase the rate and degree of hydration of sodium casei-nate.Non-heat treated sodium caseinate was resuspended from a dry powder before being subjected to foaming.This heat-treatment followed by freezing and thawing may result in improved hydration of sodium caseinate leaving it more susceptible to pH effects.Foam expansion values for these two samples increased as the pH increased from 4.0to6.0(Fig.1a).No significant difference in foam expan-sion was observed between heated and non-heat-treated sodium caseinate at pH6.0(Fig.1a).The stability of foams at pH2.0,4.0and6.0was mea-sured over time(Fig.1b).At pH2.0the mean foam stabil-ity was45%with no significant differences between controls.At pH4.0,greatest foam stability was seen for hydrolysed sodium caseinate followed by non-heated and heated with mean values of65%,52%and23%,respec-tively(Fig.1b).At pH 6.0,all samples displayed an increase in foam stability than at pH4.0(Fig.1b).The improvement in foam stability observed at pH6.0may be due to increases in solubility and viscosity of caseins as has been observed previously(Konstance&Strange, 1991).Of all samples tested the hydrolysed sodium casei-nate at pH6.0had the greatest foam expansion and stabil-ity with values of1511%and88%,respectively,of all samples at different pH values(Fig.1b).The limited degree of hydrolysis of this sample may have resulted in improve-ments in hydration,solubility and thefilm forming proper-ties of caseins.These improvements in foam stability are consistent withfindings of Slattery and FitzGerald (1998).Mohanty,Mulvihill,and Fox(1988)reported that the increased stability of acidic casein foams with increas-ing pH may be due to decreased electrostatic repulsion between protein molecules.Studies of the effect of pH have mostly shown both the foam volume and foam stability to be greatest at the p I of a protein(Clarkson et al.,2000). This is due to enhanced surface adsorption as a result of decreased repulsive force at the interface and lower solubil-ity.However,for certain proteins coagulation and aggrega-tion occurs at the p I,and in this case the amount of foaming is reduced.3.1.Effect of different sugars on foam expansionThe effect of glucose,sucrose and lactose at a concentra-tion of1%(w/v)on the foam expansion of non-heat-trea-ted,heat-treated and hydrolysed sodium caseinate at pH 2.0,4.0and6.0were investigated(Fig.2a–c).Neither sugar had any significant effect on the foam expansion of non-heat-treated sodium caseinate at the pH values tested (Fig.2a).In the case of heat-treated sodium caseinate the only significant improvement in foam expansion was observed at pH 4.0in the presence of each sugar (Fig.2b).It is possible that the presence of each sugar at a concentration of1%reduced the extent of aggregation of caseins near the p I.The foam expansion for this heat-treated sample at pH4.0increased from561%for the con-trol to an average of763%in the presence of each sugar (Fig.2b).The only significant improvement in foam expan-sion of hydrolysed sodium caseinate was observed at pH 4.0in the presence of lactose and glucose(Fig.2c).Lactose and glucose at a concentration of1%(w/v)increased the foam expansion of hydrolysed sodium caseinate from 1277%for the control to1583%and1417%,respectively (Fig.2c).These results are noteworthy in that they demon-strate that a small degree of hydrolysis is required to improve the foam expansion of sodium caseinate near the isoelectric point and that the selective addition of sugar can furthermore improve the expansion capabilities.3.2.Effect of different sugars on foam stabilityThe effect of glucose,sucrose and lactose at a concentra-tion of1%(w/v)on the foam stability of different sodium caseinate samples at pH2.0,4.0and6.0were determined (Fig.3a–c).There are few reports in the literature on the use of sugars to stabilise sodium caseinate foams.In the case of non-heat-treated sodium caseinate neither sugar yielded any significant improvement in the foam stability at pH2.0and6.0(Fig.3a).Lactose increased the foam sta-bility of non-heat-treated sodium caseinate from52%for the control to59%in the presence of this sugar at pH4.0 (Fig.3a).At pH2.0and6.0,sucrose was the only sugar capable of significantly improving the foam stability of heated sodium caseinate relative to control(Fig.3b).At pH 4.0the foam stability of heated sodium caseinate increased from23%to48%in the presence of either sugar (Fig.3b).It appears that at pH4.0,glucose,sucrose and lactose at a concentration of1%display different abilities46 D.J.Walsh et al./Food Research International41(2008)43–52to improve the foam stability of heat-treated sodium case-inate.No significant improvement in the foam stability of hydrolysed sodium caseinate was observed in the presence of either sugar at pH2.0and6.0(Fig.3c).Both glucose and lactose significantly improved the foam stability of hydrolysed sodium caseinate at pH4.0from65%for the control to75%and80%,respectively.3.3.Effect of different gums on foam expansionThe effect of different gums at a concentration of0.1% w/v on the foam expansion of different sodium caseinate samples at pH2.0,4.0and6.0was investigated(Fig.4a–c).In the case of non-heat-treated sodium caseinate onlyD.J.Walsh et al./Food Research International41(2008)43–5247xanthan gum at pH2.0and LBG at pH4.0significantly increased the foam expansion to1203%and1007%com-pared to the control foam expansion of1067%and783% at these pH values,respectively(Fig.4a).Of all the gums tested only xanthan gum and LBG at pH2.0significantly improved the foam expansion of heat-treated sodium caseinate(Fig.4b).Gum Arabic,karaya and xanthan com-pletely destroyed the ability of heated and non-heat-treated sodium caseinate to foam(Fig.4a and b).Upon the addi-tion of0.1%xanthan gum to sodium caseinate at pH4.0 insoluble white strands were formed.These strands were most likely the product of thermodynamic incompatibility (Tolstoguzov,1998,2003).Hemar,Tamehana,Munro,and Singh(2001)reported the presence of thread-like structures in0.5%xanthan gum/sodium caseinate solutions at neutral pH when visualised by confocal scanning laser microscopy. The authors explained that sodium caseinate promoted the self-association of xanthan molecules as a result of thermo-dynamic incompatability due to both casein and xanthan possessing a net negative charge at neutral pH.Although no thread-like structures were evident in sodium caseinate solutions containing gum arabic and karaya an incompat-ability was observed.The cause of this incompatibility could not be deduced from these studies.However,because Schmitt,Sanchez,Thomas,and Hardy(1999),reported gum arabic carried a net negative charge for pH values above2.0a similar interaction maybe involved for this gum.Only xanthan gum significantly improved the foam expansion of hydrolysed sodium caseinate at pH2.0and 4.0from1043%and1277%to1339%and1538%,respec-tively(Fig.4c).Of the caseinate samples studied a combi-nation of limited hydrolysis and addition of0.1% xanthan gum offered greatest improvement in the foam expansion of sodium caseinate(Fig.4c).3.4.Effect of different gums on foam stabilityThe effect of gum type on the foam stability of different sodium caseinate samples at pH2.0,4.0and6.0was deter-mined(Fig.5a–c).Only xanthan gum increased the foam stability of non-heat-treated sodium caseinate,at pH2.0, from48%to66%(Fig.5a).No improvement in foam sta-bility was observed at pH4.0in the presence of any gum (Fig.5a).Xanthan gum increased the foam stability of non-heat-treated sodium caseinate from66%to100%at pH6.0(Fig.5a).Additionally,at pH6.0,LBG,gum arabic and karaya increased the foam stability of this sample to approximately71%(Fig.5a).Similar trends were observed for heat-treated sodium caseinate where at pH2.0,xanthan gum was the only gum that significantly increased the foam stability relative to the control(Fig.5b).In contrast to the non-heat-treated sample the addition of LBG and guar gum to heat-treated sodium caseinate at pH4.0produced foams with a stability of45%compared to23%for the con-trol(Fig.5b).Purified guar and LBG have been reported not to exhibit any surface activity(Gaonkar,1991).Fur-thermore,galactomannan hydrocolloids such as guar and LBG act by modifying the rheological properties of the aqueous phase of emulsions(Dickinson,2002).Because surface-active properties are involved in emulsion and foam formation it is plausible that both gums stabilised the caseinate foams via an increase in the viscosity of the48 D.J.Walsh et al./Food Research International41(2008)43–52liquid phase.Xanthan,arabic and karaya gums all decreased the stability of foams produced at pH4.0by heat-treated sodium caseinate(Fig.5b).All gums apart from guar gum significantly increased the foam stability of heat-treated sodium caseinate at pH6.0(Fig.5b).Great-est foam stability was obtained for this sample in the pres-ence of xanthan gum where the stability increased from 51%to100%at pH6.0(Fig.5b).At pH2.0,the different gums used had little to no significant improvement on the foam stability of hydrolysed sodium caseinate(Fig.5c). However,at pH4.0both guar gum and LBG significantly increased the foam stability to79%and84%compared to the control value of65%(Fig.5c).At pH6.0,hydrolysed sodium caseinate with out any gum possessed high foam stability with a value of88%(Fig.5c).Of interest was the fact that xanthan gum was the only gum capable of increasing the foam stability of this sample at pH6.0.As with the other sodium caseinate samples the stability of hydrolysed sodium caseinate increased to100%at pH6.0 in the presence of xanthan gum(Fig.5c).3.5.Effect of gum combinations on the foam expansion of sodium caseinateThe effect of different combinations of guar gum and LBG on the foam expansion of sodium caseinate was inves-tigated at pH2.0,4.0and6.0(Fig.6a–c).Of all the sodium caseinate samples tested,only the foam expansion of non-heat-treated sodium caseinate at pH2.0in the presence of 0.2%guar gum was significantly increased from1068%to 1345%(Fig.6a).At pH4.0,a significant increase in the foam expansion of non-heat-treated sodium caseinate was observed in the presence of0.1%LBG and0.1% LBG+0.1%guar gum(Fig.6a).No significant improve-ment was observed in the foam expansion of heat-treated sodium caseinate using different combinations of gum at any of the pH values tested(Fig.6b).The differences in foam expansion between heat-treated and non-heat-treated sodium caseinate in the presence of different combinations of guar and LBG were interesting(Fig.6a and b).The results indicated that the preparation of sodium caseinate had a pronounced effect on the foam expansion of sodium caseinate.The only significant improvement in the foam expansion of hydrolysed sodium caseinate was observed in the presence of0.2%guar gum at pH4.0(Fig.6c).It is interesting to observe that at a gum concentration of 0.1%both guar and LBG had a similar effect on the foam expansion of hydrolysed sodium caseinate at pH 4.0 (Fig.6c).However,at a0.2%gum concentration only guar gum improved the foam forming properties of hydrolysed sodium caseinate(Fig.6c).It appears that although both gums are classified as galactomannans their structural differences may have had a major contributing effect on their ability to influence foam formation.There was no evidence of any synergistic interactions between guar gum and LBG to improve the foam expansion of any of the sodium caseinate samples at pH2.0,4.0and6.0(Fig.6a–c).A synergistic increase in the viscosity of xanthan–guar or xanthan–LBG suspensions has been reported(Garcı´a-Ochoa et al.,2000).Such a viscosity increase would have a significant effect on the foaming properties of sodium caseinate.In fact,an anti-synergistic effect was revealed when guar gum and LBG were used in combination.For example,non-heat-treated sodium caseinate at pH2 possessed a foam expansion of1007%and897%in theD.J.Walsh et al./Food Research International41(2008)43–5249separate presence of0.1%LBG and0.1%guar gum,respec-tively(Fig.6a).However,the foam expansion of the same sample in the presence of both gums simultaneously, yielded a foam expansion of652%at pH2.0(Fig.6a).If no synergistic interactions were present the foam expansion should at least equal the sum of the individual foam expan-sion values i.e.1904%.Similar anti-synergistic effects were evident for the other caseinate samples in the presence of the same gums at pH2.0,4.0and6.0.3.6.Effect of gum combinations on the foam stability of sodium caseinateThe effect of different combinations of guar gum and LBG on the foam stability of sodium caseinate was inves-50 D.J.Walsh et al./Food Research International41(2008)43–52tigated at pH2.0,4.0and6.0(Fig.7a–c).The concentra-tion of gum and gum type appears to be a critical factor in determining the foam stability of different sodium case-inate samples at different pH values.Non heat-treated sodium caseinate displayed significantly increased stability in the presence of0.2%guar gum at pH2.0and4.0with a mean stability of84%compared to control values of48% and52%,respectively(Fig.7a).At pH 6.0,0.2%guar gum,0.1%and0.2%LBG significantly increased the foam stability of non-heat-treated sodium caseinate(Fig.7a).In general of the combinations tested,0.2%guar gum was the optimal gum and concentration to stabilise non heat-trea-ted sodium caseinate(Fig.7a).Heat treated sodium caseinate was stabilised by0.2% guar gum and0.1%guar gum+0.1%LBG at pH 2.0 (Fig.7b).Greatest stability for this sample was obtained with0.2%guar gum where the stability increased from 42%to100%at pH2.0(Fig.7b).At pH4.0,all gums sig-nificantly improved the foam stability of heat-treated sodium caseinate(Fig.7b).Greatest stability was obtained when the caseinate was foamed in the presence0.1%guar gum+0.1%LBG where the stability increased from 22.5%to69%at pH4.0(Fig.7b).At pH6.0,all gums apart from0.05%guar gum+0.05%LBG increased the foam stability of heat treated sodium caseinate(Fig.7b).In a similar fashion to the non-heat-treated sodium caseinate 0.2%guar gum produced the most stable foam at pH6.0 (Fig.7a and b).The mixture of0.1%guar gum+0.1% LBG was the only gum component capable of improving the foam stability of hydrolysed sodium caseinate at pH 2.0(Fig.7c).Although no reports of synergism exist between guar and LBG such an increase was observed under these select set of circumstances.At pH4.0,all gums significantly improved the foam stability of hydrolysed sodium caseinate compared to control(Fig.7c).The hydrolysate generated foam in the presence of0.2%guar gum possessed a stability of100%compared to the same sample without gum which had a stability of65%at pH 4.0(Fig.7c).No significant improvement in foam stability was obtained when the hydrolysed sodium caseinate was foamed in the presence of any gum at pH6.0(Fig.7c).4.ConclusionThe foaming properties of hydrolysed and heat-treated NaCn were altered,in a pH dependent manner,on the inclusion of sugar and gum polysaccharide components. Judicious choice of these components may lead to enhanced foam expansion and drainage stability,and therefore their inclusion as foam enhancing agents merits further study from a scientific and commercial perspective. 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