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Science of Hummus

Molecular Structure

The microscopic structure of hummus is generated when the mixture of cooked chickpeas, tahini, oil, water, garlic, salt, and either lemon juice or citric acid are subjected to a shear force such as blending, grinding, and/or mixing.  Processing on an industrial scale requires large, specialized colloidal mill machines, however, hummus prepared with household food processors can also yield a high quality result.  The difference between homemade and industrially produced hummus is the quality, duration, and strength of the shear force applied to the mixture. Brand name hummus has a much longer shelf life than homemade hummus because their ingredients are blended with a stronger shear force than that which can be delivered from any household appliance.  The particle sizes of commercial hummus are smaller, and thus take longer to separate. 

On the nano-scale level, images obtained from a scanning electron microscope reveal that the microscopic structure of hummus resembles a semisolid matrix of solid aggregates that are dispersed within a continuous liquid oil phase.  This solid matrix is composed of amylose and amylopectin starch molecules that were released from the gelatinized chickpea and the disintegrated sesame starch granule that aggregated together with denatured proteins and fibers.  This characteristic semi-solid gel network of hummus is formed when the continuous oil phase surrounds these starch-protein aggregates.  The particle size distribution of hummus is bimodal with one population of particles ranging from 0.05 to 12 μm and the other ranging from 12 to 900 μm.  The diameter of the solid aggregates is much larger ranging between 50 to 250 μm compared to the small oil droplets which were less than 12 μm. Although the bimodal particle size distribution suggests that some particles are smaller than 1μm, hummus behaves as a suspension because a majority of the particles are greater than 1μm. However, it does exhibit some colloidal properties especially on the nano-scale level.

The solid particle matrix of hummus, which is composed of starch, protein, and fibers, is held together by attractive intermolecular Van Der Waals forces. Molecules do not interact at a certain far enough distance apart from one another, however, once random fluid motion brings them close enough, they begin to experience VDW attractive forces that work to pull them together until some minimum potential energy is reached. Therefore, the stability of any hummus suspension is ultimately determined on the molecular level by the distance between molecules as well as the value of charges on the surface of molecules.  According to DLVO theory, the stability of a colloidal suspension requires a balance between electrostatic repulsion and Van Der Waals attraction. Ingredients added to hummus which affect pH level or salt concentration will influence the electrostatic repulsive forces between molecular components and, therefore, directly affect the rate of particulate agglomeration and shelf life.

In foods like hummus, the pH determines the charge density at a molecular surface, while the thickness of the electric surface layer is dependent on the pH and salt concentration.  Because salt is an added ingredient, the salt concentration within hummus can change depending on the recipe formulation.  However, the amount of salt added should be carefully controlled because salt concentration in mixtures such as hummus has a direct effect on a charge carrier's net electrostatic effect in solution, and how far those electrostatic effects persist. This particular property is also known as Debye length.  It is known that the Debye length is large at low ion (low salt) concentrations and particles will be repulsive at greater distances.  However, as salt concentration increases, the effect of electrostatic repulsion over large distances is dampened, which may contribute to more rapid particulate agglomeration. Citric acid or lemon juice are also added ingredients that can influence particlulate aggregation and stability due to its pH lowering activity in hummus.  Depending on the starting molecular composition of the hummus, the lowering of pH may alter the charge of proteins or other molecules able to gain or lose a hydrogen.  After the addition of citric acid or lemon juice, the pH decreases and more molecules will exist in a protonated state.  This also affects molecular electrostatic interactions, which contributes to the stability and rate of particulate agglomeration in hummus.

Physical Properties Important for Storage and Shelf Life:

Because hummus is a minimally processed food product, its shelf life is dependent on storage conditions.  The recommended storage temperature for hummus is refrigeration at 4˚C because lower temperatures are proven to delay the two major concerns regarding hummus shelf life and stability: physical separation and biochemical/microbial degeneration.  Although hummus has a relatively low pH (5.1), its high water activity makes it a favorable environment for microbial growth.  The addition of potassium sorbate and salt mediates the propagation of particular microorganisms that thrive in such conditions, like yeasts and molds.  Yet the continuous oil phase of a hummus suspension, such as soybean oil in Sabra brand hummus, is highly susceptible to lipid oxidation because double bonds are more prone to hydrogen atom extraction and alkyl radical formation.  Lipid oxidation can cause a rancid flavor, degrade nutritional quality, and lead to undesirable changes in color and texture.  Because lipid oxidation is favored at higher temperatures, refrigeration helps to delay this degenerative mechanism.  However, the presence of salt increases the rate of lipid oxidation in foods. Therefore, it is necessary for manufacturers to formulate a product with the right amount of salt in order to balance its role in flavor, electrostatic repulsion, and physiochemical stability.

The physical separation of the oil phase from the particulate matrix may also reduce the shelf life of a hummus suspension. Since hummus is a heterogeneous mixtures of relatively large insoluble particles suspended throughout a liquid oil medium, the particles will separate out if left alone.  This occurs because the coalescence of oil droplets is energetically favorable because it reduces the surface tension between the hydrophilic and hydrophobic moieties within the microstructure of hummus.  The oil then floats to the top because it is less dense than the solid particulate aggregates.