Journal of the Science of Food and Agriculture J Sci Food Agric 79 :471–475 (1999)

Effect of sulphite and propyl gallate or ferulicacid on the thermal depolymerisation of foodpolysaccharides¹

Sandra E Hill* and David A GrayDivis ion of Food Sciences , School of Biological Sciences , Univers ity of Nottingham,Sutton Bonington Campus , Loughborough, Leics ,

LE12 5RD,UK

Abstract : Thermal degradation of gums and starches can be an important factor in the texture of food

products. This degradation can occur at neutral pH values and is thought to be due to oxidative–

reductive depolymerisation. It is thought that oxygen, a transition metal and a free radical mediate

this reaction and thus it should be controllable by the addition of an antioxidant system. Previous

studies have shown that the use of propyl gallate and sulphite, at a ratio of approximately 1 : 3,

reduces the loss in viscosity which occurs on heating galactomannans. These additives can also be used

to reduce starch conversion, which can be promoted by the inclusion of low levels of sulphite.

Although propyl gallate is an allowed food additive, its use is limited and it would be of interest if

other agents could be found to function in a similar manner. It has now been shown that ferulic acid

can replace propyl gallate and the viscosity of heated guar gum samples can still be maintained at

70% of the pre-retorted values. At the concentrations of guar and additives used, an optimum of 1:1.7

ferulic acid to sulphite was found. Preliminary work has shown that ferulic acid can also nullify the

eþ ects of sulphite addition to cassava starches. It would therefore appear that ferulic acid does have

the capacity to interfere with the mechanisms that can cause depolymerisation of polysaccharides.

The relevance of this for food systems has yet to be determined.

1999 Society of Chemical Industry(

Keywords: ferulic acid ; propyl gallate; sulphite; guar gum; cassava starch ; oxidative–reductive depolymer-isation

INTRODUCTION

Polysaccharides are often used to enhance the vis-cosity or gelation properties of foods. However, thisfunctionality may be reduced when the polymers areheated. This is due to depolymerisation and hence areduction in the average molecular weight of thepolymers. Acid degradation is often the major causeof molecular weight reduction1 and, for certain poly-mers (eg pectin), b-elimination can be important.Yet, at neutral pH values, a reaction mediated byoxygen, a transition metal and a free radical can havea major inýuence on the functionality of food macro-molecules.2 This reaction is known as oxidative–reductive depolymerisation (ORD).

The use of antioxidants for the prevention of lipidoxidation is common practice in the food industry,but they are not used to speciücally reduce or elimi-nate thermal degradation of polysaccharides. In con-trast, antioxidant addition to reduce the viscosity lossof biological macromolecules used in enhancedmineral oil recovery has been studied, and com-binations of oxygen scavengers and free radical ter-minators found to be greatly beneücial.3 Of a

number of additive systems employed to reducexanthan degradation, the combination of propylgallate and sulphite seemed of particular interest forfood use since both of these materials are allowedfood additives in some food systems.

It is known that sulphite alone does have someprotective eþect on guar gum.4 However, levels of10g kg~1 are required to retain the pre-retorted vis-cosities if sulphite alone is used.5 The amount of sul-phite can be much reduced if it is used incombination with propyl gallate: 0.8g kg~1 sodiumsulphite plus 0.5g kg~1 of propyl gallate are suffi-cient to maintain the viscosity of solutions of 2g kg~1guar gum. This combination of additives will alsohelp to maintain the viscosity of other gums used inthe food industry after a retorting procedure. Forexample locust bean gum (LBG) and tara gum donot apparently suþer any loss of viscosity, whilexanthan viscosity loss at 121¡C is reduced.6

In addition to thickening, some gums are used asgelling agents. As the combination of sulphite andpropyl gallate maintained the molecular weight oflocust bean gum on retorting it could be expected

¹ Bas ed on a paper pres ented at Ferulate ’98, IFR, Norwich, 8–11

July 1998

* Corres pondence to : E Divis ion of Food Sciences ,Sandra Hill,

School of Biological Sciences , Univers ity of Nottingham, Sutton

Bonington Campus , Loughborough, Leics , LE12 5RD, UK

(Received 13 Augus t 1998 ; revis ed vers ion received 25 Septem-

ber 1998; accepted 15 October 1998)

( 1999 Society of Chemical Industry. J Sci Food Agric 0022-5142/99/$17.50 471

SE Hill, DA Gray

that breakstrengths of gels formed after retortingLBG and carrageenan could be increased. This wasfound to be the case and these mixed gels are lessbrittle, by a factor of 4, if the combined additives areused.7

The most commonly used food thickener is starchand the viscosity of this material will also bedecreased if the product is subjected to high tem-peratures. With starches the viscosity characteristicsare more complicated. If heated in excess water thenative starch granule imbibes water and swells irre-versibly. The increase in granule size increases theviscosity, but polymers are lost from this swollengranule and the granule may break down. Amyloseand amylopectin will also be degraded and henceproduce a lower viscosity material. The continuumfrom native starch granule to degraded polymer isoften referred to as ‘starch conversion’.8 The eþectof sulphite on starches has been investigated and ithas been found that certain levels of sulphitepromote starch conversion. Compared to controls,addition of sulphite (for example 0.1g kg~1 sulphitein a starch suspension of 100g kg~1) caused theswelling volume to decrease, the solubility toincrease and the viscosity of starch pastes or thesolubilised amylose and amylopectin to decrease.9h11Aspects of increased starch conversion were seen forall the starches tested: wheat, waxy maize, potato,oat, cassava, rice. The tuber starches seemed moresusceptible to degradation than the cereal starches.Isolated amylose and amylopectin are also degradedmore if heated in the presence of sulphite.

It would appear that under the conditions used inthese experiments, sulphite acts as a prooxidant, oneof the few cases where this has been clearly demon-strated. Other materials tested have also shown thisprooxidant eþect, and inclusion of ascorbic acid orglutathione also seem to increase starch conversion.11If the prooxidants are added to starch pastes thenchanges compared to non-additive or salt controlwould seem to occur at all temperatures measured(30–120¡C).

The mechanism proposed for the degradation ofthe starch and neutral gums is that of free radicalattack. It would appear, in the limited oxygenregimes used in the starch work, that radicals aregenerated that promote ORD of the polymer. Inclu-sion in the system of a radical terminator couldreduce the loss of functionality due to the decrease inmolecular weight of the materials. The addition ofpropyl gallate to any starch system, whatever theprooxidant inclusion level, tends to maintain thestarch conversion levels to those of a non-prooxidantcontrol.

Degradation of neutral polymers by heat andprooxidants would seem to be a general phenomenonand must therefore be of relevance to food quality.However, the choice of free radical terminators suit-able for these hydrophilic systems is very limited.Propyl gallate is the most soluble in aqueous solution

within its class of food additives but it is onlyallowed in a limited number of food applications.This phenolic compound has shown that it can havea major inýuence on the functional properties ofmaterials in model systems. However, changes seenin the viscosity due to the heating of thickeners usedin complex foods are sometimes small and oftenresults are very variable. It is possible that otherfactors, such as naturally occurring radical termina-tors, are dominating the system.

Ferulic acid is a naturally occurring phenolic com-pound and is noted for its antioxidant properties inlipid systems, although its ability to control ORDreactions in aqueous environments has not beenreported. It has a structure not too dissimilar topropyl gallate and is sufficiently soluble to be used inthe aqueous gum systems. It was therefore of interestto see if the addition of ferulic acid could controlsystems designed to promote degradation of gumsand starch conversion. In this brief communicationtwo systems have been chosen to demonstrate theeþectiveness of ferulic acid in limiting ORD reac-tions. The systems are the thermal stability of guargum as measured by viscosity and the amount ofconversion of cassava starch. In the case of cassavastarch, conversion is promoted by the use of sulphiteand it has been assessed by viscosity measurementsof starch pastes (sheared gelatinised starch samples)and by the pasting proüle as shown by a Rapid ViscoAnalyser. All these ündings are compared to theresults obtained when propyl gallate is used.

EXPERIMENTAL

Materials

Guar gum from the Sigma Chemical Company andthe National Starch’s Target Brand Tapioca(cassava) were purchased. All reagents were of ana-lytical grade. Concentrations are quoted relative tothe total mass of the ünal solution. Actual sulphitelevels present, calculated using 5,5@-dithiobis [2-nitrobenzoic acid (DTNB)]12, were found to be 71%of those cited.

Methods

Guar gum viscosityAll samples were prepared in a phosphate buþer (pH7.0, 0.1M). Guar gum samples were dissolved in thebuþer at 70¡C using high shear mixing. To thesesolutions additives were added and then cans wereülled with the samples leaving no headspace.Samples were heated at 121¡C for 60min. On coolingto 25¡C the viscosity of the samples was measured bya Bohlin CS10 rheometer (Bohlin Rheology ABLund, Sweden) ütted with a 4¡ cone and plategeometry. Results are quoted at a shear rate of 2 s~1.

Starch viscosityStarch pastes. Starch suspensions were prepared byadding 50g kg~1 starch to 0.1M sodium phosphate

472 J Sci Food Agric 79 :471–475 (1999)

The thermal depolymerisation of food polysaccharides

buþer at pH 7.0. These samples were heated for30min at 90¡C with gentle stirring. These sampleswere then sheared for 2min using a Silversonhomogeniser set at the highest speed. There were nodiscernible signs of the starch granules or aggre-gations of polymers by light microscopy. However,some macromolecular association is not ruled out.

The starch pastes were held at 60¡C while sul-phite, sulphite and propyl gallate, sulphite andferulic acid or ferulic acid were added. The concen-trations used are shown in the table and ügurelegends. All samples were stored for 60min at 60¡Cin a water bath. The viscosity of pasted starch wasmeasured using the same techniques as used for theguar gum, except all measurements were made at60¡C. The data reported is at a shear rate of 27s~1.

Starch gelatinisation viscosity proüle. Native cassavastarch was added to phosphate buþer with andwithout additives. The concentration of starch (dryweight) was 107.14g per kg~1 mixture (phosphatebuþer, pH 6.5, 0.1M). This sample was mixed andthen inserted into a Rapid Visco analyser (RVA)Series 4 (Newport Scientiüc, NSW, Australia) whichwas used along with the accompanying software(Thermocline). The stirring speed was 960rpm forthe ürst 10 s then 160rpm for the remainder of thetest. A standard 1 temperature proüle was used asfollows : idle and hold at 50¡C; 0–1min \ 50¡C, 1–4:45min \ ramp up to 95¡C; 4:45–7:15min \ holdat 95¡C; 7:15–11min cooling (set at 50¡C); hold at50¡C to 13min. Each sample type was tested threetimes and the coefficient of variations of the peakvalues and ünal viscosity were less than 2%.

RESULTS AND DISCUSSION

Guar gum

Previous work has demonstrated that heating a rangeof polysaccharide gums results in a loss of vis-cosity.3,6,7 The work of Refs 6 and 7 showed that acombination of the additives sulphite and propylgallate would reduce the degradation of guar gumduring heating at 100¡C or when it was retorted at121¡C. This work has been repeated using 0.8%(8g kg~1) guar gum solubilised at 70¡C and thenheated for 1h at 121¡C. Combinations of additiveswere incorporated into the solubilised gums so thatthe total concentration of additives was 0.2g kg~1.

The results are shown in Fig 1 and the viscositybefore retorting is indicated and shows that the vis-cosity was approximately 0.5 Pa.s. Without additivesor in the presence of only one additive this wasreduced to 0.05 Pa.s. It was previously reported that,using the same methodology with a total additiveconcentration of 0.2g kg~1, a ratio of propyl gallateto sulphite of 1 : 3 gave highest post-retorting vis-cosity. For these present results, similar ratios ofthese additives gave the highest viscosity. It isnotable that the variation in the four replicate vis-cosity values increases markedly at the higher values.

Figure 1. Effect of s ulphite :phenolic compound ratio on guar gum

vis cos ity. Guar gum s olutions (8 g kgÉ1) in phos phate buffer (pH

7.0, 0.1 M) were prepared at 70¡C. Propyl gallate] s ulphiteor ferulic acid] s ulphite (– –]– –) were added to the(ÈÈ+È)

s amples s o that the total additive concentration was equivalent

(0.2 g kgÉ1). Solutions were then retorted (121¡C for 60min),cooled and the vis cos ity meas ured at 25¡C. Res ults are reportedat a s hear rate of 2 s É1. Means and s tandard deviations of five

replicates are s hown.

Ferulic acid was used in combination with sulphitefor comparison with propyl gallate. With this phe-nolic compound the maximum viscosity levels for theguar was signiücantly less (P \ 0.005) than the vis-cosity that had been achieved in the presence ofpropyl gallate. The proportion of ferulic acidrequired to achieve the maximum viscosity seemed tobe greater (1 : 1.7) than the ratio of propyl gallate tosulphite. The shift in the optimum ratios cannot beexplained by the diþerences in the molar concentra-tion of the two compounds as their molecularweights are similar (propyl gallate FWt \ 212.2;ferulic acid FWt \ 194). The deviation in the higherviscosity results was considerable, but the results doindicate that a mixture of 0.075–0.1g kg~1 ferulicacid plus sulphite (0.125–0.1g kg~1), does help tomaintain the viscosity of guar during a retortingprocess.

Cassava starch

The additive mixtures of propyl gallate and sulphitehave also been used with starch. Earlier work5,9h11showed that sulphite alone had a marked eþect onstarches but it was shown that propyl gallate couldnegate the eþects of the sulphite. Some of the experi-mental work carried out in Ref 11 has now beenrepeated and the potential of ferulic acid to reducethe amount of starch degradation has been assessed.All the starch pastes showed the expected relation-ship between shear stress and rate of strain. The vis-cosities at a speciüc shear rate can be compared andTable 1 indicates that the addition of sulphite(1g kg~1) alone does decrease the viscosity of thesamples. The use of propyl gallate and sulphite pro-duces a sample with a higher viscosity than thecontrol containing no additives, thus conürming theearly work. Ferulic acid plus the sulphite achieves a

J Sci Food Agric 79 :471–475 (1999) 473

SE Hill, DA Gray

Table 1. Effect of additives on pas ted cas s ava s tarch

vis cos itya

Additives Cas s ava s tarch vis cos ity

(mPa.s ^SD)

None 66.7 (^2.5)Sulphite (1 g kgÉ1) 53.5 (^0.7)Sulphite (1 g kgÉ1)

]ferulic acid(0.5 g kgÉ1) 66.5 (^3.5)

Sulphite (1 g kgÉ1)

]propyl gallate(0.5 g kgÉ1) 80.0 (^2.4)

Ferulate (0.5 g kgÉ1) 79.0 (^1.4)

a Meas urements were made us ing a pas ted s ample (s ee

materials and methods ) of 50 g kgÉ1 Target Brand

Tapioca s tarch (National Starch) in phos phate buffer (pH

7.0, 0.1 M). The temperature during holding and

meas urement was 60¡C and a s hear rate of 27 s É1 was

us ed. Average and s tandard deviation values of at leas t

three replicates are s hown.

viscosity of the same level as the control, whileferulic acid alone produces a viscosity greater thanthe control. Other experimental work has shown thatthe inclusion of propyl gallate alone, or propyl gallatewith other materials can produce starch samples thatshow less starch conversion than control samplescontaining no additives.10 This may well indicatethat some ORD occurs naturally in starches, but theinclusion of free radical terminators, such as propylgallate or ferulic acid, may well be able to stop this.

Table 1 indicates the eþect of sulphite and ferulicacid, and propyl gallate on the viscosity of pre-gelatinised cassava starch pastes. However, nativestarches are also widely used in the food industry andtheir characteristic swelling, viscosity–temperaturerelationship and set back are important features.Pasting proüles of starches can be followed byheating them in excess water (or buþer) and thencooled. These pasting proüles are shown in Fig 2.

Figure 2. Effect of additives on cas s ava s tarch vis cos ity over a

s pecified temperature programme. RVA profiles of target brand

tapioca s tarch (107 g kgÉ1) in phos phate buffer (pH 7.0, 0.1 M)

were recorded in the pres ence or abs ence of the additives under

s tudy. Thes e were: s ulphite (0.11 g kgÉ1) ; ferulic acid

(0.11 g kgÉ1) ; s ulphite (0.11 g kgÉ1)] propyl gallate (0.054 g kgÉ1) ;

s ulphite (0.11 g kgÉ1)] ferulic acid (0.11 g kgÉ1).

Each curve is the average of three replicates. Thelarge ürst peak indicates the swelling of the starchgranules and it is clear that the inclusion of sulphitedepresses this peak. All viscosities are lower for thesample containing sulphite compared with thecontrol. Inclusion of ferulic acid or propyl gallate, inaddition to sulphite, again negates much of the sul-phite eþect. Replicate studies indicate that ferulicacid (1g kg~1) combined with sulphite (1g kg~1)always produces a ünal viscosity slightly lower thanthe control containing no additives. Propyl gallate(1g g~1) with sulphite produces a sample that hasstatistically the same ünal viscosity as the control.

CONCLUSION

These preliminary experiments indicate that ferulicacid does protect guar gum and cassava starch fromsome forms of degradation. The ability of naturallyoccurring phenolic compounds to chelate transitionmetals and to terminate free radical chain reactionsby the formation of a stable, chain-blocking radical,could be important in a range of foodstuþs. Theiraction in hydrophilic environments may well be animportant feature, and the juxtaposition of thematerials within the food matrix as well as their con-centration may well play a role in the quality of theünal food product. The results for ferulic acidshould encourage further work in this area.

ACKNOWLEDGEMENTS

Many thanks to Fiona Barclay, Pensiri Sriburi andLiz Rogers for their practical help.

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