Sour cream

Cultured cream.[6]

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.[7]

Milk is made up of approximately 3.0-3.5% protein. The main proteins in cream are caseins and whey proteins. Of the total fraction of milk proteins, caseins make up 80% while the whey proteins make up 20%.[8] There are four main classes of caseins; β-caseins, α(s1)-caseins, α(s2)-casein and κ-caseins. These casein proteins form a multi molecular colloidal particle known as a casein micelle.[9] The proteins mentioned have an affinity to bind with other casein proteins, or to bind with calcium phosphate, and this binding is what forms the aggregates. The casein micelles are aggregates of β-caseins, α(s1)-caseins, α(s2)-caseins, that are coated with κ-caseins. The proteins are held together by small clusters of colloidal calcium phosphate, the micelle also contains lipase, citrate, minor ions, and plasmin enzymes, along with entrapped milk serum. The micelle is also coated in parts of κ-caseins which is known as the hair layer, having a lower density than the core of the micelle. Casein micelles are rather porous structures, ranging in the size of 50-250 nm in diameter and the structures on average are 6-12% of the total volume fraction of milk. The structure is porous in order to be able to hold a sufficient amount of water, its structure also assists in the reactivity of the micelle.[10] The formation of casein molecules into the micelle is very unusual due to β-casein's large amount of proline residues (the proline residues disrupt the formation of α-helixes and β-sheets ) and because κ-caseins only contain one phosphorylation residue (they are glycoproteins). The high number of proline residues inhibits the formation of close-packed secondary structures in such as α-helixes and β-pleated sheets. Due to κ-caseins being glycoproteins, they are stable in the presence of calcium ions so the κ-caseins are on the outer layer of the micelle to partially protect the non glycoproteins β-caseins, α(s1)-caseins, α(s2)-caseins from precipitating out in the presence of excess calcium ions. Due to the lack of a strong secondary or tertiary structure as a result of the proline residues, casein micelles are not heat sensitive particles. However, they are pH sensitive. 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 of milk which is a pH of 4.6.[8]

The proteins that make up the remaining 20% of the fraction of proteins in cream are known as whey proteins. Whey proteins are also widely referred to as serum proteins, which is used when the casein proteins have been precipitated out of solution.[8] The two main components of whey proteins in milk are β-lactoglobulin and α-lactalbumin. The remaining whey proteins in milk are; immunoglobulins, bovine serum albumin, and enzymes such as lysozyme.[11] Whey proteins are much more water-soluble than casein proteins.[12] The main biological function of β-lactoglobulin in milk is to serve as a way to transfer vitamin A, and the main biological function of α-lactalbumin in lactose synthesis. The whey proteins are very resistant to acids and proteolytic enzymes. However whey proteins are heat sensitive proteins, the heating of milk will cause the denaturation of the whey proteins. The denaturation of these proteins happens in two steps. The structures of β-lactoglobulin and α-lactalbumin unfold, and then the second step is the aggregation of the proteins within milk. This is one of the main factors that allows whey proteins to have such good emulsifying properties.[13] Native whey proteins are also known for their good whipping properties, and in milk products as described above their gelling properties. Upon denaturation of whey proteins, there is an increase in the water holding capacity of the product.[12]

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%.[14] During this step in the manufacturing process other dry ingredients are added to the cream; additional grade A whey for example would be added at this time. Another additive used during this processing step are 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. The reasoning behind the addition of stabilizers to fermented dairy products is to provide smoothness in the body and texture of the product. The stabilizers also assist in the gel structure of the product and reduce whey syneresis. The formation of these gel structures, leaves less free water for whey syneresis, thereby extending the shelf life.[15] Whey syneresis is the loss of moisture by the expulsion of whey. This expulsion of whey can occur during the transportation of containers holding the sour cream, due to the susceptibility to motion and agitation.[16] The next step in the manufacturing process is the acidification of the cream. Organic acids such as citric acid or sodium citrate are added to the cream prior to homogenization in order to increase the metabolic activity of the starter culture.[15] To prepare the mixture for homogenization, it is heated for a short period of time.

Homogenization is a processing method that is utilized to improve the quality of the sour cream in regards to the color, consistency, creaming stability, and creaminess of the cultured cream.[17] During homogenization larger fat globules within the cream are broken down into smaller sized globules to allow an even suspension within the system.[17] At this point in the processing the milk fat globules and the casein proteins are not interacting with each other, there is repulsion occurring. The mixture is homogenized, under high pressure homogenization above 130 bar (unit) and at a high temperature of 60 °C. The formation of the small globules (below 2 microns in size) previously mentioned allows for reducing a cream layer formation and increases the viscosity of the product. There is also a reduction in the separation of whey, enhancing the white color of the sour cream.[18]

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 allows for a sterile medium for when it is time to introduce the starter bacteria.[15]

After the process of pasteurization, there is a cooling process where the mixture is cooled down to a temperature of 20˚C. The reason that the mixture was cooled down to the temperature of 20˚C is due to the fact that this is an ideal temperature for mesophilic inoculation. After the homogenized cream has been cooled to 20˚C, it is inoculated with 1-2% active starter culture. The type of starter culture utilized is essential for the production of sour cream. The starter culture is responsible for initiating the fermentation process by enabling the homogenized cream to reach the pH of 4.5 to 4.8. Lactic acid bacteria (hereto known as LAB) ferment lactose to lactic acid, they are mesophilic, Gram-positive facultative anaerobes. The strains of LAB that are utilized to allow the fermentation of sour cream production are Lactococcus lactis subsp latic or Lactococcus lactis subsp cremoris they are lactic acid bacteria associated with producing the acid. The LAB that are known for producing the aromas in sour cream are Lactococcus lactis ssp. lactis biovar diacetyllactis. Together these bacteria produce compounds that will lower the pH of the mixture, and produce flavor compounds such as diacetyl.[19][20][21]

After the inoculation of starter culture, the cream is portioned in packages. For 18 hours a fermentation process takes place in which the pH is lowered from 6.5 to 4.6. After fermentation, one more cooling process takes place. After this cooling process, the sour cream is packaged into their final containers and sent to the market.[15]

Sour cream can also be fried in oil or fat, and used on top of noodle dishes, as in Hungarian cuisine

During the pasteurization process, temperatures are raised past the point where all the particles in the system are stable. When cream is heated to temperatures above 70 °C, there is denaturation of whey proteins. To avoid phase separation brought on by the increased surface area, the fat globules readily bind with the denatured β-lactoglobulin. The adsorption of the denatured whey proteins (and whey proteins that bound with casein micelles) increases the number of structural components in the product; the texture of sour cream can be partly attributed to this.[18][22] 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.[23]

When the cream is inoculated with starter bacteria and the bacteria begins 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 the processing step, fermentation the pH was dropped from 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 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.[8] 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. ${\displaystyle U_{E}=\left\lfloor {\frac {2\varepsilon zf(ka))}{3\eta }}\right\rfloor }$ 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.[24] 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.[15][18][25]

The interactions involved in gelation and aggregation of casein micelles are hydrogen bonds, hydrophobic interactions, electrostatic attractions and van der Waals attractions [26] These interactions are highly dependent on pH, temperature and time.[27] At the isoelectric point, the net surface charge of casein micelle is zero and a minimum of electrostatic repulsion can be expected.[28] 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.[29] 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.[30] 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).[31]

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).[32] The thixotropic properties exhibited by sour cream are what make it such a versatile product in the food industry.