The modern consumer demands a long shelf life from a cosmetic product. Cosmetics that can be stored for up to two years are valued. However, this feature is not only desired by target customers, but also by wholesalers or drugstores of all kinds. Such a product, even though it does not achieve the projected sales in a given period, is likely to perform adequately in the long term without the risk of additional costs or the need to return the goods. On the other hand, the cosmetic world has been shaped in recent years by the trends of eco, zero waste or natural ingredients. These are very legitimate trends, but often in strong opposition to a long product shelf life. Is there then a universal solution to both meet consumer expectations and eliminate the risk of a rapid product recall in the event of a lack of sales?

Preservatives

People have been using various methods to prolong the shelf life of products for centuries. The first of these were preservation methods aimed at limiting the growth of mould or other microorganisms on their products (salting, pickling, drying). The development of these measures, was the creation or discovery of suitable substances (preservatives), which made it possible to ensure a longer shelf life for products, without any special processing. By definition, preservatives are product additives designed to prolong product shelf life by inhibiting the growth of microorganisms.[1]. They are now the backbone of the tools for keeping products stable for longer. The cosmetics industry could not exist as we know it without them. In particular, products containing water and organic ingredients are mainly at risk from the growth of micro-organisms, which can then cause irritation, for example. In addition, cosmetics often contain amino acids, proteins, vitamins or components that break down quite quickly. Preservatives in cosmetics also prevent the formation of metabolic products of microorganisms. We can divide preservatives into synthetic (created through chemical synthesis) and natural. The former group includes mainly:

• Parabens, i.e. esters of phydroxybenzoic acid, such as: methylparaben (methyl parabens, methyl paraben), ethylparaben (names analogous to methylparaben), propylparaben, butylparaben, benzylparaben. Parabens in cosmetics can also appear under many other names: Glutaraldehyde, Tegosept, Mycocten, Nipasol, and designations such as E216 (propyl paraben), E218 (methyl paraben) and other symbols depending on the paraben used.[2]
• Formaldehyde and formaldehyde-releasing compounds Formaldehyde solution (formic acid aldehyde, methanal) and formaldehyde-releasing compounds (formaldehyde donors, formalin releasers). Methanal has a very low minimum inhibitory concentration against microorganisms, ranging from 0.002% to 0.05% for Gram-negative bacteria, about 0.025% for Gram-positive bacteria, 0.009% to 0.075% for fungi and moulds, about 4% for mycobacteria and about 8% for bacterial spores.[3]
• Thimerosal which is the sodium salt of 2-ethylorthothiosalicylic acid and has a high antimicrobial and antifungal activity. It is mainly used for make-up products and eye make-up removal due to its lack of irritant effects.

Among the synthetic preservatives we can still find, among others, isothiazolinones, Euxyl® K 400 or Glycasil™. The second group are natural systems. Amongst these we can include mixtures of plant extracts:

• Thyme (Thymus vulgaris) serves as an antibacterial agent and shows antioxidant properties due to its high polyphenol content. It also has a soothing effect and is therefore used in ointments or creams, especially recommended for people with sensitive skin. [4]
• Lavender (Lavandula augustifolia) has antibacterial and antiseptic properties and accelerates epidermal regeneration. This makes the plant very suitable for the treatment of acne, creams for sensitive and delicate skin or scalp care products. Lavender oil also has a regulating effect on the sebaceous glands.
• Citron (Medica limonum) also has antimicrobial and antiseptic properties. It is described as a conditioning substance - smoothing, shining, moisturising and lubricating the skin or hair. An additional advantage of the oil is its pleasant fragrance. The International Fragrance Association (IFRA) recognised Citrus Limon (Lemon) Peel Oil as a potential skin sensitiser in 2020.

We can also find extracts of Canada bitter (Hydrastis canadensis citrus), cinnamon (Cinnamomum zeylanicum), rosemary (Rosmarinus officinalis), olive (Olea europaea), oregano (Origanum vulgare), mint (Mentha piperita). All of them, when properly mixed in a safe concentration, can exhibit preservative properties for cosmetic products while offering many additional functions.

The effect of preservatives, although very well known, has acquired negative connotations in recent years. The term "preservative-free" has become firmly established in our consciousness, unequivocally associating systems that inhibit microbial growth with something undesirable. The food industry - the precursor to and largest market for the use of preservatives - is largely responsible for this state of affairs. The mythical labels on our food packaging, which range from E 200 to E 297, have for many people been equated with chemical, unnatural elements. Despite the fact that these chemicals had been tested and properly registered before use, the patch of "harmful chemicals" had already stuck. A very popular trend at present is the use of natural oils as replacements for synthetic preservatives, but we must be aware of their potential sensitising properties. A test to assess whether the preservative system used is doing its job is maintenance test (Challenge test). It determines how quickly a given micro-organism multiplies in a product and whether, at the end of the shelf life, its quantity will not exceed a set criterion.

Packaging

An indispensable element of a cosmetic product is its packaging. It is often indicative of a brand's image, product use, target group or even the price level of a product. In the case of cosmetic products, its packaging also protects the product from external influences, contamination or changes in form. It is a key element that is also responsible for the stability of the mass, and thus for determining its durability. For this to be the case, however, it must fulfil a number of basic requirements:

• Maintain tightness during transport
• Do not react with the product mass
• Enable safe and convenient dispensing of the product as intended
• Protect from undesirable external effects (e.g. uv light)
• Maintain form and shape despite changing conditions

The most versatile and accessible form that meets the above criteria at the same time is plastic packaging. In 2017, the weight of plastic packaging placed on the market amounted to 995,000 tonnes, i.e. 18% of the total weight of packaging placed[5]. It is estimated that about 68% of this (677 thousand tonnes) went to households, mainly in the form of wrappers which are the direct packaging of the product (bottles, jars, wrappers). The rest - 32% (319 thousand tonnes) moved between businesses - in industry and trade.[6] Today's customers are aware of their impact on the world around them and slogans about the environmental friendliness of products, especially in the cosmetics industry, strongly influence purchasing decisions. An additional aspect may be the versatility of such solutions, e.g. by standardising the material for the entire product line or by changing the plastic used today to a more compatible form of new packaging, thus extending the life of the product. Below are some examples:

• Use of reusable packaging - A new direction in the packaging industry is the use of less material-intensive or recycled plastics. Examples of such designs are dispensers that allow material-intensive and therefore environmentally burdensome PET bottles to be replaced by PE film pouches. Such solutions have been implemented by the British companies HCP and Twistub, which produce reusable packaging consisting of a dispenser and a decorative container into which a so-called refillable product can be inserted after emptying the contents.
• Use of organic packaging - Hydro-biodegradable materials based on biosolids (e.g. cellulose) that are designed to be recovered through composting and biodegradation, oxo-biodegradable materials based on traditional raw materials recovered through recycling and materials made from biosolids also play an important role. The American cosmetics manufacturer offers its products in packaging made of plastic with EcoPure, resins that ensure complete biodegradation within 5 years. Similarly, M&H Plastics, a UK manufacturer of containers for cosmetic products, uses oxo-biodegradable PE and PP blends in their production, the decomposition process of which is initiated by natural UV radiation and takes about 5 years. For degradable packaging for powders and lipsticks, polylactide (PLA) is used, as well as polyhydroxyesters (PHA) (e.g. from Leoplast and RPC Cresstale). Another example of a material that shows biodegradability is trimethyl polyethylene terephthalate (PTT), with the trade name Biomax PTT 1100 from DuPont, which is used to produce mascara packaging. Oxo-biodegradable cosmetics packaging (bottles or tubes) is offered by M&H Plastics, among others. [7]
• Use of glass packaging - According to The European Container Glass Federation, in 2021 the level of recycling of glass packaging in Poland was 67%. In addition, glass is one of the most durable and least susceptible to deformation and deformation material used in cosmetic packaging. In addition, by properly wrapping the packaging or using appropriately coloured glass, we can protect our product from the effects of UV light. Emphasising recycling, some companies are introducing the possibility of refilling the packaging after the contents have been used up.
• Innovative packaging - An example in this area, while at the same time increasing product shelf life, could be the work with the use of nanosilver as a component of the packaging material. Thanks to the research-proven high antibacterial properties of such a compound, its use would reduce the use of the preservative systems concerned. It has already been shown to be effective against yeasts and moulds. A very important advantage of nanosilver in packaging is that it can be easily incorporated into plastics. At present, research is still in progress in this area, although we can already see, for example, wall paints with these particles. Packaging with biomarkers, which can provide a real-time indication of a product's suitability for use, is becoming increasingly popular. Such solutions could extend and, above all, assess in real time whether a product is fit for use.

Cosmetic shelf life studies

According to Regulation (EC) No. 1223/2009 of the European Parliament and of the Council, for the time being the stability and compatibility of a cosmetic product is determined by the laboratory. There is no single standard which defines the conditions under which a product is to be tested. In MPR Labs, when performing such services for our clients, we are supported by the labelling of the stability of cosmetics in accordance with the provisions of Regulation 1223/20009/EC, the Technical Guide of the Polish Association of the Cosmetics Industry Warsaw 20 November 2013, the Guidelines on Stability Testing of Cosmetics or the Colipa/CTFA materials from 2004. On the basis of these, a methodology has been developed, which defines the various conditions (reduced temperature conditions, RT - room temperature, elevated temperature conditions, shock conditions, effect of UV light), which both the mass of the product and the packaging itself must meet. Any significant change in the mass (e.g. weight, colour, odour, pH, density, etc.) or in the packaging (deformation, colour change, lack of tightness) can prevent the cosmetic product from being placed on the market or significantly shorten its shelf life. The final decision, together with a report, is based on several or several weeks of testing and analysis, defining unequivocally whether the product is stable, whether its packaging is mass-compatible and for how many years we can issue a decision on the shelf life of the cosmetic product.

Summary

The shelf life of a product over time is made up of a large number of factors, from the stable recipe, to the appropriate microbiological protection and packaging. It is only when each element works together and does not cause significant changes to the product over time that we can speak of a stable product whose shelf life is set for years. However, there is no universal solution for every cosmetic. Each product is a different formulation, a different combination of ingredients and often different packaging.

[1] H. Gertig and J. Gawęcki 'Human Nutrition. Terminology dictionary' 2014, p. 102
[2] J. Marwicka, K.Niemyska, M.Wieczorek "Parabens as preservatives used in cosmetic preparations and their effect on apoptosis of human skin fibroblasts" Aesthetic Cosmetology 6 / 2017 / vol. 6
[3] M.Natywa, A. Górska "Use of preservatives in cosmetics - pros and cons" Aesthetic Cosmetology 2 / 2015 / vol. 4
[4] M.Ambroziak, M.Stanowska, K. Sikorska-Zimny "Thyme - a plant with many uses" Innovations in Nursing and Health Sciences 1 (4)/2020
[5] Reports from Marshal Offices
[6] Estimates by the Polish Association of Cosmetics Industry and Deloitte Advisory, based on REKOPOL and Packflow "Report of the Polish Association of Cosmetics Industry".
[7] A.Cholewa-Wójcik "Consumer evaluation of innovative cosmetic packaging" Cracow University of Economics 2015, pp 74-87