Ingredients Cultured cream. Processed sour cream can include any of the following additives and preservatives: grade A whey, modified food starch,
sodium phosphate,
sodium citrate,
guar gum,
carrageenan,
calcium sulfate,
potassium sorbate, and
locust bean gum.
Processing The manufacturing of sour cream begins with the
standardization of fat content; this step is to ensure that the desired or legal amount of milk fat is present. As previously mentioned the minimum amount of milk fat that must be present in sour cream is 18%. During this step in the manufacturing process, other dry ingredients are added to the cream, such as additional whey. Another additive used during this processing step is a series of ingredients known as stabilizers. The common stabilizers that are added to sour cream are
polysaccharides and
gelatin, including modified food starch,
guar gum, and
carrageenans. Stabilizers provide a smoother texture, create specific gel structures, and reduce whey
syneresis. This extends the product's shelf life. Synresis can occur during the transportation, when sour cream containers are jostled and agitated. The next step in the manufacturing process is acidification.
Organic acids, such as
citric acid or
sodium citrate, are added to the cream prior to
homogenization. This increases the metabolic activity of the starter culture. During homogenization, larger fat globules within the cream are broken down into smaller sized globules to allow an even suspension within the system. After homogenization of the cream, the mixture must undergo
pasteurization. Pasteurization is a mild heat treatment of the cream, with the purpose of killing any harmful bacteria in the cream. The homogenized cream undergoes
high temperature short time (HTST) pasteurization method. In this type of pasteurization the cream is heated to the high temperature of 85 °C for thirty minutes. This processing step creates a
sterile medium in which the starter bacteria can thrive. After inoculation the cream is portioned in packages and fermented for 18 hours, lowering the pH from about 6.5 to 4.6. After fermentation, one more cooling process takes place. After this cooling process, the sour cream is packaged into final containers and sent to market. The denaturation of whey proteins is also known for increasing the strength of the cross-linking within the cream system, due to the formation of whey protein
polymers. When the cream is inoculated with starter bacteria and the bacteria begin converting lactose to lactic acid, the pH begins a slow decrease. When this decrease begins, dissolution of calcium phosphate occurs, and causes a rapid drop in the pH. During fermentation the pH drops from around 6.5 to 4.6, this drop in pH brings on a physicochemical change to the casein micelles. Recall the casein proteins are heat stable, but they are not stable in certain acidic conditions. The colloidal particles are stable at the normal pH of milk which is 6.5-6.7, the micelles will precipitate at the
isoelectric point (pI) of milk which is a pH of 4.6. At a pH of 6.5 the casein micelles repulse each other due to the electronegativity of the outer layer of the micelle. During this drop in pH there is a reduction in
zeta potential, from the highly net negative charges in cream to no net charge when approaching the pI. U_E= \left \lfloor \frac{2\varepsilon zf(ka)}{3\eta} \right \rfloor The formula shown is the
Henry's equation, where z: zeta potential, Ue: electrophoretic mobility, ε: dielectric constant, η:
viscosity, and f(ka): Henry's function. This equation is used to find the zeta potential, which is calculated to find the
electrokinetic potential in colloidal dispersions. Through electrostatic interactions the casein molecules begin approaching and aggregating together. The casein proteins enter a more ordered system, attributing to a strong gel structure formation. The whey proteins that were denatured in the heating steps of processing, are insoluble at this acidic pH and are precipitated with casein. The interactions involved in gelation and aggregation of casein micelles are
hydrogen bonds,
hydrophobic interactions, electrostatic attractions and
van der Waals attractions These interactions are highly dependent on pH, temperature and time. At the isoelectric point, the net surface charge of casein micelle is zero and a minimum of electrostatic repulsion can be expected. Furthermore, aggregation is taking place due to dominating hydrophobic interactions. Differences in the
zeta potential of milk can be caused by differences in ionic strength differences, which in turn depend on the amount of calcium present in the milk. The stability of milk is largely due to the electrostatic repulsion of casein micelles. These casein micelles aggregated and precipitated when they approach the absolute zeta potential values at pH 4.0 – 4.5. When the heat treated and denatured, whey protein is covering the casein micelle, isoelectric point of the micelle elevated to the isoelectric point of β lactoglobulin (approximately pH 5.3).
Rheological properties Sour cream exhibits time-dependent
thixotropic behaviors. Thixotropic fluids reduce in viscosity as work is applied, and when the product is no longer under stress, the fluid returns to its previous viscosity. The viscosity of sour cream at room temperature is 100,000 cP, (for comparison: water has a viscosity of 1
cP at 20 °C). == Uses ==