Polyoxyethylene alkyl and nonyl phenol ethers complexation with potato starch

Food Hydrocolloids 25 (2011) 1563e1571

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Polyoxyethylene alkyl and nonyl phenol ethers complexation with potato starch Juan F. Martínez-Gallegos*, Vicente Bravo-Rodríguez, Encarnación Jurado-Alameda, Ana I. García-López Chemical Engineering Department, Faculty of Sciences, Universidad de Granada, Avd. Fuentenueva s/n, 18071 Granada, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 September 2010 Accepted 28 January 2011

Starchy soil removal efficiency when cleaning food industry equipment can be affected by surfactante starch complexes. Polyoxyethylene alkyl and nonyl phenol ethers complexation with potato starch has been studied by surface tension measurements. All surfactants assayed yield inclusion complexes with starch after a critical concentration of association was achieved. No starch saturation was observed throughout complexation until micellization, with bound surfactant being proportional to the total added surfactant. This bound/total surfactant ratio and the critical concentration of association seemed to depend mainly on the surfactant alkyl chain length. Unlike fatty alcohol ethoxylates, the nonyl phenol ethoxylate was feebly bound likely caused by its aromatic moiety. Binding isotherms and Scatchard plots suggested surfactants positive cooperative and non-cooperative binding with amylose and amylopectin respectively. Phosphate buffer raised surfactants binding capacity, ascribed either to an increase in cooperativity or to a reduction in the number of glucose units required to bind a surfactant molecule. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Starch Non-ionic surfactants Surface tension Complexes Binding isotherm

1. Introduction Starch is a common and widespread biopolymer in industrial processes being a basic raw material for the food industry, with numerous applications such as in baked goods, batters and breadings, beverages, confectionery, dressings, meat products, snacks, etc (Murphy, 2000), where it is used as a thickener, colloidal stabilizer, gelling agent, bulking agent and water retention agent (Singh, Kaur, & McCarthy, 2007). Starch is composed of two different polysaccharides, amylose and amylopectin. Whereas amylose is mainly a linear polymer consisting of 1 / 4 linked a-D-glucose units, amylopectin structure is highly branched due to a-D-(1 / 6) glycosidic bonds interlinking 1 / 4 linked a-D-glucose linear chains, with approximately one branching point per 20e25 straight chain residues (Parker & Ring, 2001). The starch residues exhibit strong soil-substrate bonds to hard surfaces (Din & Bird, 1996), hence, regular process equipment cleaning to remove starchy soils is of crucial importance for the food industry to ensure optimal process performance and high quality and hygienic products. The negative environmental effects of the technical detergents used for can be diminished by modifying the formulas of these products. For instance, these detergents can be improved by introducing enzymes and highly biodegradable surfactants. Enzymes, such as amylases, allow energy savings on using lower washing temperatures and

* Corresponding author. Tel.: þ34 958 241550; fax: þ34 958 248992. E-mail address: [email protected] (J.F. Martínez-Gallegos). 0268-005X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2011.01.010

replacement or reduction of environmentally harmful components of these cleaning products. Regarding the surfactants, non-ionic surfactants such as fatty alcohol ethoxylates (polyoxyethylene alkyl ethers), FAEO, have been found to be easily biodegradable (Battersby, Sherren, Bumpus, Eagle, & Molade, 2001; Szymanski, Wyrwas, Swit, Jaroszynski, & Lukaszewski, 2000). These fatty alcohol ethoxylates are also an alternative to the widespread alkyl phenol ethoxylates (polyoxyethylene alkyl phenol ethers), APE, which are less biodegradable. Furthermore, while APE show low toxicity their breakdown products, principally nonyl and octyl phenols adsorb readily to suspended solids and are known to exhibit oestrogen-like properties, possibly linked to a decreasing male sperm count and carcinogenic effects (Scott & Jones, 2000). In addition some FAEO have also been found not to alter a-amylases catalytic activities or even increase it (Bravo Rodríguez, Jurado Alameda, Gallegos, et al., 2006; Hoshino & Tanaka, 2003), and to stabilize enzymes in the presence of anionic surfactants, such as proteases in linear alkyl benzene sulfonates solutions (Russell & Britton, 2002), thereby preventing enzymatic activity loss. Thus detergent formulas based on amylases and fatty alcohol ethoxylates offer a feasible improvement for the cleaning of starch soiling in the food industry. The detersive efficiency of these enzymatic detergent formulas used to remove starchy soils from hard surfaces can be affected by surfactantestarch interactions. Inclusion complexes have been found in surfactantestarch solutions involving both amylose and amylopectin, the latter usually showing less complexation (Bravo Rodríguez et al., 2008; Gudmundsson, 1992; Gudmundsson &

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Eliasson, 1990; Hoshino & Tanaka, 2003; Hui, Russell, & Whitten, 1983; Kim & Robinson, 1979; Lundqvist, Eliasson, & Olofsson, 2002a, 2002b; Lundqvist, Nilsson, Eliasson, & Gorton, 2002; Svensson, Gudmundsson, & Eliasson, 1996; Tanaka & Hoshino, 2002; Wangsakan, Chinachoti, & McClements, 2004; Yamamoto, Sano, Harada, & Yasunaga, 1983). On the one hand, starchesurfactant complexes can hamper starch enzymatic hydrolysis with amylases; for example formation of surfactant complexes with amylose hindered enzymatic hydrolysis with b-amylase being only partially hydrolysable (Kim & Robinson, 1979). On the other hand, surfactantepolymer complexes may increase polymer solubility but also raise surface tension (Goddard, 1986a). Hence these inclusion complexes can alter starch removal and deserve specific study previous to detergency tests. Surface tension measurements have been used for a long time to study surfactantsepolymers binding (Goddard, 1986a, 1986b; Jones, 1967), and have also been applied to investigate the formation of these surfactantestarch complexes (Bravo Rodríguez et al., 2008; Hoshino & Tanaka, 2003; Lundqvist et al., 2002a; Svensson et al., 1996; Tanaka & Hoshino, 2002; Wangsakan et al., 2004). In addition the nature of the binding can be analysed by the same binding isotherms used for other macromolecules and ligands such as proteins and DNA (Dahlquist, 1978; McGhee & Von Hippel, 1974; Scatchard, 1949), and so the formation of complexes between starch and surfactants has been studied by means of these binding isotherms (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a; Yamamoto et al., 1983) as well as starch complexes with other ligands (Rutschmann & Solms, 1990; Yamamoto, Sano, & Yasunaga, 1982). Therefore the purpose of our investigation is to study the interactions between potato starch and different commercial FAEO and a commonly used APE, nonyl phenol ethoxylate, measuring surface tensions and analysing the binding isotherms. 2. Materials and methods

2.3. Surfactants characteristics and stock solutions The mean molecular weight, MW, the mean number of carbon atoms of the alkyl chains, Nc, the average moles of ethylene oxide per mol of surfactant, EO, the hydrophilicelipophilic balance values, HLB, and the initial moisture content of each surfactant, H (% w/w), have been summarized in Table 1. Surfactants stock solutions, 10 g/L, were daily prepared by dissolving the required weighed amount of surfactant either in Milli-Q water or phosphate buffer, 0.1 M, pH 7.5. 2.4. Surface tension measurement The surface tension was measured according to the Wilhelmy plate method (Mulqueen & Huibers, 2002) with a tensiometer K11, from KRÜSS GmbH (Hamburg, Germany), connected to thermostatic bath to maintain the temperature constant at 37  C during the measurements. Surface tension, s (mN/m), was measured in 100 mL samples at different concentrations of surfactant in the absence or presence of starch, Si ¼ 0.84 g/L. Samples were prepared by diluting in Milli-Q water the volumes of surfactant and starch stocks solutions needed to reach the desired final concentration; surfactants concentrations ranged within 4  102e1.1 104 mM. Surface tension was measured for 17 min at 1 min intervals, taking as measurement value the mean and standard deviation of the last 10 min data, thus avoiding initial measurement fluctuations. Standard deviations were always less than 0.5 mN/m, and duplicate measurements were made for each surfactant concentration (Bravo Rodríguez et al., 2008). For Findet 1214N/23, Milli-Q water was substituted by phosphate buffer in several samples to study salts effect. Finally starch and phosphate buffer were confirmed not to present surface activity by measuring separately the surface tension of a starch solution, Si ¼ 0.84 g/L, and a phosphate buffer solution, 0.1 M pH 7.5, obtaining values practically equal to those of pure water.

2.1. Starch, surfactants and other chemicals 2.5. Calculations and data analysis Soluble potato starch was purchase from Panreac Química S.A. (Barcelona, Spain). The technical surfactants assayed were the fatty alcohol ethoxylates (polyoxyethylene alkyl ethers) Findet 10/15, Findet 10/18 and Findet 124N23 and the nonyl phenol ethoxylate (polyoxyethylene nonyl phenol ether) Findet 9Q/21.5NF, which were a gift from Kao Corporation (Tokyo, Japan). Detailed composition and characterization of these surfactants are provided below in Section 2.3. Other chemicals (analytical grade) used in the present study were purchased from Panreac Química S.A. (Barcelona, Spain). 2.2. Starch characterization and stock solution Starch moisture content, 16% (w/w), was determined drying 5 g duplicated samples of starch on an infrared balance during 25 min at 100  C (Bravo Rodríguez, Jurado Alameda, Martínez Gallegos, et al., 2006), enabling to express starch concentration on dry weight basis, Si g/L. Amylose and amylopectin relative quantities in starch were 19.8% and 80.2% respectively, being determined spectrophotometrically at 600 nm by iodine staining (Bravo Rodríguez et al., 2008). Starch stock solution, 20 g/L on wet weight basis, was daily prepared (Bravo Rodríguez et al., 2008). Phosphate buffer, 0.1 M, pH 7.5, substituted Milli-Q water in those experiments aimed to study salts influence on critical micelle concentration and surfactant binding capacity.

In surfactantepolymer systems, for a not surface active polymer such as the starch assayed, a common reasonable assumption is that the surface activity of the surfactant molecule considerably exceeds that of the surfactantepolymer complex and therefore the surface tension can be taken as a measure of free surfactant in solution (Goddard, 1986a). Then in solutions with starch the free surfactant concentration, [L] mM, was calculated as that one showing equal surface tension value in surfactant solutions without starch. [L]total mM, total surfactant concentration, minus [L] gave [L]bound mM, bound surfactant concentration. This assumption and the free and bound surfactant calculus have been applied for the

Table 1 Mean molecular weight, mean number of carbon atoms of the alkyl chains, average moles of ethylene oxide per mol of surfactant, hydrophilicelipophilic balance and humidity of the different commercial polyoxyethylene alkyl and nonyl phenol ethers assayed. Surfactant

MW (g/mol)

Nc

EO

HLB

H (%)

Findet Findet Findet Findet

273a 385a 629a 638b

10a,b 10a,b 12.6a,b 9b

2.6a 5.2a 9.9a 9.5b

9.6b 12.6b 14.3b 12.8b

0.423a 0.867a 0.309a 1.4  0.1c

a

10/15 10/18 1214N/23 9Q/21.5NF

Data from Bravo Rodríguez et al. (2005). Data from manufacturer. Average value from two 5 g samples dried for 30 min at 102  C using an infrared balance (model AD-4714A from A&D Co., Ltd, Tokyo, Japan). b

c

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surfactantestarch inclusion complexes in numerous works (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a, 2002b; Svensson et al., 1996; Wangsakan et al., 2004). From surface tension data the surfactant critical concentration of association, B1, the surfactant critical micelle concentration in the absence of starch, C1, and the surfactant critical micelle concentration in the presence of starch, C2, were estimated. The maximum quantity of bound surfactant was calculated by subtracting C1 from C2 (Bravo Rodríguez et al., 2008). Surface tension values were fitted with the logarithm of the surfactant concentration, for concentrations between B1 and C2 in the presence of starch, and for concentrations lower than C1 in the absence of starch, by polynomial functions of the minimum degree sufficient for an adequate adjustment of the experimental results: second degree for Findet 10/18 and Findet 9Q/21.5NF and third degree for Findet 10/15 with or without starch, forth and second degree for Findet 1214N/23 with and without phosphate buffer respectively in the presence of starch and first order in its absence. The fittings enabled a simulation of [L] and n, mmol of bound

a

surfactant per gram of starch, values for surfactant concentrations lower than C2 in the presence of starch. The simulated values, together with the experimental ones, were represented in the isotherms and in the Scatchard plots (Dahlquist, 1978; McGhee & Von Hippel, 1974; Scatchard, 1949) to qualitatively analyze the type of binding between the starch and the different surfactants assayed. 3. Results and discussion 3.1. Surfactantestarch binding course Surface tension measurements for each surfactant, in the presence and absence of starch, are showed in Fig. 1. For all surfactants assayed, in the absence of starch the surface tension decreases with increasing surfactant concentration until the critical micelle concentration is reached, C1 Table 2, and micelles start to form. The critical micelle concentration values obtained are comparable to those found by other authors for FAEO (Corkill, Goodman, &

b

75 65

1565

Si = 0 g/L

75 65

Si = 0 g/L

Si = 0.84 g/L

55

Si = 0.84 g/L 55

45

45

35

35

25 15

25 1

c

10

100

1000

10000

100000

0.1

d

75 Si = 0 g/L

65

1

10

100

Si = 0 g/L

65

σ (mN/m)

45 35 25 0.01

e

10000

75

Si = 0.84 g/L

Si = 0.84 g/L 55

1000

55 45 35

0.1

1

10

100

1000

25 0.01

0.1

1

10

100

1000

75 Si = 0 g/L

65

Si = 0.84 g/L 55 45 35 25 0.01

0.1

1

10

100

1000

Fig. 1. Surface tension vs. the logarithm of total surfactant concentration in the presence and absence of starch at 37  C: a) Findet 10/15, b) Findet 10/18, c) Findet 1214N/23, d) Findet 1214N/23 with phosphate buffer 0.1 M, pH 7.5, e) Findet 9Q/21.5NF.

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Table 2 Critical micelle and critical association concentrations (on dry weight basis), surface tensions, and maximum amount of bound surfactant in the presence of starch. Surfactant

Findet 10/15 Findet 10/18 Findet 1214N/23 þPhosphate buffer 0.1 M, pH 7.5 Findet 9Q/21.5NF

nmax (mmol surfbound/g starch)

With starch (Si ¼ 0.84 g/L)

Without starch C1 (mM)

s1 (mN/m)

B1 (mM)

sB1 (mN/m)

C2 (mM)

s2 (mN/m)

579  58 835  87 33  5 16  4 49  2

25.4  0.8 26.5  0.8 32.3  0.9 32.5  1.3 30.2  0.3

4.83  0.48 4.18  0.44 0.62  0.09 0.12  0.03 4.40  0.21

65.7  0.8 61.2  0.8 55.9  0.9 61.9  1.3 47.3  0.3

902  79 1028  59 123  10 128  15 66  4

25.1  0.8 26.9  0.5 32.5  0.7 32.4  1.0 30.3  0.4

Harrold, 1964; Huibers, Lobanov, Katritzky, Shah, & Karelson, 1996; Meguro, Takasawa, Kawahashi, Tabata, & Ueno, 1981; Rosen, Cohen, Dahanayake, & Hua, 1982) and APE (Carrion Fite, 1985; Dai & Tam, 2003; Valea & González, 1990) with similar alkyl chain length and ethylene oxide units. For linear alkyl ethoxylates, several studies have found that the critical micelle concentration, on the one hand, diminishes as the size of the alkyl chain grows and, on the other hand, rises with increasing ethylene oxide units (Huibers et al., 1996); it has also been found that, in general, for non-ionic surfactants in aqueous solutions C1 diminishes with increasing size of the hydrophobic fragment and augments with increasing relative size of the hydrophilic fragment, being the critical micelle concentration primarily determined by the hydrophobic part of the molecule (Huibers et al., 1996). Table 2 shows that the critical micelle concentrations for the FAEO assayed, Findet 10/15, 10/18 and 1214N/23, agreed with those rules since it mainly depended on the alkyl change length and to a lesser extend on the number of ethylene oxide units. In the presence of starch, Fig. 1, surface tension decrease follow the same tendency as in its absence until the critical concentration of association, B1 Table 2, is attained. From this point, for a given surfactant concentration the surface tension rises with respect to the experiments in the absence of starch and therefore a surfactantepolymer complex is formed (Goddard, 1986a; Jones, 1967). These surfactantestarch inclusion complexes have also been found with other non-ionic (Bravo Rodríguez et al., 2008; de Miranda, Cacita, & Okano, 2007; Hoshino & Tanaka, 2003; Wangsakan et al., 2004), cationic (Lundqvist et al., 2002a; Wangsakan et al., 2004) and anionic surfactants (Svensson et al., 1996; Tanaka & Hoshino, 2002; Wangsakan et al., 2004), although it is not possible to observe the existence of B1 except in the case of other FAEO, Brij 35, Unitol L-230 and Unitol L-70 (de Miranda et al., 2007; Hoshino & Tanaka, 2003; Wangsakan et al., 2004),or in the case of another APE, Renex 95 (de Miranda et al., 2007). Brij 35 and Unitol L-230 are both a polyoxyethylene (EO ¼ 23) mono-Ndodecyl ether, Unitol L-70 is a polyoxyethylene (EO ¼ 7) mono-Ndodecyl ether, and Renex is a polyoxyethylene (EO ¼ 9.5) nonyl phenol ether which is analogous to Findet 9Q/21.5NF. Factors such as surfactant nature (i.e. head-group properties, hydrocarbon tail length, etc.), starch polymer nature (i.e. amylose and amylopectin contents, maltodextrins, botanical source, etc.) and binding conditions (i.e. buffer, temperature, etc.) influence the formation of these complexes (Goddard, 1986a, 1986b; Svensson et al., 1996; Wangsakan et al., 2004; Wulff & Kubik, 1992) making difficult to establish comparisons for B1 but might justify the differences appointed above. Anyway it is noteworthy to remark that a previous work with alkyl polyglycosides, APG, and the same starch and binding conditions (Bravo Rodríguez et al., 2008) did not show the presence of B1; then these non-ionic surfactants were assumed to bind to starch since the early beginning of their addition. Meanwhile, the FAEO and APE assayed in the present study exhibit B1 concentrations and therefore it can be inferred that these surfactants have less affinity to form complexes with starch compared with those alkyl polyglycosides, since the surfactant

384 230 106 133 20

critical concentration of association is a measure of the surfactantepolymer interaction strength (Goddard, 1986a). In addition, from those studies based on surface tension measurements for Brij 35 (Hoshino & Tanaka, 2003; Wangsakan et al., 2004) it can be observed that B1 and its corresponding surface tension values are in the same range of values, 0.25e1.25 mM and 60e62 mN/m respectively, as those ones obtained here for Findet 1214N/23, B1 and sB1 Table 2, despite the differences in starch substrates and experimental conditions, having both surfactants approximately equal Nc. Therefore, considering all these facts, it could be generally assumed that FAEO and APE show less tendency to form complexes with starch compared with the other non-ionic, cationic and anionic surfactants previously referred (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a; Svensson et al., 1996; Tanaka & Hoshino, 2002; Wangsakan et al., 2004); nevertheless this general assumption could only be strictly stated if experiments were carried out in the same conditions and with the same starch substrate. B1 values for the FAEO assayed, Table 2, show an increasing value with decreasing alkyl chain length, Table 1, as Findet 10/15 and 10/ 18 have equal Nc and similar B1 values whereas Findet 1214N/23 have a lower B1 value due to its longer alkyl chain length. Sodium alkyl sulfates homologous series have been found to exhibit the same behaviour showing a decreasing linear relationship between the logarithm of surfactant critical concentration of association and the alkyl chain length (Arai, Murata, & Shinoda, 1971; Lange, 1971; Shirahama & Ide, 1976). So, apparently, the difference in number of ethylene oxide moieties, EO Table 1, would have little influence in the surfactant critical concentration of association for the FAEO, a fact also supported by the B1 range of values for Brij 35 already mentioned before, 0.25e1.25 mM, which is similar to the B1 value obtained for Findet 1214N/23, Table 2, having both surfactants close average alkyl chain lengths, 12 and 12.6 respectively, but far different average number of ethylene oxide moieties, 23 and 9.9 respectively. Moreover, Findet 9Q/21.5NF, being an APE, also shows similar Nc and B1 values to Findet 10/15 and 10/18, Tables 1 and 2, despite their different chemical nature and hydrophilic group length. All these facts would point the alkyl chain length as the key factor for the B1 value of these ethoxylated surfactants. When the surfactant critical concentration of association is exceeded, Fig. 1, thereupon complexation takes place. [L]bound was calculated and plotted vs. [L]total, Fig. 2. A direct proportional relationship between [L]bound and [L]total is found and so the bound surfactant ratio, f, quotient between bound and total surfactant concentrations, [L]bound/[L]total, kept constant along the complexation process until the critical micelle concentration in the presence of starch is reached, C2, Fig. 2. At C2 a competition between incorporation of non-polar surfactant tails into micelles and into starch helical coils takes place (Wangsakan et al., 2004) and micellization occurs as it is likely more energetically favourable than complexation (Bravo Rodríguez et al., 2008). Therefore, in all these non-ionic surfactantestarch systems, the formation of the surfactantestarch complex persists from the surfactant critical concentration of association to the critical micelle concentration in the presence of starch with no appreciable starch saturation previous to micellization.

J.F. Martínez-Gallegos et al. / Food Hydrocolloids 25 (2011) 1563e1571

a

b

400

250 200

f = 0.366 R2 = 0.9969

300

1567

f = 0.229 R2 = 0.9612

150

200 100

100 50

C2

0

c

200

400

600

800

1000

0

d

100 f = 0.787 R2 = 0.9962

80

200

400

600

800

1000

1200

125 f = 0.881 R 2 = 0.999

100

60

75

40

50

20

25 C2

0

C2

0 0

e

C2

0

0

25

50

75

100

125

0

25

50

75

100

125

150

20

15

f = 0.264 R2 = 0.9872

10

5 C2 0 0

10

20

30

40

50

60

70

Fig. 2. Bound surfactant concentration vs. total surfactant concentration at 37  C in the presence of starch, Si ¼ 0.84 g/L: a) Findet 10/15, b) Findet 10/18, c) Findet 1214N/23, d) Findet 1214N/23 with phosphate buffer 0.1 M, pH 7.5, e) Findet 9Q/21.5NF.

FAEO

1

APG APE

0.8 0.6

f

The same relationship between [L]bound and [L]total can be found from Brij 35 surface tension experimental data (Hoshino & Tanaka, 2003; Wangsakan et al., 2004), f been approximately 0.8, and with alkyl polyglycosides (Bravo Rodríguez et al., 2008), APG. Then, all these f values have been plotted vs. Nc together with those ones found for the FAEO and APE assayed, Fig. 3, showing that f roughly raises with increasing Nc; thus, even considering different nonionic surfactants and experimental conditions, once again the alkyl chain length seems to be one of the main factors affecting the f values, similarly as commented before for the critical association concentration. The maximum amount of bound surfactant, expressed as moles of bound surfactant per gram of starch, nmax, has been calculated from C1 and C2 values, Table 2. Several works have pointed out the hydrophobic interaction as the cause for complexes formation (Goddard, 1986a; Hui et al., 1983). This assumption is completely satisfied by the FAEO assayed as increasing HLB values, i.e. increasing hydrophilicity, Table 1, gave decreasing nmax values,

0.4 0.2 0 9

10

11

12

13

Nc Fig. 3. Bound surfactant ratio vs. alkyl chain length for different non-ionic surfactants and experimental conditions.

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Table 2. Similar results have also been previously reported with alkyl polyglycosides (Bravo Rodríguez et al., 2008). Regarding the APE Findet 9Q/21.5NF, it shows the lowest nmax value, Table 2. A weaker APEeamylopectin interaction has been found compared with FAEO, and was likely caused by the APE aromatic moiety which hampered a close approaching of surfactant monomer and amylopectin glucose units (de Miranda et al., 2007). Thus Findet 9Q/21.5NF chemical structure is thought to be responsible for its low binding capacity. Regarding salts influence on surfactantestarch binding course, experiments with Findet 1214/N23 were carried out with or without phosphate buffer 0.1 M pH 7.5, Fig. 1c, d; the following effects have been observed, Figs. 1c, d and 2c, d, and Table 2: buffer, on the one hand, diminished B1 and C1 values, and, on the other hand, increased f, C2 and nmax values. Thus buffer eased surfactantestarch interaction and complexation lowering the surfactant critical concentration of association, raising the amount of bound surfactant per total surfactant added and increasing C1 to C2 range, i.e. nmax. Analogous behaviour has been reported with other surfactantepolymer complexes, such as sodium dodecyl sulfateepolyvinyl pyrrolidone or sodium dodecyl sulfateepolyethylene oxide complexes, when salts were added (Cabane & Duplessix, 1982; Goddard, 1986a; Murata & Arai, 1973). Finally, it is also noteworthy to remark that, for all the surfactants assayed, the assumption of lack of surface activity of the surfactantestarch complexes is supported by the approximately equal surface tension values attained at the critical micelle concentrations in the absence and presence of starch, s1 and s2 respectively, Table 2, which implies negligible surfactantestarch complex adsorption at the airewater interface. 3.2. Surfactantestarch binding nature As previously mentioned, both amylose and amylopectin have been found to form inclusion complexes with surfactants (Bravo Rodríguez et al., 2008; Gudmundsson, 1992; Gudmundsson & Eliasson, 1990; Hoshino & Tanaka, 2003; Hui et al., 1983; Kim & Robinson, 1979; Lundqvist et al., 2002a, 2002b, 2002; Svensson et al., 1996; Tanaka & Hoshino, 2002; Yamamoto et al., 1983). The surfactant hydrocarbon tails length and head-group properties influences the characteristics of these surfactantestarch complexes (French & Murphy, 1977; Hui et al., 1983; Wangsakan et al., 2004; Wulff & Kubik, 1992). The linear structure of the amylose enables to form these helical inclusion complexes where amylose winds around the hydrophobic part of the ligand with six to eight glucose units per turn depending on the type of ligand (French & Murphy, 1977; Hui et al., 1983). Amylopectin, due to its branched structured, is more restricted to form these complexes but it has been proposed that its most outer chains are also able to bind surfactants and lipids giving these helical inclusion complexes (Bravo Rodríguez et al., 2008; Gudmundsson, 1992; Gudmundsson & Eliasson, 1990; Lundqvist et al., 2002a; Svensson et al., 1996). When the polysaccharide winds around the hydrophobic tail of the surfactant the outside of the coil has a hydrophilic character while the inside has a hydrophobic one (Immel & Lichtenthaler, 2000; Lundqvist et al., 2002b). Then the hydrophobic part of the starchesurfactant complex is inside the helix protected from the surrounding water which would explain why the complex does not adsorb at the airewater interface; thus the surfactantestarch complex is not surface active similarly to surfactant micelles. The binding of surfactants to starch can be positive cooperative, i.e. the binding of the first ligands make easier ulterior binding, or Langmuir type, i.e. non-cooperative as bound ligands does not affect succeeding binding. Several studies have demonstrated that

amylose binds different ligands, including various surfactants, by positive cooperative binding (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a; Rutschmann & Solms, 1990; Yamamoto et al., 1982, 1983) and have also proposed a Langmuir-type binding for surfactanteamylopectin systems (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a). On the one hand, the amylose positive cooperative effect have been attributed to a configuration change of amylose in solution from random coil to helix when one inclusion complex is formed thus lowering the resistance to form further complexes on the amylose chain (Lundqvist et al., 2002b). On the other hand, amylopectin could only bind surfactants with its most external chains (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002; Svensson et al., 1996), and due to the short average length of amylopectin chains, 20e25 glucose residues (Adkins, Banks, & Greenwood, 1966; Whistler & Daniel, 1984), this binding ought to be non-cooperative (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a; Yamamoto et al., 1983). It is also known that the length of the ramified chains of amylopectin shows a bimodal and in some cases a polymodal distribution (Hizukuri, 1985, 1986), and that the capacity of amylopectin to form complexes with a ligand rises with increasing external chain length (Banks, Greenwood, & Khan, 1970); therefore, it would be predictable that the surfactants would bind to amylopectin within a broad concentration range, being bound before all else to the largest amylopectin-sized chains (Lundqvist et al., 2002a). In this sense it is also noteworthy to point out that surfactants at low concentrations are preferentially bound to amylose than to amylopectin (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a; Rutschmann & Solms, 1990). Binding isotherms and Scatchard plots (Dahlquist, 1978; McGhee & Von Hippel, 1974; Scatchard, 1949) were obtained from surface tension measurements to analyze the nature of the binding between the starch and the ethoxylated non-ionic surfactants assayed; Fig. 4 shows the binding isotherms for all the surfactants assayed; meanwhile in Fig. 5, as an example, only the Scatchard plots for Findet 1214N/23 with and without buffer are supplied (Scatchard plots for Findet 10/15, 10/18 and 9Q/21.5NF are available in Supplementary Data). A theoretic binding isotherm have been previously proposed for the complexation of the cationic surfactant hexadecyltrimethylammonium bromide, CTAB, and a potato starch composed of 75% amylopectin and 25% amylose, been this theoretic isotherm the result of adding amylose and amylopectin individual contributions (Lundqvist et al., 2002a). Regarding the potato starch assayed, with analogous amylopectin

400

mol surf bound/g starch)

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Findet 10/15 Findet 10/18

300

Findet 1214N/23 Findet 1214N/23 + buffer Findet 9Q/21.5NF

200

100

0 0.1

1

10

100

1000

[L] ( Fig. 4. Binding isotherms of Findet 10/15, 10/18, 1214N/23 (with and without phosphate buffer 0.1 M, pH 7.5), and Findet 9Q/21.5NF to potato starch. Experimental data (symbols), simulated values (solid lines).

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30

4. Conclusions

Findet 1214N/23 Findet 1214N/23 + buffer

25

ν / [L]

20 15 10 5 0 0

20

40

60

80

100

bound/g

starch)

120

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140

Fig. 5. Scatchard plots for Findet 1214N/23 with and without phosphate buffer 0.1 M, pH 7.5. Experimental data (symbols), simulated values (solid lines).

and amylose composition, 80.2% and 19.8% respectively, for the all the surfactant tested the shapes of the binding isotherms, Fig. 4, were similar to that of the theoretic one proposed (Lundqvist et al., 2002a). In addition, binding isotherms similar to the theoretic one can also be observed for different ligandepotato starch systems, including other non-ionic surfactants (Bravo Rodríguez et al., 2008; Rutschmann & Solms, 1990). For all the surfactants tested the final part of the binding isotherms is markedly ascendant, Fig. 4, so according to the theoretic isotherm (Lundqvist et al., 2002a) and to the experimental data obtained with other non-ionic surfactants (Bravo Rodríguez et al., 2008) and ligands (Rutschmann & Solms, 1990) the rising end of the binding isotherm at high surfactant concentrations is due to the Langmuirtype binding of the surfactants assayed to the amylopectin. Concerning amylose binding, for all the surfactant tested no sigmoidal shape, typical of the cooperative binding (Lundqvist et al., 2002a), can be seen on the binding isotherms, Fig. 4, but the Scatchard plots, Fig. 5 and Figures in Supplementary data, exhibit at low surfactant concentrations the characteristic convex upward shape of positive cooperative binding (Dahlquist, 1978; McGhee & Von Hippel, 1974); then taken in account the aforementioned works (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a; Rutschmann & Solms, 1990; Yamamoto et al., 1982, 1983) and the facts that amylopectin binding must be non-cooperative (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a; Yamamoto et al., 1983) and that surfactants at low concentrations are preferentially bound to amylose (Bravo Rodríguez et al., 2008; Lundqvist et al., 2002a; Rutschmann & Solms, 1990), the cooperative binding corresponds to the surfactants binding to amylose. Thus, at the complexation beginning, the FAEO and APE tested will be bound to amylose by positive cooperative binding and afterwards, at higher surfactant concentrations, to amylopectin by non-cooperative binding until the critical micelle concentration is reached and surfactant micellization occurs. Regarding the phosphate buffer effect on Findet 1214N/23 binding isotherms, Fig. 4, the amount of bound surfactant for a fixed free surfactant concentration increased in the presence of phosphate buffer, i.e. salts augmented the binding capacity as already mentioned before in surfactantestarch binding course. In addition, from the analysis of their corresponding Scatchard plots, Fig. 5, the ordinate at the maximum value of the curve substantially increased when phosphate buffer was present in the solution. Thus, according to McGhee and Von Hippel (1974), that increase can be ascribed either to an increment of the cooperativity or to a reduction in the number of glucose units required to bind a molecule of surfactant by amylose.

The surface tension of the ethoxylated surfactants assayed increased in the presence of starch due to the formation of surfactantestarch complexes. A critical concentration of association was found for all the FAEO and APE suggesting a lower affinity to form complexes with starch in comparison with other non ethoxylated surfactants. The surfactant bound to starch was always directly proportional to the total added surfactant, with no starch saturation detectable previous to micellization. The hydrophobic interaction was responsible for this complexation and the FAEO were quantitatively more bound compared with Findet 9Q/21.5NF since APE chemical structure was assumed to interfere surfactantestarch approaching. The alkyl chain length of the FAEO and APE assayed seems to be a primary factor that determines the critical association concentration, and the bound surfactant ratio. Comparisons done with other non-ionic surfactants appear to support this fact. Binding isotherms and Scatchard plots confirmed that the FAEO and APE assayed at first preferentially bind to amylose by positive cooperative binding and later to amylopectin by non-cooperative Langmuir binding. The presence of salts proved to increase the surfactant binding capacity, being likely caused by an increase in the cooperativity or a reduction in the number of glucose units required to bind a molecule of surfactant by amylose. Acknowledgments This work was financed by the excellence project P07-TEP02603 of the Consejería de Innovación Ciencia y Empresa of the Junta de Andalucía, Spain. Appendix A. Supplementary data Scatchard plots for Findet 10/15, 10/18 and 9Q/21.5NF. This material is available free of charge via the Internet at http://www. sciencedirect.com. Scatchard plots for Findet 10/15, 10/18 and 9Q/21.5NF can also be found, in the online version, at doi:10.1016/j.foodhyd.2011.01.010. Notation APE: alkyl phenol ethoxylates APG: alkyl polyglycosides B1: critical concentration of association, mmol/L C1: critical micelle concentration in the absence of starch, mmol/L C2: critical micelle concentration in the presence of starch, mmol/L EO: average moles of ethylene oxide per mol of surfactant f: bound surfactant ratio FAEO: fatty alcohol ethoxylates H: initial surfactant humidity, % (w/w) HLB: hydrophilicelipophilic balance [L]: free surfactant concentration, mmol/L [L]bound: bound surfactant concentration, mmol/L [L]total: total surfactant concentration, mmol/L MW: average molecular weight, g/mol Nc: average number of carbon atoms of the surfactants’ alkyl chains Si: starch concentration, g/L on a dry weight basis Greek letters n: amount of surfactant bound to starch, mmol surfactant/g starch

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nmax: maximum amount of surfactant bound to starch, mmol surfactant/g starch s: surface tension, mN/m sB1: surface tension corresponding to critical concentration of association B1, mN/m s1: surface tension corresponding to critical micelle concentration C1, mN/m s2: surface tension corresponding to critical micelle concentration C2, mN/m

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