The History Of Rheological Characteristics

Published Date: 02 Nov 2017

Generally, enzymatic protein hydrolysates 'have a bitter taste which may be one of the main factors limiting the spread of their utilization in food. However, a low level of bitterness can, to a certain degree, be accepted as a necessary consequence of

proteolysis because it may be masked by the use of different products: 1) bitterness has been masked through the use of glutamic acid and glutamyl rich oligopeptides (Desmazeaud and Hermier (1972); 2) addition of polyphosphates has successfully masked the bitterness of casein hydrolysates (Tokita, 1969b); 3) gelatin can also achieve a similar effect, although not as weil as glycine (Stanley, 1981; Kilara, 1985); 4) Gly-Gly residues added to the Cterminal or N-terminal positions of bitter

peptides (Shinoda et al, 1987); 5) cycledextrin, because of its capacity for wrapping the hydrophobie functions of bitter amino acids. However, a large excess of a-cyclodextrin solution (150 g.l-1) is necessary to substantially mask the bitterness of synthesised peptides (Tamura et al, 1990).Addition of 10% ~-cyciodextrin is also effective in masking bitterness of skim milk hydrolysates (Helbig et al, 1980). However,further experiments are needed to establish its safety as a food additive. Until

1981, Japan was the only country where the use of cyciodextrin in food products was not limited (Szejtli, 1981). Monomeric and polymerie ~-cyclodextrin are commercially available; however, among polymers from oe-, ~- and y-cyciodextrins, ~- seems

to be the best in studies involving debittering of grapefruit juices (Shaw and Wilson,1985; Shaw and Buslig, 1986); 6) starch: this sugar polymer is expected to cover the bitter compounds with "its net structure" and to prevent them from reaching the bitter taste receptor sites. However, for this purpose it is necessary to heat the mixtures of starch and bitter peptides at 100 "C overnight (Tarnura et al, 1990); 7) proteins and peptides. Due to their affinity, addition of peptide compounds such as skim milk, soybean, casein, whey protein concentrate and casein hydrolysate to bitter amino acids and peptides is effective in decreasing or masking their bitterness (Tamura et al, 1990); 8) acidic amino acids. Bitterness can be masked by aspartic acid or glutamic acid but, on the negative side,they produce sourness (Noguchi et al,1975). However, an acidic solution of taurine reduces amino acid bitterness as effectively as other acidic ami no acids and that, without sourness. Bitterness of peptides is masked only when a large excess of amino acids is added (Tamura et al,1990).Casein digests obtained with the ficin/ pepsin system have a very bitter f1avour.Addition of pig's kidney homogenate as a source of exopeptidases (which remove amino acids singly from the ends of the peptide molecules) to the enzyme system raises the degree of hydrolysis, and yields casein digests relatively free of bitter ness, containing small peptides and over 50% free amino acids (Clegg, 1973). Clegg and Mc Millan (1974) th us concluded that within the spectrum of the proteolytic enzymes in pig's kidney there is a system capable of

hydrolysing the peptide responsible for the bitter flavour. lt was even possible to modify this last method for the production of kilogram quantities of the relatively non-bitter casein hydrolysate for use in medical trials (Clegg et al, 1974b). Although Japanese Bitter f1avourin dairy products: part Il 369 workers have been more concerned about bitterness in soybean than in milk products,it is possible to extrapolate their results ta milk proteins. Thus bitterness appearing during an enzymic proteolysis was found to be caused by bitter peptides bearing hydrophobic ami no acid residues (especially leucine) in the C-terminal structures, and to be lessened by decomposing these C-terminal structures with exopeptidases such as bovine pancreas carboxypeptidase A, Aspergillus acid carboxypeptidase and leucine aminopeptidase (Fujimaki et al, 1968; Yamashita et al,1969; Arai et al, 1970a, b). However, according to Fujimaki et al (1970b), debittering methods using these exopeptidases

encounter certain limitation, since such enzymes produce significant amounts of free ami no acids, mainly hydrophobic, that may affect the food quality of the proteolysates (eg if the exopeptidases are not effective in r

Zeta (ζ)-potential

The charge on an emulsion droplet can influence the rheological properties of an emulsion.The charge determines whether the droplets are aggregated or unaggregated; and thus affects the viscosity. The ζ -potential represents the charge of the droplets with adsorbed protein and/or biopolymer, plus the charge associated with any ions that move along with the droplet in the electric field (Surh et al. 2006). Table 2 summarizes the ζ –potential of the emulsion droplets as a function of concentration of the encapsulating agent (WPC-80). Whey proteins concentrate being negatively charged at neutral pH, showed negative ζ-potential on emulsion droplets, and ranged from 28.6 to -33.5 mV. There was no significant difference between the ζ-potential of 7.5 and 10% WPC emulsions, which was comparatively higher than that of other emulsions studied. It suggested that 7.5 and 10% WPC emulsions were the most stable systems in terms of ζ –potential. Khalloufi et al. (2008) reported around -50mV ζ –potential on the droplets of soybean oil based emulsions stabilized by WPI. The ζ -potential results lead to the hypothesis that electrostatic repulsion occurs between the oil droplets covered by negatively charged whey proteins. The relatively higher negative ζ -potential of whey protein concentrate coated droplets may account for greater intensity of the electrostatic repulsion force and superior stability of emulsion (Taherian et al. 2011). It can be concluded from the results of particle size distribution and ζ –potential that flaxseed oil emulsion produced by using 7.5% WPC-80 was the most stable showing narrow droplets size distribution and highest zeta potential.

Rheological characteristics

Depending upon the composition, particle size and its charge and viscosity, etc., emulsions show different rheological properties like Newtonian (ideal fluid) and Non-Newtonian (shear thinning, shear thickening, bingham plastics, etc.). Rheological properties of an emulsion play a significant role in determining the optimized conditions during the processing conditions (like pumping, mixing, flowing in pipes) or in designing a delivery system for a particular food application. Certain food systems like juices and beverages, which have very low viscosity, should not be changed during mixing with other ingredients, or during flowing & filling. Other food systems are highly viscous or gel like (for example, dressings, desserts) and in these cases the delivery system should not decrease the viscosity or disrupt the gel network (McClements, 2007).

Figure 3 represents the apparent viscosity (cP) under the shear rate (5-150 s-1) for emulsions having different concentration of WPC-80 during 28 days of storage. All the emulsions showed non-Newtonian, shear thinning (Pseudoplastic) behavior as viscosity decreased with increase in shear rate. Pseudoplastic behavior is the most common type of non-ideal behavior exhibited by food emulsions. Shear-thinning may occur for a variety of reasons in food emulsions (e.g., the spatial distribution of the particles may be altered by the shear field, or flocs may be deformed and disrupted) (Hunter 1993, Mewis and Macosko 1994). Viscosity increased with increase in protein concentration, maximum and minimum for emulsions containing 12.5 and 5% WPC, respectively. Normally, the viscosity of an emulsion increases with increasing droplet or total solids concentration. Viscosity also increased during the storage period; ranged from 7.85 to 23.7 cP at shear rate of 150 s-1.

For most non-Newtonian liquids, the viscosity decreases with an increase in shear rate, giving rise to what is known as shear thinning behavior or pseudoplasticity (Rao, 1977). Debowska, (2011) reported that rapeseed oil (30%) : WPC emulsions showed a non-Newtonian, shear thinning behavior. Shear thinning behavior was observed due to irreversible deformation and breakdown of flocs under the shear stress (McClements, 2005). The flow curves data for all emulsions fitted well to the power law model equation. The values for various coefficients at a shear rate of 5-150 s-1 are shown in Table 3. These parameters were evaluated under the said shear range as it is typical of food processes, such as flow through a pipe, stirring or mastication. It is clear from the table that all the emulsions showed very low pseudoplasticity, since the flow behavior index (n) of all the emulsions was in the range from 0.206 to 0.591. Similarly, Kuhn and Cunha, (2012) studied the flaxseed oil emulsions stabilized by whey protein isolates (total solids 33%) and reported that all o/w emulsions showed low pseudoplasticity with flow behavior index in the range of 0.78-0.95. It can be observed that consistency index (k) (Pa.sn) increased (from 0.154 to 0.511 Pa.sn) with increase in concentration of whey proteins, suggesting the increase in viscosity. Shear thinning behavior was also observed by Taherian et al. (2011) in o/w emulsion containing fish oil (10%): whey protein isolate (1%) emulsion. However, Lizarraga et al. (2008) found that corn oil-in-water emulsions (50g oil/100g) stabilized by WPC presented a Newtonian behavior.

Conclusions

The results of this study revealed that emulsions stabilized by whey protein concentrate showed good physical stability with no sign of phase separation, when homogenized at 3000 psi and stored at low temperature (4-7ºC) for 28 days. Higher pressure (4500 psi) with higher concentration of whey proteins led to gelation emulsion spontaneously. Results also showed that emulsion containing 7.5% WPC was the most stable in terms of particle size distribution, Z-average and ζ-potential. Rheological data revealed that all the emulsions showed shear thinning behavior, which is a characteristic of food emulsions. Pseudoplastic behavior suggested that emulsions were suitable during processing conditions, such as flowing in pipes, shearing or stirring, etc. From the data it could be concluded that flaxseed oil-in-water emulsions stabilized by WPC may be used to produce oxidatively and physically stable ω-3 fatty acid delivery systems for incorporation of nutritionally significant amounts of these important bioactive lipids into foods.

Conflicts of interest

The authors have no conflicts of interest.