Protein Structure All proteins are made up of many amino acids joined by peptide bonds. Peptide bonds are strong bonds and are not easily disrupted. (dipeptide = two amino acids, polypeptide = several amino acids) Each protein has a complex and unique conformation, which is determined by the specific amino acids and the sequence in which they occur along the chain. To understand the function of proteins in food systems and the changes that occur in proteins during processing, it is important to understand the basics of protein structure. Proteins are described as having four types of structure :
a. primary, b. secondary, c. tertiary, d. quaternary Primary Structure Struktur ini merupakan struktur yang palingn sederhana, berupa suatu susunan linier asam amino R1 NH3
C R2 COO H NH3
R2 C CO H Peptide bond
F G NH N S C
R3 CO NH C H Peptide bond H T
D K G S A
A carboxylic acid condenses with an amino group with the release of a water H2O R1 A
H H 2O NH3 COO C
CO The amino acid sequence is called as primary structure Secondary Structure asam amino yang menyusun protein dihubungkan oleh ikatan peptida dan ikatan hidrogen
Tertiary Structure Ikatan-ikatan yang mungkin ada adalah : a. ikatan ionik (elektrostatik), b. ikatan hidrogen, c. ikatan hidrophobik dan d. ikatan dipole atau ikatan hidrofilik e. ikatan disulfide, Quartenery Structure
terbentuk dari beberapa unit molekul protein tersier, membentuk satu moleku protein. Ikatan yang ada sama dengan pada struktur tersier Reactions and Properties of Proteins Amphoteric Like amino acids, proteins are amphoteric (being able to act as an acid or a base) depending on the pH. This enables them to resist small changes in pH (buffering capacity).
Isoelectric Point The isoelectric point (pI) is the pH at which the protein is electrically neutral. At this pH, the global or overall charge on the protein is zero. the number of positive charges is equal to the number of negative charges. At the isoelectric point, the protein molecules usually precipitate because they do not carry a net charge. The pH of the isoelectric point differs for each protein. It depends on the ratio of free ionized carboxyl groups to free ionized amino groups in the protein. The isoelectric point is important in food processing. For
example, cottage cheese is made by adding lactic acid to milk to bring the pH to the isoelectric point of the major milk proteins (the caseins). The proteins precipitate at this pH, forming curds. These are separated from the rest of the milk and may be pressed and/or mildly salted before being packaged as cottage cheese. Water-Binding Capacity Water molecules can bind to the backbone and to polar and charged side chains of a protein. Depending on the nature of their side chains, proteins may bind varying amounts of waterthey have a water-binding capacity. As proteins get closer to their isoelectric point, they tend to bind
less water, because reduced charge on the protein molecules results in reduced affinity for water molecules. The presence of bound water helps to maintain the stability of a protein dispersion. Salting-In and Salting-Out Some proteins cannot be dispersed in pure water but are readily dispersed in dilute salt solutions. When a salt solution increases the dispersibility of a protein, this is termed salting-in. It occurs because charged groups on a protein bind the anions and cations of the salt solution more strongly than water. The ions, in turn, bind water; thus, the protein is dispersed in water
more easily. Salting-in is important in food processing. For example, brine may be injected into ham to increase the dispersibility of the proteins. This has the effect of increasing their water-binding capacity, and so the ham is moister and its weight is increased. The same is true for poultry to which polyphosphates are added. Salting-out occurs at high salt concentrations, when salts compete with the protein for water. The result is that there is insufficient water available to bind to the protein, and so the protein precipitates. This is not normally a problem in food processing. However, it may be a contributing factor to the
deterioration of food quality during freezing of foods; during the freezing process, water is effectively removed as ice crystals, and so the concentration of liquid water decreases and the solute concentration increases dramatically. Denaturation Denaturation is the change in the secondary, tertiary, and or quaternary structure of a protein. There is no change in the primary structure. Denaturation does not involve breaking of peptide bonds. The protein unfolds, but there is no change in its amino acid sequence.
Perubahan yg terjadi karena denaturasi 1. Ikatan peptida mudah diserang enzim proteolitik 2. Penurunan kelarutan 3. Aktivitas sebagai enzim turun/hilang 4. Tidak dapat terkristalisasi kembali 5. viskositas meningkat 6. Rotasi optis meningkat Denaturation may occur as a result of the following: A. Physical agent
1) Heat 2) Cold 3) Mechanical treatment 4) Hydrostatic pressure 5) Irradiation 6) Interfaces B. Chemical agent 1) Acid and alkali 2) metal 3) Organic solvent 4) Aqueous solution of organic compounds
C. Energetics of denaturation The changes that produce denaturation usually are mild changes (mild heat treatment), such as pasteurization or blanching, or small changes in pH are sufficient to change the conformation of a protein. Denatured proteins normally lose their functional properties; that is, they are unable to perform their normal function in a food. Enzymes are inactivated and so the reactions that they catalyzed no longer can take place. This has important implications in food processing.
Denaturation may be desirable and can be deliberately brought about by food processing. Examples : heating beaten egg white foams to form meringues, adding acid to milk to form cottage cheese, inactivating enzymes by heat (blanching) that would cause rancidity or discoloration during frozen storage. Sometimes denaturation is undesirable. For example : frozen egg yolks are lumpy and unacceptable when thawed because the lipoproteins denature and aggregate. Overheating of foods also can cause unwanted
denaturation. Hydrolysis of Peptides and Proteins Break down of peptide bonds Requires acid or base, water and heat Gives smaller peptides and amino acids Similar to digestion of proteins using enzymes Occurs in cells to provide amino acids to synthesize other proteins and tissues OH CH3 O
CH2 O + H3N CH C N CH C OH H H2O, H+ heat OH
CH3 O + H3N CH COH + CH2 O + H3N CH C OH
Maillard browning is the reaction that is responsible for the brown color of baked products. A free carbonyl group of a reducing sugar reacts with a free amino group on a protein when heated and the result is a brown color. The reaction is highly complex and has a significant effect on the flavor of foods as well as the color. It is known as nonenzymatic browning, because the reaction is not catalyzed by an enzyme The Maillard reaction is favored by the following:
The Maillard reaction is favored by the following: High sugar content High protein concentration High temperatures High pH Low water content Maillard browning is responsible for the discoloration of food products such as powdered milk and powdered egg. The reaction causes loss of the amino acids lysine, arginine, tryptophan, and histidine, as these are the amino acids with free
amino groups that are able to react with reducing sugars. Enzymes All enzymes are proteins. Enzymes are important in foods, because they catalyze various reactions that affect color, flavor, or texture, and hence quality of foods. Some of these reactions may be desirable, whereas others are undesirable and produce unwanted discoloration or off-flavors in foods. Each enzyme has a unique structure or conformation, which enables it to attach to its specific substrate and catalyze the
reaction. When the reaction is complete, the enzyme is released to act as a catalyst again. All enzymes have an optimal temperature and pH range, within which the reaction will proceed most rapidly. Heat or changes in pH denature the enzymes, making it difficult or impossible for them to attach to their respective substrates, thus inactivating them. Protein Binding
Properties H2O Fat Fat binding Protein and bound Fat
Textural Properties Sorption Protein and sorbed H2O H2O energy H2O
Texturization Textured protein H+,OH- Solubility M+,A-
Surface Properties Emulsification fat energy Protein-Lipid Emulsion Protein in solution
Air Foaming energy Protein-Air Foam energy H2O Coagulum Coagulation
H+,OHM+, A- Gelation Gel Gambar 1. Aspect of protein functionality and its interalionships Isoelectric Point Each protein has a unique Isoelectric Point Affects protein stability in dispersions
Occurs when pH of the aqueous environment around protein creates a net charge of Zero within the protein Same number of positive and negative charges (number of NH3+ = COO-) Least stable situation for protein dispersion Don't have like charges repelling proteins anymore Proteins tend to clump together Property used to make Cottage Cheese pH lowered until the net charge on casein molecules = Zero Casein proteins precipitate, forming curds Whey proteins in milk don't precipitate under these conditions
Functional Sifat fungsional protein Isolasidanpemurnianprotein Teknologi pengolahan kedelai dan kacang tanah Nabati hewani
Outline Sifat zwibterion Fungsi asam amino sebagai buffer Denaturasi dan hidrolisis protein sifat fungsional protein Isolasi dan pemurnian protein Teknologi pengolahan kedelai dan kacang tanah Teknologi pengolahan serealia (gandum, jagung, beras)
Struktur protein Denaturasi dan faktor-faktor penyebab Hidrolisis protein dan faktor-faktor penyebabnya Hidrasi Gel protein Surface active Foaning dll Teknik isolasi protein Teknik pemurnian protein
Tujuan isolasi dan pemurnian Fermentasi tempe Pengolahan tahu Pembuatan daging firan Proses-proses yang terjadi pada protein selama pengolahan tersebut Protein peda gandum dan pengolahan gandum Protein pada jagung dan padi Terima Kasih
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