Microemulsions and Emulsions in Foods - American Chemical Society

Chapter 12 Function of a-Tending Emulsifiers and Proteins in Whippable Emulsions 1

J. M . M . Westerbeek , and A. Prins

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Product Research and Analysis Laboratory, Campina-Melkunie, P.O. Box 13, 5460 BA Veghel, The Netherlands Department of Food Science, Agricultural University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands

Downloaded by MONASH UNIV on April 2, 2017 | http://pubs.acs.org Publication Date: December 26, 1991 | doi: 10.1021/bk-1991-0448.ch012

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In whippable emulsions a combination of proteins and small molecular emulsifiers is often applied as surface active agents in order to obtain desired whipping characteristics like e.g. firmness and volume. The consistency of these products is closely related to the aggregation of the dispersed fat globules. This study is performed on model systems, consisting of dispersions of the water insoluble a-tending emulsifier glycerol lacto palmitate (GLP) in sodium caseinate solutions. Though it is well known that the presence of relatively large amounts of so-called a-tending emulsifiers strongly promote aggregation of the fat globules the mechanism which explains this phenomenon is s t i l l unknown. The study included DSC, x-ray and neutron diffraction measurements to elucidate the micro-structure of the dispersed particles interfaces. Based on the obtained results a hypothetical model was developed explaining the colloidal stability of the fat particles in these whippable systems at different temperatures.

Whipped emulsions should always have solid-like properties, because the aerated product has to be stable against flow for a long period of time. In the case of whipped products a yield stress, caused by the presence of a particle network, is very often the way to prevent the product from flowing. The formation of such a network is frequently induced by the addition of smallmolecular emulsifiers. Lipophilic, so-called a-tending 0097-6156/91/0448-0146$06.00/0 © 1991 American Chemical Society El-Nokaly and Cornell; Microemulsions and Emulsions in Foods ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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e m u l s i f i e r s , l i k e propylene g l y c o l monostearate (PGMS), a c e t y l a t e d monoglycerides (ACTM) o r l a c t y l a t e d monoglyc e r i d e s (GLP) are e s p e c i a l l y e f f e c t i v e i n promoting aggregation o f f a t g l o b u l e s (1,2). These a-tending emuls i f i e r s may be c h a r a c t e r i z e d by the f a c t t h a t they a r e non-polymorphic, can o n l y e x i s t i n the a - c r y s t a l l i n e form below the m e l t i n g p o i n t o f the hydrocarbon c h a i n s and are p r a c t i c a l l y i n s o l u b l e i n water (2,3). I t i s not understood what mechanism i s r e s p o n s i b l e f o r this extensive f a t p a r t i c l e aggregation phenomenon. Recently, Buchheim and Krog (4) suggested t h a t c r y s t a l l i z a t i o n o f supercooled f a t may p l a y an important r o l e i n the occurrence o f f a t p a r t i c l e aggregation i n whippable emulsions. I t i s indeed t r u e t h a t c r y s t a l l i z a t i o n phenomena p l a y an important r o l e i n the formation of a p a r t i c l e network i n these emulsions. In whipped cream, the network probably consists of partly crystallized o i l droplets. However, s i n c e whippable emulsions, which c o n t a i n r e l a t i v e l y l a r g e amounts o f an a-tending e m u l s i f i e r , do not churn d u r i n g whipping ( 5 ) , i t seems l i k e l y that *another mechanism i s r e s p o n s i b l e f o r the i n s t a b i l i t y o f these emulsions. I t i s well-known that mixtures o f water and e m u l s i f i e r s such as p h o s p h o l i p i d s (6,7) o r monoglycerides (8,9) may form l a m e l l a r , c u b i c o r hexagonal mesomorphic phases (10) above the c r y s t a l l i z a t i o n temperature o f the hydrocarbon chains o f the e m u l s i f i e r . When these systems are cooled down a l a m e l l a r g e l phase may form. T h i s g e l phase i s c h a r a c t e r i z e d by a l a m e l l a r s t r u c t u r e o f a l t e r n a t i n g l a y e r s o f e m u l s i f i e r and water molecules (10,11). The l i p i d molecules are c r y s t a l l i z e d i n the a-polymorphic form. In F i g u r e 1 t h r e e types o f a-gel phase s t r u c t u r e s a r e presented (10). L i t e r a t u r e o f t e n s t a t e that a-tending e m u l s i f i e r s do not show lyotropic mesomorphism (12-14). However the p h y s i c a l behaviour o f GLP/water mixtures and g l y c e r o l monostearate/water mixtures i s very s i m i l a r below the c r y s t a l l i z a t i o n temperature o f the hydrocarbon c h a i n s o f the l i p i d molecules (1,5). Depending on the amphiphile concentration, these systems both g e l under these c o n d i t i o n s , and show e x c e l l e n t whipping p r o p e r t i e s . Our hypothesis i s t h a t a-tending e m u l s i f i e r s a r e a b l e t o form an a-gel phase with water a t the i n t e r f a c e o f f a t p a r t i c l e s below the c r y s t a l l i z a t i o n temperature o f the e m u l s i f i e r hydrocarbon c h a i n s . The aggregated f a t p a r t i c l e s are l i n k e d t o each other by t h i s g e l phase. In t h i s paper a study on the p h y s i c a l behaviour o f g l y c e r o l l a c t o p a l m i t a t e (GLP), as an example o f an a-tending e m u l s i f i e r , i s presented. Our aim has been t o propose a s a t i s f y i n g mechanism f o r the s t r u c t u r e formation i n emulsions c o n t a i n i n g an a-tending e m u l s i f i e r . The proper technique t o study l i p i d polymorphism and mesomorphic behaviour i s a combination o f small and wide angle X-ray

American Chemical Society Library El-Nokaly and Cornell; and Emulsions in Foods 1155Microemulsions 15th St., N.W.

ACS Symposium Series; American Chemical WtthiiMrtnn ft 0 Society: fMtaff Washington, DC, 1991.


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f IT mm UiUUM

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B F i g u r e 1 : The schematic s t r u c t u r e o f three p o s s i b l e l a m e l l a r g e l phases formed by s u r f a c t a n t s i n contact with water below the c r y s t a l l i z a t i o n temperature o f t h e i r hydrocarbon chains.

El-Nokaly and Cornell; Microemulsions and Emulsions in Foods ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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d i f f r a c t i o n (SAXD and WAXD) i n a d d i t i o n t o d i f f e r e n t i a l scanning calorimetry (DSC). Furthermore, neutron d i f f r a c t i o n has proved t o be a u s e f u l technique f o r s t r u c t u r a l a n a l y s i s of mesomorphic phases as was the case f o r p h o s p h o l i p i d s (15-17).


EXPERIMENTAL Materials. G l y c e r o l l a c t o p a l m i t a t e ( GLP ) was purchased from Grindsted, Denmark. The sample i s a very complex mixture of mono-, d i - and t r i g l y c e r i d e s of l a c t i c and p a l m i t i c a c i d . According t o the s u p p l i e r the product c o n t a i n s about 15% l a c t i c a c i d . Sodium c a s e i n a t e was obtained from DMV, Veghel, The Netherlands. T h i s sample i s a s p r a y - d r i e d milk p r o t e i n i n powder form c o n t a i n i n g 94.5% p r o t e i n ( N x 6.38 ) on moisture f r e e b a s i s , 5.2% moisture, 4.1% ash and 0.8% f a t . Glucose syrup was obtained from Cerestar, The Netherlands. I t i s a combined acid/enzymatic hydrolysed c o r n s t a r c h which mean Dextrose E q u i v a l e n t (DE) = 35. P r e p a r a t i o n of Spray-Dried GLP Powders. P r i o r t o sprayd r y i n g a concentrated emulsion of GLP p a r t i c l e s i n water was prepared c o n t a i n i n g 30% GLP ( w/w ), 5% sodium c a s e i n a t e , 15% glucose syrup and 50% demineralized water. H i t h e r t o the dry p r o t e i n powder was d i s p e r s e d d i r e c t l y i n the melted e m u l s i f i e r at 70°C. T h i s protein/emuls i f i e r mixture was added t o the water phase a t 70°C. Subsequently t h i s d i s p e r s i o n was homogenized a t a cons t a n t pressure of 100 atmosphere a t 70°C i n a high-pressure homogenizer ( Rannie, 100 1/hr ). T h i s emulsion was s p r a y - d r i e d with an A/S NIRO Atomizer as d e s c r i b e d prev i o u s l y (18). Then the powders were s t o r e d a t room temperature f o r DSC o r d i f f r a c t i o n s t u d i e s . P r e p a r a t i o n of Freeze-Dried GLP Powders. Melted GLP was mixed with demineralized water a t a temperature o f about 60°C with a S o r v a l l mixing apparatus. During t h i s mixing procedure the sample was g r a d u a l l y c o o l e d down t o a temperature below the c r y s t a l l i z a t i o n p o i n t of the emuls i f i e r mixture. Then the obtained GLP g e l was f r e e z e d r i e d and s t o r e d at room temperature. D i f f e r e n t i a l Scanning C a l o r i m e t r y . DSC was performed with a M e t t l e r TA-3000 system. In the DSC-cup the sample amount v a r i e d between 10 and 20 mg, depending on the c o n c e n t r a t i o n of p o t e n t i a l l y c r y s t a l l i z i n g matter present i n the sample. The heating curves which are shown together i n one Figure, were not c o r r e c t e d f o r d i f f e r e n ces i n the amount o f sample i n the DSC-cup. Therefore the peak areas below the curves may not be i n t e r p r e t e d as being r e p r e s e n t a t i v e of the heat values per u n i t amount of sample.


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X-Ray D i f f r a c t i o n . SAXD-measurements have been p e r f o r med w i t h the s p r a y - d r i e d GLP powder samples. Part o f these samples were d i s p e r s e d i n d e m i n e r a l i z e d water the day b e f o r e the x-ray d i f f r a c t i o n experiments. The SAXDmeasurements were conducted with a Kratky camera, manuf a c t u r e d by A. Paar. The camera was equipped with a Braun one-dimensional p o s i t i o n s e n s i t i v e d e t e c t o r which was connected t o a Braun multi-channel a n a l y z e r . The r a d i a t i o n source was a PW-1729 x-ray generator, produc i n g N i - f i l t e r e d CuKa-rays, i t s wavelength being 0.154 nm. Each channel of the multi-channel a n a l y z e r c o r responded t o a c e r t a i n d i f f r a c t i o n angle. The samples were put i n t o small g l a s s c a p i l l a r i e s (0 e = 1.0 mm) with a w a l l t h i c k n e s s o f about 0.01 mm. C a l i b r a t i o n of the apparatus was carried out with l e a d stearate. Measurements were u s u a l l y performed w i t h i n 30 minutes. C o r r e c t i o n s were made f o r background n o i s e and the c u r ves were subsequently desmeared according t o a program based on Lake's theory (19.).


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Neutron D i f f r a c t i o n . Neutron d i f f r a c t i o n experiments were performed a t ISIS, (Rutherford Appleton Laboratory, Didcot, England). The s m a l l angle d i f f r a c t i o n measurements were conducted with the LOQ spectrometer. The s p a l l a t i o n neutron source ISIS produces i n t e n s e neutron b u r s t s 50 times per second by means of c o l l i s i o n s of a h i g h l y e n e r g e t i c , pulsed proton beam with a Uranium t a r g e t . The f a s t neutrons from the t a r g e t s t a t i o n are slowed down i n a hydrogen moderator at a working temper a t u r e of 25 K i n order t o o b t a i n a c o l d neutron enhanced thermal neutron spectrum. A r o t a t i n g d i s k chopper a t 25 Hz removes a l t e r n a t e p u l s e s from ISIS t o avoid frame o v e r l a p from adjacent p u l s e s . Neutrons o f wavelengths v a r y i n g from 2 t o 10 A are recorded by a two-dimensional p o s i t i o n s e n s i t i v e neutron d e t e c t o r . The area d e t e c t o r i s a m u l t i w i r e (128x128), BF filled, proportional d e t e c t o r . I t has 64x64 channels, i t s r e s o l u t i o n being 1 cm i n both d i r e c t i o n s , each one of them c o n t a i n i n g about 80 time of f l i g h t channels. They are a l l handled by an in-house data a c q u i s i t i o n system and a Microvax computer. Reduction of the raw LOQ's time of f l i g h t data t o a composite c r o s s s e c t i o n I(Q) i s done by accurate t r a n s m i s s i o n c o r r e c t i o n s over a wide range of wavelengths . Dry and hydrated GLP powder samples, c o n t a i n i n g v a r i a b l e amounts of D 0 were brought i n t o c i r c u l a r shaped quartz cuvets. The weight of the samples i n s i d e the c e l l s was determined i n order t o be a b l e t o c a l c u l a t e the average c r o s s s e c t i o n of each sample. The c e l l t h i c k n e s s c o u l d be e i t h e r one or two m i l l i m e t e r s , depending on the amount of D 0 and the amount of a i r present i n each g e l sample. Each measurement l a s t e d between 1 and 3 hours 10

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f o r each sample depending upon whether 1*10 or 3*10 counts were r e q u i r e d f o r acceptable data s t a t i s t i c s .

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RESULTS D i f f r a c t i o n Studies. The e x i s t e n c e of a-gel phases can e a s i l y be c h a r a c t e r i z e d with d i f f r a c t i o n techniques. The s w e l l i n g of a c r y s t a l l a t t i c e caused by the uptake of water should c l e a r l y appear from an i n c r e a s e of the long spacing which can e a s i l y be measured with SAXD. The s w e l l i n g of c r y s t a l l i z e d GLP was s t u d i e d by means of h y d r a t i o n experiments with s p r a y - d r i e d GLP samples. To t h i s end the powder, c o n t a i n i n g c r y s t a l l i z e d GLP part i c l e s with an average s i z e s m a l l e r than 1 urn, was brought i n t o contact with demineralized water at room temperature. A f t e r about 24 hours we performed SAXD experiments on both dry and wet samples. In F i g u r e 2 two examples of SAXD s p e c t r a are represented. Comparing the two d i f f r a c t i o n s p e c t r a i t i s obvious from the i n c r e a s e of the long spacing t h a t hydration, though not complete, has indeed occurred at room temperature. Comparable i n f o r m a t i o n may be obtained with neutron d i f f r a c t i o n . In p r i n c i p a l water molecules can be l o c a t e d w i t h i n the s t r u c t u r e of a m u l t i l a y e r i f h y d r a t i o n i s performed with D 0 (15,17). In order t o o b t a i n improved knowledge on the h y d r a t i o n p r o p e r t i e s of GLP, we p e r f o r med neutron d i f f r a c t i o n on three d i f f e r e n t types of GLP samples hydrated with v a r i a b l e amounts of D 0. The f i r s t s e r i e s was prepared from the s p r a y - d r i e d GLP-powder. The second s e r i e s of experiments was performed with a freeze-dried GLP powder sample, f r e e of any other a d d i t i o n a l component. The f i r s t and second s e r i e s of samples were hydrated at ambient temperature. The t h i r d s e r i e s of samples c o n s i s t e d of GLP g e l s , which were obtained by heating v a r i a b l e amounts of D 0 and GLP t o a temperature of about 60 °C i n s e a l e d b o t t l e s followed by c o o l i n g t o ambient temperature under vigorous mixing. The d i f f e r e n c e s between the three samples were thus related to composition, average particle size and temperature at which the samples were hydrated with D 0. The r e s u l t s of these measurements are represented i n F i g u r e 3. T h i s F i g u r e shows the measured long spacing of the g e l phase of GLP as a f u n c t i o n of the D 0 concentrat i o n i n weight percentages. I t i s obvious t h a t the values f o r the long spacings of the f u l l y hydrated g e l phases depend on the p r e p a r a t i o n method. The d i f f e r e n c e s are not due t o experimental e r r o r , because t h i s F i g u r e c l e a r l y shows t h a t the values of the long spacing f o r sample B and C are quite constant at high D0 c o n c e n t r a t i o n s . Our e x p l a n a t i o n f o r t h i s e f f e c t i s t h a t fractionation of specific GLP components at the i n t e r f a c e of the p a r t i c l e s probably causes d i f f e r e n c e s i n the g e l phase composition which, i n t u r n , may cause 2

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20/ (deg) F i g u r e 2 : SAXD curves obtained a t 20°C f o r d i f f e r e n t GLP samples. A : Dry, s p r a y - d r i e d GLP powder sample. B : Hydrated, GLP powder sample c o n t a i n i n g 67% water. Both smeared (- - -) and desmeared ( ) curves are represented.


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d i f f e r e n c e s i n the v a l u e o f the long p o i n t o f maximum h y d r a t i o n .

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Temperature Dependence. A proper technique t o d e t e c t phase t r a n s i t i o n s i n a system l i k e mixtures o f an extending e m u l s i f i e r and water i s d i f f e r e n t i a l scanning c a l o r i m e t r y . The i n f l u e n c e o f water on t h e phase behav i o u r o f a-tending e m u l s i f i e r s has been s t u d i e d by means of both c o o l i n g and h e a t i n g experiments ( 5 ) . In F i g u r e 4 the i n f l u e n c e o f the presence o f water on the m e l t i n g p r o p e r t i e s o f s p r a y - d r i e d GLP i s shown. I t i s obvious t h a t the complete m e l t i n g curve i s s h i f t e d a few degrees towards a higher temperature, when the GLP p a r t i c l e s a r e brought i n t o c o n t a c t with water a t ambient temperature. T h i s experiment confirms our hypothesis o f h y d r a t i o n o f a-tending e m u l s i f i e r s below the c r y s t a l l i z a t i o n temperat u r e o f the a m p h i p h i l i c components hydrocarbon c h a i n s . The h e a t i n g curves suggest t h a t a l a r g e p a r t o f the e m u l s i f i e r molecules p a r t i c i p a t e s i n the h y d r a t i o n process, because the s h i f t observed almost accounts f o r the complete m e l t i n g curve. T h i s e f f e c t was not expected t o occur so e x p l i c i t l y . I f the g e l phase would c o n s i s t o f o n l y p a r t o f the t o t a l e m u l s i f i e r sample, i t would have been l i k e l y t h a t the melting peak o f t h e hydrated emuls i f i e r , i n comparison with the d r y sample, would have been broader o r would even have been s p l i t up i n two peaks. Instead the endothermic heat peaks o f the hydrat e d samples a r e s i g n i f i c a n t l y sharper than those o f t h e dry samples which may be i n d i c a t i v e o f a b e t t e r f i t t i n g o f molecules with the bulky head groups i n t o the c r y s t a l l a t t i c e o f t h e a-gel phase. F i n a l l y we w i l l show that with temperature dependent SAXD measurements i t can be proved t h a t the long spac i n g s we have shown indeed a r e r e l a t e d t o a g e l phase s t r u c t u r e . In F i g u r e 4 we showed t h a t t h e main GLP f r a c t i o n o f the hydrated sample melts a t a temperature o f about 46°C, whereas the d r y sample melts a t a temperatur e o f about 43°C. In F i g u r e 5 the SAXD curves o f the hydrated GLP powder a t d i f f e r e n t temperatures a r e shown. These curves a r e not c o r r e c t e d i n such a way t h a t the peak h e i g h t s can be compared with each o t h e r . The l o n g spacing o f about 62A disappears a t i n c r e a s i n g temperat u r e , i n d i c a t i n g t h a t the a-gel phase melts over a wide temperature range. At 47°C t h i s long spacing i s no l o n ger present i n the SAXD spectrum. Furthermore, this experiment g i v e s evidence f o r t h e a s s e r t i o n t h a t GLP does not show mesomorphism above t h e m e l t i n g temperature of i t s hydrocarbon chains (12,14). The weak r e f l e c t i o n a t a d-value o f about 48 A i s probably r e l a t e d t o c r y s t a l s formed by a r e l a t i v e l y s m a l l p a r t o f t h e GLP mixt u r e ( a l s o compare with F i g u r e 4 ) .


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F i g u r e 3 : The long spacing o f three d i f f e r e n t GLP samples as f u n c t i o n o f the weight percentage deuterium oxide as determined with neutron d i f f r a c t i o n a t amb i e n t temperature. A : Spray-dried GLP powder c o n t a i n i n g 60% GLP. B : F r e e z e - d r i e d GLP powder c o n t a i n i n g 100% GLP. C : melted GLP.

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T/(°C) F i g u r e 4 : Heating curves o f completely c r y s t a l l i z e d dry and hydrated s p r a y - d r i e d e m u l s i f i e r powders d e t e r mined with DSC (Heating r a t e = l°C/minute).


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DISCUSSION From the r e s u l t s shown i n t h i s paper i t may be concluded without doubt t h a t GLP i s able t o form a hydrated g e l phase s t r u c t u r e below the melting temperature of i t s hydrocarbon c h a i n s . The s t a b i l i t y of t h i s g e l phase i s e x c e l l e n t . We have proved with wide angle x-ray d i f f r a c t i o n t h a t the a-polymorphic form of both dry and hydrated GLP samples i s very s t a b l e between 4 and 25°C ( 5 ) . T h i s may seem to be c o n t r a d i c t o r y t o r e s u l t s obtained f o r the s t a b i l i t y of the a-gel phase of other emulsif i e r s l i k e d i s t i l l e d monoglycerides. The s t a b i l i t y of the a-gel phase of monoglycerides l i k e g l y c e r o l monostearate i s relatively poor ( 9 ) . Dependent on the c o n d i t i o n s (pH, temperature, s a l t c o n c e n t r a t i o n ) a t r a n s i t i o n of t h i s g e l phase w i l l occur i n t o the B-polymorp h i c form which i s accompanied with water e x c l u s i o n from the c r y s t a l l a t t i c e . I t has been reported the s t a b i l i t y of the g e l phase of monoglycerides may be prolonged, when small amounts of f a t t y a c i d s (20) or l e c i t h i n (21) are added t o the system. Presumably the formation of mixed c r y s t a l s i s favourable f o r the s t a b i l i t y of the ag e l phase of an amphiphile. C r y s t a l l i z a t i o n i n the a - m o d i f i c a t i o n i s a p r e r e q u i s i t e f o r the formation of g e l phases. Krog and Lauridsen (22) a s s e r t t h a t e m u l s i f i e r s which are non-polymorphic and s t a b l e i n the a - m o d i f i c a t i o n l i k e sodium s t e a r o y l l a c t y l a t e or t e t r a g l y c e r o l monostearate may form g e l s with water e x h i b i t i n g long-term s t a b i l i t y . Therefore, i t i s c l e a r t h a t a-tending e m u l s i f i e r s being non-polymorphic are p o t e n t i a l g e l phase forming amphiphiles. The s t a b i l i t y of the a-polymorphic form of GLP i s probably d e t e r mined mainly by s t e r i c r e p u l s i o n of the bulky head groups of the mono- and d i g l y c e r i d e s and the l a c t a t e e s t e r s of these molecules. Hydration of the p o l a r head groups may provide an a d d i t i o n a l s t a b i l i z i n g effect which w i l l be d i s c u s s e d l a t e r . The s t r u c t u r a l parameters of the a-gel phase of GLP have not been d i s c u s s e d y e t . I t i s not known which amphiphill i c components p a r t i c i p a t e as s t r u c t u r e elements of t h i s g e l phase s t r u c t u r e . Probably i t c o n t a i n s both mono- and d i g l y c e r i d e s and l a c t a t e d e s t e r s of these components. We have shown t h a t the long spacing of the hydrated g e l phase i s dependent on the p r e p a r a t i o n method. Therefore a-gel phase i n f a c t i s not a completely c o r r e c t term f o r the hydrated c r y s t a l l i n e s t r u c t u r e of GLP. However f o r reasons of s i m p l i c i t y we w i l l d e f i n e t h i s s t r u c t u r e as being an a-gel phase. F i g u r e 6 represents a h y p o t h e t i c a l schematic molecular s t r u c t u r e f o r the g e l phase of GLP, which was brought i n contact with water above the t r a n s i t i o n p o i n t of the e m u l s i f i e r . In t h i s model i t i s assumed t h a t the dry b i l i p i d l a y e r t h i c k n e s s amounts to a value of approximately 55 A. I f the dry b i l i p i d l a y e r


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INT/(-) Downloaded by MONASH UNIV on April 2, 2017 | http://pubs.acs.org Publication Date: December 26, 1991 | doi: 10.1021/bk-1991-0448.ch012

43°C'

-I /

/

10

/

D-value/nm

F i g u r e 5 : X-ray d i f f r a c t i o n curves o f hydrated sprayd r i e d GLP powders a t i n c r e a s i n g temperature. Both smeared (- - -) and desmeared ( ) curves are r e presented .

F i g u r e 6 : A schematic molecular model f o r the a-gel phase o f g l y c e r o l l a c t o p a l m i t a t e hydrated above the c r y s t a l l i z a t i o n temperature of the e m u l s i f i e r .



12.



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t h i c k n e s s i s c o r r e c t , the water l a y e r t h i c k n e s s o f the g e l phase would o n l y be 8 A or 9 A. In the case o f s p r a y - d r i e d GLP samples, which are hydrated a t room temperature, the value f o r t h i s water l a y e r t h i c k n e s s probably i s a few Angstroms l a r g e r . DSC-measurements have i n d i c a t e d t h a t the m e l t i n g temper a t u r e of GLP i s s h i f t e d towards a higher temperature i n the presence o f water. Such an e f f e c t has been observed with s e v e r a l other amphiphile/water mixtures (23,24) On the other hand, the melting temperature o f monoglycerides (22) o r p h o s p h o l i p i d s (25) decreases i n the presence of water. I t i s obvious that the m e l t i n g temperature of c r y s t a l l i z e d hydrocarbon chains o f an amphiphile depends not o n l y on the water c o n c e n t r a t i o n and hydrocarbon c h a i n l e n g t h but e s p e c i a l l y on the nature of the p o l a r head group. In our o p i n i o n h y d r a t i o n o f such a group may cause both an i n c r e a s e or a decrease i n m e l t i n g temperat u r e of the chains, dependent on whether h y d r a t i o n l e a d s t o an i n c r e a s e or a decrease i n the l a t t i c e energy o f the c r y s t a l l i n e amphiphile. In the case o f GLP both a decrease i n s t e r i c head group r e p u l s i o n between molecul e s i n o p p o s i t e l a y e r s and formation o f hydrogen bonds may p l a y an important role i n the m e l t i n g point increase. What does t h i s phase behaviour of GLP mean f o r the s t a b i l i t y of emulsions which c o n t a i n both p r o t e i n s l i k e sodium c a s e i n a t e and an a-tending e m u l s i f i e r ? In the case of GLP c o n t a i n i n g emulsions we suggest a temperatur e dependent model f o r the s t r u c t u r e o f the o / w - i n t e r f a ce o f the emulsion d r o p l e t s as represented i n F i g u r e 7 . At a temperature exceeding the m e l t i n g p o i n t o f the amphiphile mixture the emulsion i s s t a b i l i z e d by the p r o t e i n s and perhaps by the r e l a t i v e l y more h y d r o p h i l i c components o f the added e m u l s i f i e r . Dependent on i t s c o n c e n t r a t i o n below the m e l t i n g p o i n t o f the e m u l s i f i e r an a-gel phase i s formed a t the o/w-interface. I t i s l i k e l y that the s t a b i l i z i n g p r o t e i n s w i l l a t l e a s t p a r t l y desorb spontaneously from the o/w-interface (26) and subsequently f l o c c u l a t i o n w i l l then occur. The p a r t i c l e s w i l l not c o a l e s c e because o f the s t a b i l i z i n g h y d r a t i o n f o r c e exerted by the hydrated e m u l s i f i e r molecules. The l a s t t e n years a b e t t e r i n s i g h t i n t o h y d r a t i o n f o r c e s has been obtained (27). Hydration f o r ces are o n l y a c t i v e over a very s h o r t range o f about 30A. I f e l e c t r o s t a t i c r e p u l s i o n i s o f no importance, thus i n the case of n e u t r a l amphiphiles o r i o n i c s at high s a l t levels, amphiphiles may form mesomorphic s t r u c t u r e s as a r e s u l t of van der Waals f o r c e s . I t i s b e l i e v e d t h a t the van der Waals f o r c e s are counterbalanced by the s h o r t range h y d r a t i o n f o r c e s . In t h i s r e s p e c t i t i s s i g n i f i c a n t t o mention t h a t i t has been shown t h a t h y d r a t i o n f o r c e s may form a s t r o n g b a r r i e r t o both b i l a y e r aggregation and f u s i o n o f p h o s p h o l i p i d membranes



158


T>45°C

T

Microemulsions and Emulsions in Foods - American Chemical Society

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