Cosmetic preservatives

In the cosmetic industry, preservatives are vital additives whose primary function is to limit or prevent the proliferation of microorganisms in a product. The effectiveness of these cosmetic preservatives relies on them reaching the specific concentration required to inhibit microorganisms and interacting directly with microbial cells.

In terms of mechanism of action, preservatives can be divided into two broad categories: the first disrupts the integrity of the cell membrane, leading to the loss of cellular contents and interrupting its energy-producing processes in a manner usually associated with an increase in the permeability of the cell membrane; and the second involves interactions with nucleic acids, the core components of the cell, and acts by inhibiting or inactivating key enzymes, which is usually accomplished by interfering with specific chemical groups such as sulfhydryl or hydroxyl groups.

Specifically for commonly used preservative ingredients such as phenoxyethanol, various organic acids and parabens, they perform their preservative function mainly through the first mode of action, i.e., affecting the integrity of the cell membrane. In contrast, substances like imidazolidinyl urea, isothiazolinone, and iodopropynol butylcarbamate (IPBC) inhibit microbial growth primarily through the second mechanism, i.e., by reacting with nucleic acids within the cell. Depending on their chemical structure and mode of action, these preservatives show differences in their application range and inhibitory effect on different types of microorganisms.

1. Common preservatives

1.1 Chemical preservatives

1.1.1 Parabens

Nipagin esters is a general term for parabens, which mainly include methyl ester, ethyl ester, propyl ester, butyl ester, etc. With the growth of the carbon chain, its water solubility gradually deteriorates, which affects its distribution rate in the aqueous phase; methyl nipagin has the best water solubility, and can often be added directly in the aqueous phase, whereas nipagin ethyl ester, propyl ester, and butyl ester tend to be dissolved in the oil phase. Preservatives of this family are widely used to inhibit the growth of bacteria, yeasts and molds. They work by interfering with the synthesis of microbial cell membranes, thereby inhibiting microbial growth. Despite their acclaimed effectiveness, discussions about their safety have raised concerns in recent years. The Technical Specification stipulates that the maximum permitted concentration of parabens, their salts and esters is 0.4% (by acid) for a single ester; 0.8% (by acid) for a total of mixed esters; and that the sum of propyl ester and its salts, butyl ester and its salts, shall not exceed 0.14% (by acid), respectively.

1.1.2 Phenoxyethanol

Phenoxyethanol, also known as 2-phenoxyethanol, molecular formula for C: HO, colorless transparent slightly viscous liquid at room temperature, slightly soluble in water, easily soluble in ethanol and sodium hydroxide aqueous solution, with an aromatic odor. Phenoxyethanol inhibits cell growth and achieves antimicrobial effect by acting on the cell membrane to increase the passage rate of potassium ions and reduce enzyme activity. It is an internationally recognized safe preservative and the most frequently used preservative in cosmetics worldwide. The Technical Specification stipulates that the maximum permissible use concentration is 1.0%.

1.1.3 Benzoic acid and its salts and esters

Benzoic acid, also known as benzoic acid, molecular formula C7H6O2, slightly soluble in cold water, hexane, soluble in hot water, ethanol, ether, chloroform, benzene, carbon disulfide and turpentine and other wpcodeself. [18]Benzoic acid and its sodium salt is a class of acidic preservatives used earlier. Sodium benzoate preservative system also has its defects, in the system of higher pH value, its bacteriostatic effect decreases significantly wpcodeself.[19] The Technical Specification stipulates that the maximum allowable total use concentration is 0.5% (in terms of acid).

1.1.4 Poly(aminopropylidenebiguanide)

Poly(aminopropylidenebiguanide), also known as polyhexamethylenebiguanide hydrochloride and polyhexylidenebiguanide, abbreviated as PAPB, is a cationic polymer. Physical and chemical properties are colorless to light yellow liquid, completely soluble in water, alcohols, insoluble in oil phase and hydrocarbons. The guanidine group in bisguanidine is the active group, and its positively charged active group is easily adsorbed by various bacteria and viruses that are negatively charged, thus inhibiting the splitting function of the bacteria and viruses, together with the film formed by the polymer blocking the respiratory channel of the microorganisms, causing the microorganisms to suffocate and die rapidly. The Technical Specification stipulates that the total maximum allowable use concentration is 0.3%.

1.2 Preservatives of natural origin

1.2.1 Sorbic acid

Sorbic acid is also known as refreshing tea acid, 2,4-hexadienoic acid, 2-propenyl acrylic acid, with the molecular formula C6H8O2, this is a preservative extracted from natural substances such as pears, slightly soluble in water, soluble in a wide range of organic solvents, and effective against a wide range of fungi and yeasts. It exerts its preservative effect by interfering with the metabolic pathways of microorganisms. The Technical Specification provides for a maximum allowable use concentration of 0.6% of the total (as acid).

1.2.2 Tea tree oil

The main active components of tea tree oil are terpenoids, the most important of which is terpinen-4-ol (also known as terpineol). These compounds are able to penetrate the cell membranes of microorganisms, enter the cell interior and disrupt its structure and function. Specifically, they can interfere with microbial metabolic pathways, disrupt enzyme systems, affect gene expression, and ultimately lead to microbial death or inhibition of growth.

2. No added preservatives

As consumer demand for natural and additive-free products increases, some cosmetic brands have introduced products that do not contain traditional preservatives. These products usually rely on the natural preservative properties of other ingredients, and although they are not included in the list of preservatives in the Technical Specifications, these ingredients have broad-spectrum antimicrobial properties in addition to their primary uses, and the same preservative efficacy as that of added preservatives can be achieved through the combination of these ingredients.

Common raw materials with antimicrobial properties such as quaternary ammonium salts, 1,2-alkanediols, monofatty acid glycerides, cinchona, sorbitan caprylate, P-p-anisic acid, etc., can be applied to shampooing and conditioning products according to the need to use as a substitute for preservatives.

However, it is worth noting that many products are currently advertised as preservative-free, but their essence is the addition of raw materials with preservative systems that are not listed in the preservative catalog.

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