Protein adsorption in the food industry
Encyclopedia
Protein adsorption
refers to the adhesion of proteins to solid surfaces. This phenomenon is an important issue in the food processing industry, particularly in milk processing and wine and beer making. Excessive adsorption, or protein fouling, can lead to health and sanitation issues, as the adsorbed protein is very difficult to clean and can harbor bacteria, as is the case in biofilms. Product quality can be adversely affected if the adsorbed material interferes with processing steps, like pasteurization. However, in some cases protein adsorption is used to improve food quality, as is the case in fining of wines.
clays, are used to clarify wine by removing these proteins. Also, proteinaceous agents such as albumin, casein, or gelatin are used in wine clarification to remove tannins or other phenols.
is a community of microorganisms adsorbed to a surface. Microorganisms in biofilms are enclosed in a polymeric matrix consisting of exopolysaccharides, extracellular DNA and proteins. Seconds after a surface (usually metal) is placed in a solution, inorganic and organic molecules adsorb onto the surface. These molecules are attracted mainly by Coulombic forces (see above section), and can adhere very strongly to the surface. This first layer is called the conditioning layer, and is necessary for the microorganisms to bind to the surface. These microorganisms then attach reversibly by Van der Waals force
s, followed by irreversible adhesion through self-produced attachment structures such as pili or flagella. Biofilms form on solid substrates such as stainless steel. A biofilm's enclosing polymeric matrix offers protection to its microbes, increasing their resistance to detergents and cleaning agents. Biofilms on food processing surfaces can be a biological hazard to food safety. Increased chemical resistance in biofilms can lead to a persistent contamination condition.
. Heat exchanger surfaces can become fouled by adsorbed milk protein deposits. Fouling is initiated by formation of a protein monolayer at room temperature, followed by heat induced aggregation and deposition of whey protein and calcium phosphate deposits. Adsorbed proteins decrease efficiency of heat transfer and potentially affect product quality by preventing adequate heating of milk.
. When a surface in a fluid has a net charge, ions in the fluid will adsorb to the surface. Proteins also have charged surfaces due to charge amino acid residues on the surface of the protein. The surface and the protein are then attracted by Coulombic forces.
The attraction a protein feels from a charged surface () depends exponentially on the surface's charge, as described by the following formula:
Where
A protein's surface's potential is given by the number of charged amino acids it has and its isoelectric point
, pI.
.
As milk is heated during pasteurization
many of the proteins in the milk are denatured. Pasteurization temperatures can reach 161°F (71.7°C). This temperature is high enough to denature the proteins below, lowering the nutritional value of the milk and causing fouling. Milk is heated to these high temperatures for a short time (15–20 seconds) to reduce the amount of denaturization. However fouling from denatured proteins is still a significant problem.Denaturation
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
refers to the adhesion of proteins to solid surfaces. This phenomenon is an important issue in the food processing industry, particularly in milk processing and wine and beer making. Excessive adsorption, or protein fouling, can lead to health and sanitation issues, as the adsorbed protein is very difficult to clean and can harbor bacteria, as is the case in biofilms. Product quality can be adversely affected if the adsorbed material interferes with processing steps, like pasteurization. However, in some cases protein adsorption is used to improve food quality, as is the case in fining of wines.
Protein Adsorption
Protein adsorption and protein fouling can cause major problems in the food industry (particularly the dairy industry) when proteins from food adsorb to processing surfaces, such as stainless steel or plastic (e.g. polypropylene). Protein fouling is the gathering of protein aggregates on a surface. This is most common in heating processes that create a temperature gradient between the equipment and the bulk substance being heated. In protein-fouled heating equipment, adsorbed proteins can create an insulating layer between the heater and the bulk material, reducing heating efficiency. This leads to inefficient sterilization and pasteurization. Also, proteins stuck to the heater may cause a burned taste or color in the bulk material. Additionally, in processes that employ filtration, protein aggregates that gather on the surface of the filter can block the flow of the bulk material and greatly reduce filter efficiency.Beer Stone
Beer stone is a buildup that forms when oxalate, proteins, and calcium or magnesium salts from the grains and water in the beer brewing process precipitate and form scale on kegs, barrels and tap lines. The minerals adsorb to the surface of the container first, driven by charge attractions. Proteins are often coordinated to these minerals in the solution and can bind with them to the surface. In other cases proteins also adsorb to the minerals on the surface, making deposits difficult to remove, as well as providing a surface that can easily harbor microorganisms. If built-up beer stone inside tap lines flakes off, it can negatively affect the quality of the finished product by making beer hazy and contributing "off" flavors. It is also harmful from a nutritional standpoint: oxalates can decrease absorption of calcium in the body, in addition to increasing risk of kidney stone formation.Wine Making
Grape and wine proteins tend to aggregate and form hazes and sediment in finished wines, especially white wines. Haze-causing proteins can persist in wine due to low settling velocities or charge repulsion on individual particles. Fining agents, such as bentoniteBentonite
Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...
clays, are used to clarify wine by removing these proteins. Also, proteinaceous agents such as albumin, casein, or gelatin are used in wine clarification to remove tannins or other phenols.
Biofilms
A biofilmBiofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
is a community of microorganisms adsorbed to a surface. Microorganisms in biofilms are enclosed in a polymeric matrix consisting of exopolysaccharides, extracellular DNA and proteins. Seconds after a surface (usually metal) is placed in a solution, inorganic and organic molecules adsorb onto the surface. These molecules are attracted mainly by Coulombic forces (see above section), and can adhere very strongly to the surface. This first layer is called the conditioning layer, and is necessary for the microorganisms to bind to the surface. These microorganisms then attach reversibly by Van der Waals force
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
s, followed by irreversible adhesion through self-produced attachment structures such as pili or flagella. Biofilms form on solid substrates such as stainless steel. A biofilm's enclosing polymeric matrix offers protection to its microbes, increasing their resistance to detergents and cleaning agents. Biofilms on food processing surfaces can be a biological hazard to food safety. Increased chemical resistance in biofilms can lead to a persistent contamination condition.
Dairy Industry
Thermal treatment of milk by indirect heating (e.g. pasteurization) to reduce microbial load and increase shelf life is generally performed by a plate heat exchangerPlate heat exchanger
A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids spread out over the plates. This facilitates...
. Heat exchanger surfaces can become fouled by adsorbed milk protein deposits. Fouling is initiated by formation of a protein monolayer at room temperature, followed by heat induced aggregation and deposition of whey protein and calcium phosphate deposits. Adsorbed proteins decrease efficiency of heat transfer and potentially affect product quality by preventing adequate heating of milk.
Mechanisms for Protein Adsorption
The common trend in all examples of protein adsorption in the food industry is that of adsorption to minerals adsorbed to the surface first. This phenomenon has been studied but it is not well understood. Spectroscopy of proteins adsorbed onto clay-like minerals show variations in the C=O and N-H bond stretches, meaning that these bonds are involved in the protein binding.Coulombic
In some cases proteins are attracted to surfaces by an excessive surface chargeSurface charge
Surface charge is the electric charge present at an interface. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field...
. When a surface in a fluid has a net charge, ions in the fluid will adsorb to the surface. Proteins also have charged surfaces due to charge amino acid residues on the surface of the protein. The surface and the protein are then attracted by Coulombic forces.
The attraction a protein feels from a charged surface () depends exponentially on the surface's charge, as described by the following formula:
Where
- is the potential felt by the protein
- is the actual potential of the surface
- x is the distance from the protein to the surface, and
- is the Debye lengthDebye lengthIn plasma physics, the Debye length , named after the Dutch physicist and physical chemist Peter Debye, is the scale over which mobile charge carriers screen out electric fields in plasmas and other conductors. In other words, the Debye length is the distance over which significant charge...
.
A protein's surface's potential is given by the number of charged amino acids it has and its isoelectric point
Isoelectric point
The isoelectric point , sometimes abbreviated to IEP, is the pH at which a particular molecule or surface carries no net electrical charge....
, pI.
Thermodynamic
Protein adsorption can also occur as a direct result of heating a mixture. Protein adsorption in milk processing is often used as a model for this type of adsorption in other situations. Milk is composed mainly of water, with less than 20% of suspended solids or dissolved proteins. Proteins make up only 3.6% of milk in total, and only 26% of the components that are not water. These proteins are all responsible for fouling that occurs during pasteurizationPasteurization
Pasteurization is a process of heating a food, usually liquid, to a specific temperature for a definite length of time, and then cooling it immediately. This process slows microbial growth in food...
.
As milk is heated during pasteurization
Pasteurization
Pasteurization is a process of heating a food, usually liquid, to a specific temperature for a definite length of time, and then cooling it immediately. This process slows microbial growth in food...
many of the proteins in the milk are denatured. Pasteurization temperatures can reach 161°F (71.7°C). This temperature is high enough to denature the proteins below, lowering the nutritional value of the milk and causing fouling. Milk is heated to these high temperatures for a short time (15–20 seconds) to reduce the amount of denaturization. However fouling from denatured proteins is still a significant problem.