Multi-scale properties of protein-stabilized foams

Investigation of the factors affecting foamability and foam stability of cold brew coffee

BACKGROUND

In this study, the foamability and foam stability of nitrogen-infused cold brew coffee, as affected by coffee variety (Arabica and Robusta), degree of roast (light, medium, dark), brewing temperature (4, 20, 35 °C), brew ratio (1:5-1:15 w/w; coffee/water), ground particle size (712, 647 and 437 μm volume mean diameter) and beverage temperature (4, 20 and 35 °C), were investigated.

RESULTS

Dynamic surface tension of cold brew, as determined from bubble tensiometry, decreased from 65-70 mN m^-1 to about 60 mN m^-1 as the bubble lifetime increased from 0.1 s to 1 s. Infusing the cold brew coffee (70 mL) with nitrogen gas for 30 s at 50 mL min^-1 generated 30-40 mL of foam head. At the same degree of roast, brews prepared from Arabica beans had more stable foam than those from Robusta. Foam stability increased with increasing degree of roast, increasing brewing temperature, decreasing particle size, and decreasing the beverage temperature. By contrast, brew ratio had relatively less effect on foaming properties. Nitrogen-containing constituents present in the 80% (v/v) ethanol-soluble fraction (55.9% of total dissolved solids) of the brew samples were important contributors to foaming, while the 80% (v/v) ethanol-insoluble fraction (42.3% of total dissolved solids) that contained polysaccharides was important in stabilizing the foam.

CONCLUSION

The foamability and foam stability of cold brew coffee are significantly affected by coffee variety, degree of roast, brewing temperature, ground particle size, and beverage temperatures. The foam properties are dictated by the low-molecular-weight nitrogen-containing compounds and high-molecular-weight polysaccharides present in the cold brew coffee. © 2022 Society of Chemical Industry.

Improving the colloidal and sensory properties of a caseinate hydrolysate using particular exopeptidases

Enzymatic hydrolysis with endopeptidases can be used to modify the colloidal properties of food proteins. In this study, sodium caseinate was hydrolyzed with Sternzym BP 25201, containing a thermolysin-like endopeptidase from Geobacillus stearothermophilus as the only peptidase, to a DH of 2.3 ± 1%. The hydrolysate (pre-hydrolysate) obtained was increased in its foam (+35%) and emulsion stability (+200%) compared to untreated sodium caseinate but showed a bitter taste. This hydrolysate was further treated with the exopeptidases PepN, PepX or PepA, acting on the N-terminus of peptides. Depending on the specificity of the exopeptidase used, changes regarding the hydrolysate properties (hydrophobicity, size), colloidal behavior (emulsions, foams) and taste were observed. No changes regarding the bitterness but further improvements regarding the colloidal stability (foam: +69%, emulsion: +29%) were determined after the application of PepA, which is specific for the hydrophilic amino acids Asp, Glu and Ser. By contrast, treatment with the general aminopeptidase PepN resulted in a non-bitter product, with no significant changes regarding the colloidal properties compared to the pre-hydrolysate (p < 0.05). Similar results to those for PepN (reduced bitterness compared to the pre-hydrolysate, enhanced colloidal stability compared to sodium caseinate) were also obtained using commercial Flavourzyme, which was reduced in its endopeptidase activity (exo-flavourzyme). In conclusion, the modifications obtained with the applied exopeptidases offer a potent tool for researchers and the industry to produce non-bitter protein hydrolysates with increased colloidal properties.

Foam rheology at large deformation

Large deformations are prone to cause irreversible changes in materials structure, generally leading to either material hardening or softening. Aqueous foam is a metastable disordered structure of densely packed gas bubbles. We report on the mechanical response of a foam layer subjected to quasistatic periodic shear at large amplitude. We observe that, upon increasing shear, the shear stress follows a universal curve that is nearly exponential and tends to an asymptotic stress value interpreted as the critical yield stress at which the foam structure is completely remodeled. Relevant trends of the foam mechanical response to cycling are mathematically reproduced through a simple law accounting for the amount of plastic deformation upon increasing stress. This view provides a natural interpretation to stress hardening in foams, demonstrating that plastic effects are present in this material even for minute deformation.

Effect of back mixing on thin-layer drying characteristics of sewage sludge by the appropriate foaming pretreatment

Herein, we report the creative combination of foaming and back mixing.