2000 [Doi 10.1002_1521-3803(20000501)44!3!152__aid-Food152_3.0.Co;2-8] F. Mancini; T. H. McHugh --...

8/10/2019 2000 [Doi 10.1002_1521-3803(20000501)44!3!152__aid-Food152_3.0.Co;2-8] F. Mancini; T. H. McHugh -- Fruit-Algi

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Mancini/McHugh: Fruit-alginate interactions in novel restructured products

Fruit-alginate interactions in novel restructured products

F. Mancini and T. H. McHugh*

1 Rationale for development of novelrestructured fruit products

The USDA Food Guide Pyramid advised mature adults toconsume 24 servings of fruit per day bringing this category,after cereal products, to the largest recommended for food con-sumption. To increase the likelihood of consumers reachingthese dietary goals, it is necessary to offer them more conveni-ence and variety. Current markets for many fruit products arelimited because the traditional technologies for economicallyprocessing them are restricted to relatively few forms or styles.In the case of peaches, 89% of the processed peaches arecanned, 5% are frozen, 4% are used in jams and other pre-serves, and 2% are dried [1].

According to a recent report published by the NationalResearch Council of the National Academy of Sciences, post-

harvest losses of fruits may be as high as 3040% in bothdeveloped and developing nations [1]. These losses can bereduced by developing processing systems capable of manu-facturing large amounts of fruit materials into a shelf stableform within a short harvest season. These shelf stable productscan be subsequently made into a variety of desirable, value-added final products throughout the remainder of the year.Aseptically processed concentrated fruit purees are the leastcostly option; however, at present the market for concentratedpurees is limited. There is a need to develop new processingstrategies that will increase the value of the fruit purees inorder to make production for this market more profitable.Mechanically harvested and off-grade fruit offer potential rawmaterial for the concentrated puree market.

By combining fruit purees with various gelling agents, novelproducts can be developed. Alginates are one common type ofgelling agent used in restructured products.

2 Simple gel systems

2.1 Pure alginate gels formation of calciumalginate and acid gels

Alginates are a group of naturally occurring polysaccharidesextracted from some marine brown algae (Phaeophyceae). Che-mically they are a family of unbranched binary copolymers of1(4)-linked (-D-mannuronic acid) (M) and (-L-guluronic acid)(G) residues arranged in a block-wise fashion along the polymerchain forming homopolymeric regions (M-blocks, G-blocks)and heteropolymeric regions (MG, MMG, GGM) [2].

Alginates gel forming properties are mainly due to theircapacity to bind a number of divalent ions like calcium and arestrongly correlated with the proportion and length of theguluronic acid blocks (G-blocks) in their polymeric chains [3

5]. Following the addition of calcium ions, alginate undergoesconformational changes, giving rise to the well known eggbox model of alginate gelation. This is based on chain dimer-ization and eventually further aggregation of the dimers [35].The strength of alginate gels was found to depend on the num-ber of cross-links formed, on the type of cross-linking ion, andon the length and stiffness of the blocks between links, while itwas proven to be independent of molecular mass above a cer-tain threshold value. The compression modulus of calciumalginate gels was dependent on the proportion of G-blocksalong the alginate chain and their length. G-blocks led to theformation of large voids that acted as preferential sites for cal-cium ions. The formation of crosslinkages was highly coopera-tive even though these regions were punctuated by D-man-

nuronic acid residues. This effect was not observed for M-blocks and alternating sequences [3, 4]. Crosslinks enable theCa-alginate gel to form at any temperature. The optimum pHrange is large, from pH 3.8 to pH 10. The setting time may beadjusted from a few seconds to many minutes. Final gels areheat irreversible, i.e. once set will not melt on reheating [6].

There are three different methods to initiate controlled algi-nate gelation [7]: By diffusion setting gels are formed simply by diffusing cal-

cium ions into an alginate solution. A critical stage in thegelling process is the correct hydration of the alginate; aneffective dispersion to facilitate hydration is given by blend-ing the alginate with other powdered ingredients (sugar,starch, oil, alcohol) using high speed or high shear mixers.

Readily soluble calcium salts can be used to prepare the set-ting bath solution (calcium chloride, calcium lactate) intowhich the sodium alginate solution is dropped or extruded.

152 Nahrung44(2000) Nr. 3, S. 152 157 WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 0027-769X/2000/0305-0152$17.50+.50/0

Novel, healthy, value-added restructured fruit products meet consu-mer demand for an improved diet containing increasing amounts offruit. As primary ingredients, fruit purees promise to provide new out-lets for visually imperfect fruit or fruit that is too small for the fresh orcanned markets. Generally these new product forms require a texturiz-ing agent such as alginate to control the functional properties of thefinal restructured fruit products. Traditional alginate and pectin gel

systems are reviewed in this manuscript as are mixed gel systems.Recent research results describing the production and properties ofnovel restructured products containing high-guluronic alginate andpeach puree without any additional calcium or sugar source arereviewed. Effects of fruit/alginate interactions on gel formation condi-tions and texture profile results are evaluated.

Universita degli Studi di Perugia, Istituto di Industrie Agrarie, Perugia,Italy, and *United States Department of Agriculture, Agricultural Re-search Service, Western Regional Research Center, Albany, CA, USA.Correspondence to:

Dr.T. H. McHugh, United States Department of Agriculture, Agricul-

tural Research Service, Western Regional Research Center, 800 Bu-chanan Street, Albany, CA 94710, USA(e-mail: [email protected]).


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By internal setting the calcium is released under controlledconditions simultaneously throughout the system. The mostfrequently used calcium salts are calcium sulphate dihydratein neutral gels and dicalcium phosphate in acid products.Generally a sequestrant is included to eliminate the effects

of water hardness on alginate hydration and to control cal-cium release during the early stages of processing. Gel formation by cooling occurs when a hot solution, which

contains all the components, is used. Thermal energy of thealginate prevents chain alignment and only after cooling canthe calcium-induced associations take place. Gels preparedin this way are more stable to syneresis. This stability is dueto the fact that calcium required for gel formation is avail-able to all of the alginate molecules at the same time, allow-ing the calcium and the alginate to react to form a thermody-namically stable network. By contrast, in diffusion setting,the alginate molecules closest to the calcium ions in the set-ting bath react first. Similarly with internal setting, at roomtemperature the molecules closest to the macroscopic parti-

cles of dissolving calcium react first. These two methods,diffusion and internal setting, result in a certain amount ofinherent instability in the gel network, which give rise togreater gel shrinkage and syneresis.

Another mechanism for alginate gelation occurs through agradual pH reduction below the pKa value of the uronic acidresidues, resulting in the formation of the what are oftenreferred to as acid gels [8] that have been proposed to be sta-bilized by intermolecular hydrogen bonds [9]. The pKa valuesof the two monomers have been found to be 3.38 for mannuro-nic and 3.65 for guluronic in 0.1 N NaCl [10]. An abruptdecrease in pH causes the precipitation of alginic acid mole-cules rather than the formation of a three-dimensional gel net-work. Precipitation of alginic acid molecules has been exten-

sively studied [11] as has gel formation by lowering the pHbelow the pKa values. However, the formation and propertiesof the acid gel seem to be poorly understood and few reportsin the literature describe the effect of polymer variables on theproperties of these gels [8]. The best way to develop an alginicacid gel is by diffusion setting using an acidic setting bath.Alginic acid gels are generally grainy and unstable, resultingin a high level of syneresis; therefore, they possess little com-mercial interest [4, 12]. These gels are turbid and break at lowlevels of deformation. Maximum gel strength was reached atthe final pH of 2.5. Chemical composition strongly determinedthe mechanical properties of the final gel. Guluronic acidblocks are the most effective building blocks for junction for-mation. Homopolymeric mannuronic acid blocks were also

able to support the formation of stable intermolecular cross-links, although they were much less effective than polyguluro-nate. In extreme cases, strictly alternating MG/GM-blocks canact as repeating sequences capable of forming crosslinks.Molecular weight increases result in increasing ability for algi-nic acid gel formation. Kinetic measurements showed an equi-librium in the dynamic storage moduli within 2448 h,depending on the chemical composition of the alginate sampleused. Mechanical spectroscopy revealed that highly solid gelsexhibited an increasing frequency dependence with decreasingmolecular weight [8].

2.2 Alginate fruit gels containing added calcium

Research on structured fruit products began in the 1940swhen Peschardt [13] developed a process using alginates forthe formation of structured cherries. Droplets, containing

cherry puree and alginate, were dropped into a bath of calciumsalt to form a skin. Several patents resulted from fruit analogpreparation using alginates [1315] and several reviews havebeen written on this topic [1719].

Many studies apply internal setting to make alginate fruit

gels. A method for producing a food matrix system that simu-lates fruit texture with good sensory quality and processing sta-bility was reported by Luh et al. [20]. The modification andcharacterization of texture of this fabricated calcium alginategel system were also reported [21]. Pelaez and Karel [22]developed a similar method to prepare fruit-simulating algi-nate gels. Kaletunc et al. [23] studied alginate gelling proper-ties as texturizing agents in apple pulp and reconstituted grape-fruit juice. Nussinovitch and Pelag [24] studied the texturiza-tion of raspberry-alginate products exploring the effect of pulpconcentration on selected mechanical parameters. Followingthe same gel preparation procedure, succulent texturized prod-ucts were prepared using pasteurized grapefruit vesiclesentrapped within the alginate gel structure [25]. Truong et al.

[26] optimized levels of tetrasodium pyrophosphate alginateand calcium sulfate as to their effects on the physical and sen-sory characteristics of texturized sweet potato products. Mon-quetet al. [27] studied gelation kinetics in texturized fruit con-taining sweetened passion fruit pulp and alginate. All of thesestudies required the addition of calcium for gel formation.

2.3 Pure pectin gels formation of high and lowmethoxyl pectin gels

Pectins are another common type of gelling agent used forthe formation of restructured fruit products. They are animportant constituent of the cell wall and soft tissue of higherplants, where they contribute to the mechanical properties of

the cell wall and influence cell adhesion. Commercially theyare extracted from apple waste or from the peel of citrus fruits[28, 29].

Pectin is composed of long, regular sequences of 1,4-linked-D-galacturonate residues which in nature may be partiallymethyl esterified. Inserted into the main uronide chain arerhamnose units. Ester content varies with the source of the rawmaterial and may also be varied during extraction. Dependingon the degree of methyl esterification, pectins are classified intolow methyl esterified (LM) pectins (25 to 50%) and high methylesterified (HM) pectins (50 to 80%). Pectin ability to form gelsin the presence of calcium ions or sugar and acid makes them animportant ingredient of many food products. Degree of pectinpolymerization as well as degree of esterification, attached

chains of neutral sugars, acetylation, and crosslinking of pectinmolecules affect gel strength and texture, and the consumeracceptability of the gel product [28, 30].

Two distinct types of gels can be formed from pectins [28,29]: Pectins with low levels of methyl esterification (LM) form

firm gels in the presence of calcium ions in a manner verysimilar to alginates. The interactions between calcium ionsand carboxyl groups of the pectin are described by the eggbox model involving a two-stage process of initial dimeriza-tion and subsequent aggregation of preformed egg boxes [5,31]. Calcium acts as a bridge between pairs of carboxylgroups of pectin molecules. The junctions are formedbetween unbranched nonesterified galacturonan blocks

bound together noncovalently by coordinated calcium ions.Gel strength is reported to increase with decreasing contentof methyl esterified sequences in LM pectin.


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Pectins with high levels of methyl esterification (HM)require low pH and high solids levels to gel. In particular,they require a pH below 3.6 and the presence of a cosolute,generally sucrose at a concentration greater than 55% byweight. These conditions for gelation are likely to encourage

interactions between pectin chains by minimizing both elec-trostatic repulsion (low pH) and chain-solvent interactions(low water activity, high solids) [32]. Crosslinking of poly-mer chains involves extensive segments from two or morepectin molecules to form junction zones. The junction zonesare stabilized by a combination of hydrogen bonds andhydrophobic interactions between pectin molecules [33].The hydrophobic effects arise from the unfavorable interac-tions between water molecules and the nonpolar methoxylgroups of pectin molecules. Temperature of storage, pH,pectin concentration and the sugar used affect the firmnessof the gel and its structure development. HM pectin/glucosegels were shown to be firmer than fructose gels [34]. Meas-urements of gel yield have demonstrated that gel strength

increases with ester content up to a level of around 70%.Further increases in esterification do not result in concomi-tant increases in gel strength [35].

HM pectin systems gel after a time lag and once formedcannot be remelted (are heat irreversible), but LM-pectin gelscan, in most cases, be remelted and reformed repeatedly (areusually thermoreversible) [28]. HM-pectin gels have an opti-mum pH range from 2.5 to 4.0, whereas LM-pectin gels rangefrom pH 2.5 to 5.5. Low pH values tend to increase thestrength of both HM- and LM-pectin gels.

2.4 Pectin based fruit gels containing added acidand/or calcium

Pectin gels have been extensively studied and are commonly

used in the food industry to make jams, jellies and marma-lades. Pectin has a dominant position as a gelling agent in jamsand jellies because: 1) the natural pectin content in the fruitused for jam making is responsible for the gelation of tradi-tional jam that has been produced domestically for centuries,2) pectin is compatible with a natural image of the product,and 3) pectin has a good stability at the pH of jams and jellies,even when hot. The selection of a suitable pectin for a particu-lar application is dependent upon the desired texture and gel-ling temperature. These properties are determined by pectintype and product composition.

LM-pectins must be used if the product pH is above approxi-mately 3.5 and/or the soluble solids (SS) concentration is belowapproximately 55%. To produce jams and jellies, with little or

no sugar, for low energy products and to fill the need for sugar-free products for diabetics, LM-pectins can be used [29].

HM-pectins are used in many food applications. Bakery prod-ucts areoften glazed with a cold setting flan jelly. This is a HM-pectin containing preparation where gelling conditions existexcept for the pH, which is too high. The preparation is acidified

just prior to use and poured over the baked goods while it is stillliquid [28]. HM pectin, being thermally stable, makes jellies thatare placed in the batter of a dough and baked without melting.HMpectinsare alsoused to make flavoredcandies.

3 Mixed gel systems

3.1 Alginate pectin gels

Novel textures can be developed using mixtures of polysac-charides [36]. Many food products include in their formulation

more than one hydrocolloid to achieve the desired physicalstructure, perceived eating quality and behavior during pro-cessing. In some cases, the results are similar to those expectedfrom the single polymers; however, it is common that the prop-erties of mixtures are superior or are qualitatively different

from those of either component alone. For instance, it may bepossible to obtain similar results using lower concentrations ofgelling agents with obvious cost advantages. In other situationssome polymers that are individually non-gelling are able toform gels on mixing. This behavior is called synergism [37].The discovery of new synergistic gelling systems affordspotential commercial value, as have the well-known synergis-tic interactions of certain galactomannans with xanthan, agarand kappa-carragenan [38].

Mixed gel systems containing alginates and pectins havebeen studied by several scientists [36, 3942]. Mixtures of thetwo polymers give firm, cuttable gels at low pH. Such gels canbe formed without the addition of calcium required for alginategels and without the high sugar concentrations required for

HM-pectin gels. The presence of calcium ions in the initialsolutions in antagonistic to gelation. Under the same condi-tions alginates alone produce gelatinous precipitates and thepectin does not gel. The interaction was reported to have beendiscovered during a search for thermoreversible gelling sys-tems that could be used in low sugar, low energy jams and jel-lies [41].

X-ray diffraction analysis showed that the nature of theinteraction is a genuine heterologous association between spe-cific chain sequences of the two polymers. Inspection ofspace-filling molecular models indicates that poly-L-guluro-nate and esterified poly-D-galacturonate (e.g. with low chargedensity) are capable of packing together in a parallel, 2-foldcrystalline array [32]. The near mirror image chains can form

a close-packed, nested structure with opportunities for favor-able noncovalent interactions (e.g. between methyl estergroups of pectin and H-1 and H-2 of polyguluronate). In theabsence of counter-ions to balance the charge on the polyuro-nate chains, an assembly of this type would form by esterifica-tion (consistent with the preferential interactions of pectins ofhigh methyl-ester content) or by protonation (consistent withthe requirement of progressively lower pH with increasingcontent of unesterified carboxyl groups).

Mixtures of HM-pectin (HM) and high-G alginates form thestrongest gels. Gels formed by cold-setting using, in combina-tion with a HM pectin (70% methyl esterified), a typical com-mercial high-G alginate (70% guluronate) are about 2 3times stronger, in terms of both rigidity and break point than

those formed at equivalent pH by a typical high-M sample(60% mannuronate). Gel melting points for these samples dif-fer substantially. Mixed gels involving high-G alginate exhi-bit greater stability to their high-M equivalents [32]. At roomtemperature, pH values close to 3.8 represented the upper limitat which gel formation occurs [32, 36, 40, 41]. Gel rigidity,breaking stress and melting point increase with decreasing pH.It has been shown that the rigidity of mixed gels incorporatingthe same pectin increased systematically with increasing con-tent of long polyguluronate sequences in the alginate. Gel sta-bility, whether characterized by the modulus of rigidity, break-ing load or the melting temperature, increased as alginate L-guluronic acid residues increased. The average number of G-units in the blocks had to exceed 4 for the interaction with pec-

tin. A ratio of 1:1 between alginate and pectin was shown tobe the optimum, generally providing optimal gel strength [32,41]. Sugar also affected gel texture being not essential for gela-


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tion but affecting gel strength, setting characteristics and melt-ing temperature [32]. LM-pectin was able to form mixed gelswith high-G alginates; however, a much lower pH wasrequired (2.7 for typical commercial low-methoxyl pectin)[32]. In contrast to simple alginate or HM pectin gels, the

mixed gels are usually thermoreversible, and, under suffi-ciently acidic conditions, gel structure may be retained at100 C [40].

The mixed gels form at lower pHs (3.0 3.8) than calcium-alginate gels, enabling numerous potential applications. Mixedsystems could be used for cold-setting fruit gels or flans, stabi-lization of acidic emulsions such as salad cream or mayon-naise, and preparation of novel multi-textured products. It hasalso been suggested that the interaction could have medical orpharmaceutical applications, for example by using the acidicenvironment of the stomach to set up a gelled or semigelledstructure in situ [29].

Initial studies produced gels by cooling hot, acidified mixedsystems, but cold-setting gels can also be prepared using the

dissociation of glucono-(-lactone) (GDL) to lower pH in situ[36, 40]. A typical base formulation for a hot-mix gel mightuse high-G alginate (3 g/kg product) and high-methoxyl pectin(3 g/kg) dissolved together in cold, soft water and brought tothe boil, with subsequent addition of citric acid (3 g/kg) andsodium citrate (0.5 g/kg). In the cold-set procedure, GDL(typically 10 15 g/kg) would be added as an aqueous disper-sion to the cold, mixed solution of alginate and pectin (ratio of1:1) and stirred rapidly. It is essential that the polymers arefully dissolved and that the GDL is completely dispersed.Once the mixed-gel network is established, subsequent addi-tion of calcium has no deleterious effect, and indeed can sig-nificantly enhance the strength of the gel. This might indicatethat outer faces of the participating polyguluronate sequences

may be capable of normal egg-box binding of calcium withconsequent consolidation of the gel network [32].Gel properties of LM-pectin-alginate mixed gels have been

studied [42]. The effect of total polymer concentration, compo-nent ratio, pH and concentration of calcium ions on the mixedgels strength were investigated. The highest strength was pro-duced using a concentration of LM-pectin fraction below 30%and keeping the other variables (total polymer concentration,pH of buffer, and calcium concentration) constant. It has beenalso confirmed that the mechanism of gel formation dependson the pH of LM-pectin-alginate mixture.

Table 1 summarizes the simple and mixed gel systemsdescribed above and the operative conditions required for theformation of each gel type.

3.2 Novel fruit alginate mixed gel systems noadded calcium or sugar

The potential interactions between fruit and alginates havebeen suggested [40]. Mixed systems, involving interactionswith pectins or calcium naturally present in fruit, have beenhypothesized [37, 40, 43] to form novel fruit/alginate products,without the additional sugar required for pectin gel formationand the addition of calcium required for alginate gel formation.However, no formal scientific studies have been performed onthis subject.

Recently the potential formation and properties of novelrestructured fruit products utilizing synergistic interactions

between high-G alginates and peach puree without any addi-tional calcium or sugar source were studied. The effects ofpeach, alginate and acid concentration on final product moist-

ure, pH, color and texture were characterized. In addition, thestructure/function relationships between alginate composition(MW, composition and block frequency) and final productproperties were evaluated [44, 45].

Novel alginate peach mixed gel systems were developedthrough this research, eliminating the need for additional cal-cium or sugar. Final products contained up to 99% fruit andexhibited favorable characteristics. Figure 1 reports the proce-dure used for gel formation. Both citric acid and D-glucono-(-lactone) were good acidifying agents for this system. GDLwas selected for its ability to hydrolyse gradually into the sys-tem and slowly reduce pH to the values necessary for gelation[36]. Fully characterized commercial alginates were utilized inthese experiments. All had high guluronic content and wereextracted from Laminaria hyperborea. They differed in mole-cular weight and block distribution.

A complete 3333 factorial design was employedusing different concentrations of dried peach puree (15%, 26%

and 37%), alginate (0.30%, 0.55% and 0.80%), GDL (0.50%,0.75% and 1.0%) and alginate type. Moisture, color, pH andtexture profile analysis (TPA) were determined to evaluateeach formulate. The data was analysed as described by Bourne[46] for fracturability, hardness, chewiness, adhesiveness,cohesiveness and gumminess, and measurements expressed astexture profile values.

Acid (GDL) addition was not necessary for gel formation atthe intermediate and highest fruit concentrations tested (26%,37%). Both alginate and peach addition were necessary for gelformation. Elimination of alginate or replacement of it withsucrose prevented gel formation. Tests without fruit resulted inviscous solutions but no gel was formed, excluding the possi-bility of acid gels forming in the mixed gel system. These

observations suggest that the main interactions involved in gelformation were calcium-alginate and/or alginate-pectin, withthe peach puree contributed both the pectin and calcium to the

Table 1. Gel types and characteristics.

Gel type Range of pH forgel formation

Thermalstability

Necessary conditions

Calcium alg inate 3.8 1 0 Irreversib le Presence of calcium

Alginic acid 2.8 3.8 Irreversible pH 3.8HM Pectin 2.5 3.5 Irreversible Soluble solids 55 80%,

pH 3.6LM Pectin 2.5 5.5 Reversible Presence of calcium,

pH 6.5Alginate HM Pectin 3 .0 3 .8 Reversib le pH 4

Figure 1. Procedure for manufacture of standard peach alginate gels.


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mixed gel system. The following tests were performed todetermine which of these two types of interactions was predo-minant in the formation of structure in mixed alginate fruitgels.

Formulation pH values ranged from 3.54 to 3.85 suggesting

that the predominant interactions found in these systemsoccurred between high-G alginates and HM-pectins. The addi-tion of 3% NaOH (1.0 N) was used to increase the pH to 4.5.This resulted in weaker gels, supporting our hypothesis.

The addition of 0.50% calcium citrate before GDL additionalso resulted in decreased gel strength. This again suggeststhat HM-pectin/high-G-alginate interactions are important ingel formation. As reported from the literature, calcium addi-tion before acidification results in reduced interactionsbetween pectin and alginate; whereas, it would be expected tostrengthen calcium-alginate interactions [32].

The involvement of pectin alginate interactions in gel for-mation was also demonstrated through the addition of highmethoxyl pectin to the formulation. Pectin addition increased

fracturability and hardness of the resultant gels, confirming itsrole in gelation.Thermoreversibility of peach/alginate gels was also tested

to see at what temperature the gels would melt and if theywould reform after cool. The slurry was submitted to a tem-perature program set up to increase the temperature 0.5 C/minfrom 25 C to 90 C. A weight was placed on the top of thealginate gel to see when it sunk into the gel. Melting began at74C and continued to 87 C. After cooling the gels reformed.Thermoreversibility of gels also implicates the importance ofalginate-pectin interactions in this system.

To better understand the role of pectin in gelation other fruitand vegetable purees were tested using the proceduredescribed in Figure 1. Tomato showed same behavior of peach

forming gels with a final pH of 3.9. Tomato puree containslarge amount of high methoxyl pectins. Formulations contain-ing pea or carrot puree, neither of which contains much highmethoxyl pectin, did not gel. The final pH was 4.9 for pea and4.6 for carrot, too high for alginate and pectin interactions.Increasing the GDL concentration from 1% to 3%, the pHdecreased to 3.8 and 3.9, respectively, but formulates were stillunable to gel. Two different brands of apple puree were alsotested, one gelled (the higher quality puree, less brown) andthe other did not. The final pH of the two experiments was thesame (pH 3.7). This result suggested that these two applepurees possessed different pectin contents. Perhaps one wasformed from lower grade, older apples containing less highmethoxyl pectin than the higher grade puree. These results

demonstrated a correlation between gel properties and pureecomposition, more so than pH conditions. Better knowledgeand characterization of fruit and vegetable puree compositionand pectin content is necessary to better understand theirbehavior in gel formation and to evaluate the possibility ofusing them in this mixed gel system.

Another interesting observation was that gel formationoccurred only when the mixture was heated before the additionof GDL and alginate solution. Mouquetet al. [47] also noticedthe need for heat treatment for the gelation process in the textur-ization of sweetened mango pulp. It was hypothesized thatchanges in the puree during heating, either chemical or physicaloccurred. Attempts to make gels by cooling the heated drumdried puree before the addition of GDL or alginate were per-

formed, but gels did not form. By heating the alginate to 80 Cand pouring it into room temperature puree the gel did set. Afterdiscovering that gels could be formed by adding elevated con-

centrations of acid without the addition of heat we realized thatheat treatment was not indispensable. By increasing the GDLconcentration up to 2%4% (pH 3.13.4) gels formed at roomtemperature. At high GDL concentrations, gel strengthincreased with increasing alginate concentrations (from 0.25 to

1.5%). Additional research is required to characterize the invol-vement of specific interactions in these gel systems.Interactions between alginates and peach puree resulted in

improvements in most of the TPA parameters. Particularlyalginate and peach resulted in large synergistic increases infracturability, hardness, gumminess and chewiness values. Thehighest values of fracturability and hardness, gumminess andchewiness were obtained for the formulates containing 0.8%alginate and 37% peach. Adhesiveness highest values wereobtained at the lowest alginate concentration and at the highestpeach concentration. Cohesiveness did not change signifi-cantly for the combinations of variables tested. Peach, alginateand GDL concentrations as well as alginate type (MW and G-blocks content) significantly affected hardness and fracturabil-

ity [44, 45].Increases in MW and G-block content of polymer alsoresulted in increased values of hardness and fracturability. AsMW increased from 79 KDa to 295 KDa the two parametersincreased, and the same trend was found when G-block contentincreased from 0.55 to 0.57 [44, 45].

Preliminary tests on gels stability were made. After 72 h ofstorage at 5C, peach-alginate gels showed good stability andsamples with higher water content showed very low syneresis.In preliminary tests gels were subjected to freezing for 12days and thawing at room temperature and once again exhib-ited good stability. Cold storage could be used to increase gelshelf-life. These results suggest possible applications as ingre-dients in frozen products or ice cream.

4 Conclusions

Research on simple alginate and pectin gel systems wasreviewed, as were restructured fruit products developed fromeach of these systems. More complex mixed gel systems werealso reviewed. Novel gel systems were formed from alginatesand peach puree, without additional calcium or sugar. Thesegels were shown to be the result of pectin alginate interactions.Novel alginate-peach gels required minimal preparation. Thedeveloped gel system could be easily scaled up and utilized bythe food industry for a variety of final purposes. Final fruitproducts offer potential in the marketplace as eat out-of-hand

snack foods or as ingredients in baked, frozen and/or cannedfoods. They contain up to 99% fruit and are highly nutritious.

Disclaimer

Names are necessary to report factually on available data; however,the USDA neither guarantees nor warrants the standard of the product,and the use of the name by USDA implies no approval of the productto the exclusion of others that may also be suitable.

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Received: 15 November 1999.Accepted: 24 January 2000.

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