L-Alanyl-L-cystine is a dipeptide formed by the connection of alanine and L-cystine through a peptide bond. Its stability in food is affected by various factors, as follows:
I. Influence of Temperature
High Temperature: Generally, excessively high temperatures have an adverse effect on the stability of L-Alanyl-L-cystine. When food is heated to a relatively high temperature, it may lead to the breakage of peptide bonds, causing it to decompose into alanine and L-cystine, and even further degradation. For example, in the case of baked goods, if the temperature is too high and the baking time is too long, the content of L-Alanyl-L-cystine will decrease significantly.
Low Temperature: In a low-temperature environment, L-Alanyl-L-cystine is usually more stable. Low temperature can reduce the thermal motion of molecules and slow down possible chemical reactions, which is beneficial for maintaining its structural integrity. Therefore, storing foods containing L-Alanyl-L-cystine under refrigeration or freezing conditions helps to extend their shelf life and maintain their stability.
II. Influence of pH Value
Acidic Environment: In a strongly acidic environment, the stability of L-Alanyl-L-cystine may be affected. Under acidic conditions, the amino group may be protonated, affecting the charge distribution and spatial structure of the molecule, making the peptide bond more vulnerable to attack and hydrolysis. For instance, in an acidic beverage with a pH value of 2 - 3, it may gradually decompose, resulting in a decrease in its content.
Alkaline Environment: Under alkaline conditions, L-Alanyl-L-cystine may also be unstable. In an alkaline environment, the carboxyl group may dissociate, which also changes the properties of the molecule and makes the peptide bond more likely to break. When the pH value of the food is higher than 8, its decomposition rate may accelerate.
Neutral Environment: Relatively speaking, L-Alanyl-L-cystine is usually more stable under near-neutral pH conditions. When the pH value of general foods is between 6 - 7, its structure and properties can be well maintained, and its decomposition rate is relatively slow.
III. Influence of Oxygen
The cystine in alanyl - L - cystine contains a sulfhydryl group (-SH), which has strong reducibility and is prone to an oxidation reaction with oxygen in the air. The oxidation may cause the sulfhydryl group of cystine to be oxidized to a disulfide bond (-S-S-), and even further oxidized to other forms such as a sulfonic acid group, thus changing its structure and properties and affecting its stability in food. Therefore, during food processing and storage, reducing contact with oxygen, such as using vacuum packaging or filling with inert gases, helps to improve the stability of L-Alanyl-L-cystine.
IV. Influence of Metal Ions
Some metal ions, such as copper ions (Cu²⁺), iron ions (Fe³⁺), etc., may have a catalytic effect on the stability of L-Alanyl-L-cystine. These metal ions can act as catalysts for redox reactions, accelerating its oxidation or other chemical reactions, leading to its decomposition or deterioration. For example, when trace amounts of copper ions are present in food, it may significantly accelerate its oxidation rate and reduce its stability.
V. Influence of Water Activity
Water activity (Aw) is an important factor affecting various chemical reactions and microbial growth in food. For L-Alanyl-L-cystine, both too high and too low water activity may affect its stability. When the water activity is too high, it is conducive to the growth of microorganisms and the activity of enzymes, which may lead to its decomposition by microorganisms or hydrolysis by enzymes. When the water activity is too low, physical changes such as crystallization or vitrification of the components in the food may occur, which may also affect the stability of L-Alanyl-L-cystine. Generally, controlling the water activity of food within an appropriate range (such as 0.3 - 0.6) helps to maintain its stability.
