Physical Stability And Fat Reduction

Fat reduction can have a profound effect on the physical stability of a product. One of the important roles of fat replacing ingredients, therefore, is their ability to maintain physical stability while at the same time providing acceptable quality in sensory terms. The importance of this issue was demonstrated when some products launched in the early 1990s suffered from apparent physical instability and had to be withdrawn from the market (e.g., some low-fat spreads and hard cheeses). Little published data exist on physical characteristics and stability in relation to fat reduction in foods, and virtually none at all that attempts to relate fat content and physical characteristics and stability to sensory characteristics. In practice, the issue of physical stability is compounded when moving from water-continuous liquid products to oil-continuous semi-solid products. For present purposes, the subject of changes in physical characteristics in relation to fat replacement will be illustrated with water-in-oil emulsions — i.e., spreads.

Various test and characterization procedures for assessing low-fat spreads were investigated by Bavington et al. (1992) using ten commercially available spreads ranging in fat content from 20 to 40%, and containing different aqueous and fat phases. All the techniques employed were able to distinguish between the spreads, with the results from conductivity measurements and stability tests correlating well with their observed microstructure. Other techniques such as differential scanning calorimetry, and solid fat content, spreadability, and texture measurements measured by penetrometry, were primarily related to the hardness of the spreads, although the spread microstructure also influenced the results obtained from these tests. Most of the physical tests performed were found to be related to the results obtained from sensory analysis of the spreads, since the textural characteristics tended to dominate sensory discrimination (see Section 4.5.1). Spreads with no aqueous phase stabilizers were found to have the smallest aqueous phase droplet size, while hardness and spreadability of the spreads were primarily, though not exclusively, related to the solid fat content. Other spread characteristics, such as stickiness, cloying character, the rate of breakdown in the mouth, etc., were dependent upon the type of aqueous phases stabilizers and the openness of the spread microstructure (Bav-ington et al., 1992). Figure 4.1 gives an example of some of the differences in selected physical characteristics for three of the ten commercial low-fat spreads investigated in relation to the composition of the aqueous phase as indicated on the labels. It is apparent, therefore, that low-fat spreads produced commercially differ considerably in terms of their physical characteristics. Overall, the differences in physical characteristics can be attributed in general terms to four main factors: fat content; composition of the aqueous phase; composition of the fat phase; and processing methods/conditions used.

Figure 4.1 Differences in selected physical characteristics of some of the commercial 40% fat spreads: Sample 2, containing sodium caseinate and sodium alginate; Sample 5, containing modified starch and milk proteins; Sample 6, containing gelatin and milk proteins. (Compiled from Bavington et al., 1992.)

Figure 4.1 Differences in selected physical characteristics of some of the commercial 40% fat spreads: Sample 2, containing sodium caseinate and sodium alginate; Sample 5, containing modified starch and milk proteins; Sample 6, containing gelatin and milk proteins. (Compiled from Bavington et al., 1992.)

In low-fat spreads containing 30% fat or less, the primary concern is to maintain a water-in-oil emulsion structure. In standard processing methods, this can easily be achieved using an appropriate emulsification system, which will allow the formation of the emulsion and the maintenance of the water-in-oil emulsion structure during processing (Clegg et al., 1993). However, while in physical terms an excellent emulsion can be achieved, with small water droplets (0.5 to 2.0 |m) homogeneously distributed throughout the continuous fat phase, the emulsifiers necessary for obtaining such a stable system give rise to undesirable organoleptic characteristics which manifest themselves predominantly in textural attributes (e.g., the slow rate of emulsion breakdown in the mouth), and these, in turn, affect flavor release characteristics of the spreads. Thus, certain changes in product formulation are necessary if an acceptable quality is to be achieved.

It is now an established commercial practice to use hydrocolloid stabilizers, either polysaccharide-based (e.g., starch, maltodextrin, sodium alginate) or protein-based (e.g., gelatin or sodium caseinate), in order to structure the aqueous phase of the low-fat spreads (Bavington et al., 1991). Such ingredients thicken and gel the aqueous phase droplets, giving rise to rheological changes that affect the different stages of manufacture, as well as the physical stability of the final product, which will be dependent on the type and concentration of the hydrocolloid(s) used.

Clegg et al. (1993), in a study of the role of aqueous phase stabilizers in low-fat spreads containing 30% fat, demonstrated an apparent destabilizing effect of hydrocol-loids on the physical characteristics of the final product. This was manifested in an increase in droplet size, changes in droplet size distribution, and a decrease in thermal and shear stability compared with spreads containing no aqueous phase stabilizers. A confocal laser scanning microscopy technique was used to study changes in microstructure in order to ensure minimum disruption of the spread systems, and also to be able to view the structure in three dimensions. While slight destabilization was observed for spreads containing gelatin and those containing gelatin and maltodextrin, a marked increase in droplet size was found in spreads containing gelatin and sodium alginate. In the case of spreads containing gelatin and modified starch, a significant destabilization was found, with evidence that some of the starch was disrupting the crystalline fat phase structure. Extensive destabilization was found in spreads containing gelatin and sodium caseinate as a result of the surface active properties of the sodium caseinate, and its tendency to promote a water-continuous emulsion, which was therefore counteracting the action of the emulsifiers present. However, while at a lower concentration of sodium caseinate the microstructure of the spread was more or less bi-continuous in nature, at higher concentrations of sodium caseinate the increased viscosity of the aqueous phase tended to counteract to a certain degree the surface active properties of the protein, thus limiting the extent to which the aqueous droplets coalesced during and after processing. As a result, some restabilization was apparent, and the presence of aqueous lakes and large droplets was evident. Figure 4.2 shows confocal images of selected low-fat spreads from these studies (Clegg et al., 1993).

On the other hand, destabilization of the water-in-oil low-fat emulsion systems, as affected by the presence of hydrocolloids, has an important positive effect on sensory characteristics in terms of the rate of emulsion breakdown in the mouth, melt-down properties and flavor release. In other words, a certain degree of instability needs to be introduced into a necessarily tight and stable low-fat oil-continuous emulsion system (which is needed to enable processing) in order to mimic the sensorily perceived characteristics of the full-fat spread. In addition, the source of the fat blend, the melting profile of the fat, and the ratio of liquid to crystalline fat affect the structure of the emulsion and organoleptic characteristics. A more extensive discussion of the sensory implications of fat reduction is given in Section 4.5.

Figure 4.2 Effect of aqueous phase stabilizers on microstructure of 30% fat spreads obtained using confocal laser scanning microscopy technique: (a) no aqueous phase stabilizers; (b) 2% gelatin/15% maltodextrin; and (c) 2% gelating/8% sodium caseinate. (From Clegg, S. M., Moore, A. K., and Jones, S. A., Leatherhead Food Res. Assoc. Res. Rep. No. 715, 1993. With permission.)

Figure 4.2 Effect of aqueous phase stabilizers on microstructure of 30% fat spreads obtained using confocal laser scanning microscopy technique: (a) no aqueous phase stabilizers; (b) 2% gelatin/15% maltodextrin; and (c) 2% gelating/8% sodium caseinate. (From Clegg, S. M., Moore, A. K., and Jones, S. A., Leatherhead Food Res. Assoc. Res. Rep. No. 715, 1993. With permission.)

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