In Part 1 of Preserving Botanical Formulations Naturally we introduced you to strategies used to preserve products. Now we will take a closer look at the hurdle approach which can be used in formulating natural and non-natural skin, hair and body care products.

HURDLE 1: Good Manufacturing Practice

Preservatives should not be added to negate unclean manufacturing practices. (Dweck 2005) They are used to prevent any contamination after manufacture – there are bacteria in the air and carried on peoples skin so it is important to protect the product from environmental and human contact.  In short, the product should be clean at the point of which the preservative is added. Dweck (2005) states;

There is a common misconception among many manufacturers who use preservatives as a fail safe option due to poor plant hygiene and manufacturing systems. This strategy often fails miserably, resulting in many product recalls and contamination incidents due to the preservative system being overloaded and used up fighting contamination before the product has even left the manufacturing vessel.’’

So how do we ensure our products start off clean?

• If possible, test all raw materials to ensure they are clean (at minimum you need the certificate of analysis.)

• Water used in products must be clean and screened or sterilised before use. You need to use de-ionised and/or purified water and if you can, make sure it is  pasteurised before use in a cold processed product.

• All equipment including pumps and valves need to be sterilised and if possible checked for contamination.

• There should be good procedures and processes in place to ensure that cleaning is carried out to a high standard

• Staff need to be trained on maintaining adequate personal hygiene and regular checks need to be carried out to ensure compliance to procedures and processes. (Dweck, 2005, Personal Care)

‘’We can protect cosmetics from contamination during manufacturing by strict enforcement of sanitation procedures. We should rigidly require and monitor cleanliness of process water, other raw materials and manufacturing processes, and maintain high standards of personal hygiene.’’ (5)

Another important part of GMP is having procedures in place to ensure your preservative has been included. There is no point having a great preservative system if the finished product does not contain it.   Many manufacturers ensure that two people are present to observe and record the preservative being added. (6)

HURDLE 2: pH

Using pH to control the growth of microbes works in a number of ways. Firstly, some pathogenic and product spoiling microbes do not survive at certain pH ranges. Secondly, some preservatives are more efficient within a certain pH range.

pH and Germ Survival

Normally when we talk about microorganisms we refer to bacteria and fungus (mould and yeast). There are many bugs that do not survive extreme pH ranges. Most microbes, and especially pathogenic ones, can survive and grow in a pH of between 5 and 8, so slightly acidic, neutral and slightly alkaline. Most bacteria prefer a neutral pH of between 6.5-7.5 and most gram positive and gram negative bacteria cannot survive below pH 4.5. However many pathogenic yeast and moulds can thrive in acidic pH levels below pH 4.5.

Most organisms die off at pH above 10. Traditional solid soap is probably the most common example of a product preserved by its pH. Soaps pH is normally around 9.5-10 which will not support the growth of most dangerous bacteria, yeast and mould. It also has a low amount of water which means that it has a few hurdles for micro-organisms to overcome.

Similarly, nearly all gram positive and negative bacteria are not able to survive acidic environments. Dropping the pH of a product to 4.7 will create an additional hurdle for bacteria. Dropping under 4.5 will eliminate most if not all harmful bacteria but will not kill yeast and mould. It is also known that as the pH falls below pH 3,0, the conditions for growth of yeast becomes hostel. (7)

There are of course organisms that survive extreme pH ranges. These are called extremophiles. These are organisms that can survive extreme environmental conditions. This could be extreme heat or extreme pH and even high salt/sugar levels. Acidophiles like Lactobacillus (the lactic acid producers) can live quite happily in very low pH ranges of 0.1-4.  On the opposite end of the spectrum there are Alkaliphilies, alkali loving organisms like Vibrio cholerge and Alkaligenes faecalis who love a high pH of 9. However, these extremophiles are not generally pathogenic to humans who are naturally of a neutral pH – our skin is between pH 4.7 and 5.4 , our saliva is about 7.4 and our blood is neutral - between pH 7.35-7.45. Part of the reason for them not being a threat to humans is because they do not like our pH so they will not live or multiply in us (8)

pH and preservatives

pH can also have an effect on the preservative efficacy and stability. For example, weak acids like sorbic and benzoic acid become increasingly more potent as the pH drops. The lower the pH, the more  undissociated the acid becomes, which has the biocidal effect; that which will kill harmful pathogens. (9)

A low pH can thus have a two-fold action, it can simultaneously kill many harmful bacteria as well as improve the efficacy of the preservative.

HURDLE 3. Water activity

All organisms need water and nutrients to survive. Limiting the available water is another strategy that can be used to help stop microbial growth.

Water activity/available water is not the same as the percentage of water in a product. Instead, it is normally used to refer to water that is not bound with solutes, particularly glycols, polyols or salts. By definition, if there is no water in a product, if it is non-aqueous/anhydrous then there is no available water.

However, there can be no available water even if the product contains a fairly large amount of water. One good example of this would be honey which if it is to be defined as honey (and legally to be sold as such) contains about 16-22% water. Another example would be brine water used to preserve food – the water is tied up with the salt and is not available for cells to grow. So how much available water is needed for different organisms to survive?

In terms of available water, 1.00 would be 100% available water and 0.00 would be bone dry. The approximate amount of water needed for organisms to thrive are as follows;

Bacteria 0.94-0.99

Yeast 0.7

Mold 0.6

As Dweck (10) states, reducing available water, '' can be achieved by the addition of salts, polyols, sugars, protein hydrolysates and amino acids and is more effective in controlling gram-negative bacteria than yeast and moulds.’’

There have been cases where humectants have been used at sufficient levels alone to increase the formulas resistance to contamination. In a dental cream, a mixture of sorbitol and glycerine, at respective levels of 10% and 12%, is often enough to protect the formula. (Antibacterial Agents and Preservatives Francoise Siquet, Colgate-Palmolive Technology Center, Milmort)

Likewise, Dekker et al 1997) states that, ‘Sorbitol and glycerol, in concentrations of around 20% w/w, are most commonly used to reduce water activity’’ (11)  

Typically we would only need 20-35% glycerin in a product to effectively preserve it, however, this would probably make for a very sticky product.

To measure water activity in a product special instruments are needed such as  vapour pressure manometry, electric hygrometry, hair hygrometry and dew point. Indeed, testing water activity of finished formulations can be valuable in understanding what type of preservative to include, for instance, if it is found that only fungi can grow, only preservatives with activity against fungi may need to be used.

It is probably important to note that although you cannot always measure water activity, there are some formulations you can be pretty certain do not need a preservative. For instance, there are a lot of salt sprays on the market that do not have preservatives. There are also products preserved with high amounts of ethanol (alcohol) which will not spoil i.e. >20%. In fact ISO 29621-2017 states,’’Products containing alcohol levels >20% by volume mass do not require microbiological testing (challenge testing and end product testing.)”(2010)

HURDLE 4. Packaging

How you package your products can have an impact on the type of preservative you use and even the amount of preservative you will need.

Packaging material and chemical reactions

It is thought that oil soluble preservatives can be absorbed into containers and closures. It is advised that products should be tested in their intended containers to determine the preservatives effectiveness under normal storage conditions because, ‘absorption, complexation, and volatility can erode antimicrobial activity….certain containers are not compatible with certain preservatives, such as nylon and parabens or polyethylene and certain phenolics, mercurial and benzoates.’ (12)

Packaging and consumer abuse

The main challenge, after manufacturing considerations, to the cleanliness of a product is how it is used by the consumer. Brannan and Dill highlighted that un-preserved shampoo bottles with a flip-top lid showed the highest degree of protection from contamination. With regards to lotions, pump top dispensers gave the greatest protection from contamination. They found that screw cap and slit caps provided the least amount of protection from consumer use and contamination. The study showed that the container is an intrinsic aspect in formulation and that preservation is not just about adding a preservative, but it is about the how the whole product functions and interacts with the container and the end user.(13, 14)

HURDLE 5 - Type of formulation

The use of different formulation types can help and hinder preservation. For instance, if surfactants are present at levels above their critical micelle concentration (CMC) then they could lock around the preservative inhibiting its effectiveness. Conversely, if they are evident at a lesser level then they could help solubilise the preservative and have a detrimental effect on the cell wall of the organism (the germ), making it weak and  vulnerable to attack. (15)

Emulsion Types

Different types of emulsion are more susceptible to microbial growth. Typically water in oil emulsions are easier to preserve than oil in water emulsions.  This is because the water droplets are in the internal or dispersed phase, surrounded by oil (in the continuous phase) making it difficult for microbes to migrate into other water droplets.  Theoretically, bacteria and mould would be introduced in the manufacturing process but find it hard to migrate once there. Furthermore, the smaller the droplet and the more densely packed the emulsion the harder it is for germs to move around and multiply.  Some sources suggest that once the oil content of the emulsion reaches over 83%, it is very difficult for microbes to multiply and cause problems.    That doesn't mean that it is impossible for them to grow so they may still need a preservative.  Due to all this, it is likely that less preservative is needed in a water in oil emulsion as opposed to the oil in water type.

Cationic Surfactants

Many cationic surfactants are very effective, however some are classified as preservatives and have very low permitted levels as they can be irritating and cause sensitisation (16).

HURDLE 6 - Using multifunctional anti-microbial ingredients

Using other ingredients not classified as preservatives

You can create a 'preservative free' or 'self-preserving' product by using actives and ingredients that have secondary preserving benefit.  There are a number of 'multifunctional' agents that are based on or marketed as humectants, fragrance materials, conditioners, antioxidants (to name a few) that are not officially classified as preservatives under the EU Directive.  Although they are not classified as preservatives, they do have preserving qualities to them that could be considered just as powerful as more traditional types of preservation i.e. phenethyl alcohol (fragrance found in rose oil) or propanediol (humectant), glyceryl caprylate and levulinic acid and its salts are a few. Sodium lauroyl lactylate is not only a detergent/surfactant, at 0.5% it can be used to significantly decrease the anti-fungal activity of the overall formulation. (17)

Hurdle 7: Using Chelating Agents to Support your Preservative 

Metal ions are normally found in water and raw materials. They are also an important part of the cells of all living organisms including bacteria and fungus. However, they can have negative effects on a cosmetic formulation. They can cause chemical degradation, discolouration, emulsion instability, rancidity, changes in fragrance and all sorts of other problems which can impact the shelf life of the product and manufacturing processes.

Chelating agents have an effect on micro-organisms and can work with the preservative in order to make them more efficient. Metals such as Ca and Mg are very important for stabilising the outer membrane of cellular organisms like bacteria and fungi. They are part of the building blocks that make up their cell wall. Chelating agents (also referred to as sequestering agents) tie up metal ions, pulling them away from the cell by forming complexes. This contributes to the cells wall being weakened or compromised. This ‘partial solubilisation of the cell membrane’ allows preservatives (biocides) a way in, causing it to die. (18,19)

Preservatives on the market

Typically preservatives on the market are blends, whether they are designed for the natural market, whether they are legally classified as preservatives; or whether they are multifunctional actives with microbial actions. Preservatives are normally in blends because they work to provide a wide coverage against fungi and bacteria. By working together they work synergistically so typically less is needed for each preservative in the blend, using less of each individual preservative means less likelihood of irritation.

The Myth about Mild Preservatives

Some say ‘natural preservatives’ are not as efficient as the traditional paraben based or formaldehyde donor types of preservatives. Essentially, it does not matter how you preserve your formula as long as you ensure it is safe. Whether a preservative works has no bearing on whether it came from a natural source or not – its effectiveness at keeping your product clean and it’s rate of irritation are what is important. Some preservatives have a higher use rate to work than others; there are preservatives that are effective at killing at 0.1% in the formulation and those that are effective at 0.5 and now, there are newer blends on the market that might need to be added 3 or 4 %. I think microbiologist Dene Godfrey summed it up perfectly when he stated,

The concept of a “mild” preservative is based on the compound having a good toxicity profile, especially regarding skin irritation/sensitisation. In other words, a higher concentration of the “mild” preservative is required to elicit an adverse reaction. To illustrate this, parabens are considered to be relatively mild preservatives, whereas the isothiazolinones are not. This is reflected in the use concentrations; parabens are required at concentrations varying from 0.2 – 0.4%; isothiazolinones from 5 – 15 parts per million (0.0005 – 0.0015%)…The difference in the margin of safety between an “aggressive” preservative at 0.0005% and a more “mild” preservative at 1% is very little, as the end result is a concentration high enough to exhibit biological activity – killing micro-organisms! ‘’ (20)

Predicting the Efficacy of your Preservative

Every formulation is different and just because you have used a preservative successfully in one type of formulation, does not mean that it will work in another. This is because it is impossible to know how it will interact with both the microorganism, the formula and the packaging. So, in order to know if your preservative works and that your product is safe, you will need the assurance of a preservative efficacy test on each and every formulation. It doesn’t matter if you use a tried or tested preservative, like the traditional paraben blend, or a new one such as Geogard ECT or any that are considered 'natural', they all need to be tested in every product bought to market. Furthermore, advice can be given on the best preservative that can be used but this still does not negate the need for thorough testing.

Selecting a Preservative for your Botanical Formulation

It can be a bit tricky selecting the right preservative but thankfully there are many small-scale suppliers that sell preservatives acceptable to the organic market. They will normally give detailed information on how to incorporate your preservative. Having said this, it is a good idea to seek out the manufacturer’s advice.  You can do this easily by googling the INCI of the product or the brand/manufacturers assigned name and adding ‘PDF’ to your search. Once you have the information they will tell you some important things about your preservative, mainly;

1. How wonderful it is.

2. The pH you need to add it at – most of them require an effective pH range – this is important. Most organic acids will require a pH of 5.5 or less. In which case you need to cross-reference with your other ingredients. Interestingly many natural emulsifiers available to the home-crafter or small scale manufacturer do not work well under pH 6 which can be a problem, in which case you will need to experiment more. It might be okay but you may need to support your emulsifier with additional fatty alcohols or another emulsifier. You may need to change your emulsifier entirely.

3. They may say that it is ‘broad spectrum’. This may be true but if you read carefully they may hint that it is ‘weak’ with certain germs in which case you would need to consider how you will tackle it's deficiencies.

4. The phase in manufacturing you need to add it – the supplier could say that it needs to be added to the cool-down phase. They may also say that it could be added up to 80 Celsius with no problem. It is ideal to put your preservative in the water phase and heat it but it may not work out well with your formula. It is worthwhile testing to see if adding at different phases could have an effect on stability or aesthetics.

5. What it is compatible with and what it doesn’t agree with – if the incompatibility is due to safety reasons or the preservatives performance then follow the advice. But if it is to do with aesthetics it might be worthwhile trying it to see how it affects the overall formula if added at different phases of manufacturing.

6. The percentage range to use. For instance, a manufacturer could advise to add it between 0.8-1.2% of the overall formula. Ideally, it is better to add it at the lowest percentage rate and test however if you do not have the luxury of infinite testing it is probably better to start mid range - again you must test.

Selecting a preservative and testing can be problematic. But when you know what you are doing and you experiment it is far better than being spoon-fed information. I have chosen not to give percentage rates of different preservatives as they are readily available online by manufacturers along with their advice which I feel is the better way to go.  Also, it doesn't give a false sense of security, helps you understand what you are doing and why you are doing it.

REFERENCES:

1. Ayliffe et al 1966 in Hiom, 2008, Chapter 14. Preservation of Medicines and Cosmetics).

2. W. Siegert, 2012 Microbiological Quality Management for the Production of Cosmetics and Detergents

3. S. Hiom, 2008

4. Kabara, Jon J., 1999, HURDLE TECHNOLOGY: ARE BIOCIDES ALWAYS NECESSARY FOR PRODUCT PROTECTION?

5. Janet C. Curry, Daniel K. Brannan, with Philip A. Geis, 2006 History of cosmetic microbiology in cosmetic Microbiology: A practical approach

6. W. Siegert Microbiological Quality Management for the Production of Cosmetics and Detergents, SOWF, 2012

7. PITT, J.I., Resistance of some food spoilage yeasts to preservatives, Food Technology, 26(6), pp. 238, 239, 241, 1975)

8. Int J Cosmet Sci. 2006 Oct;28(5):359-70. doi: 10.1111/j.1467-2494.2006.00344.x.Natural skin surface pH is on average below 5, which is beneficial for its resident flora.Lambers H1, Piessens S, Bloem A, Pronk H, Finkel P

9. Eklund 1989 in R.J. Lambert and M. Stratford Microbiology Section, Unilever Research, Sharnbrook, Bedford UK, journal of microbiology

10. http://www.creative-developments.co.uk/papers/Preservatives%20SPC%202000.htm

11. KABARA, J.J. and ORTH, D.S., Principles for product preservation in preservative-free and self preserving cosmetics and drugs, Marcel Dekker, New York, pp. 1-14, 1997).

12. Scott, V.W. Sutton, 2006 Antimicrobial preservative efficacy and microbial content testing in Cosmetic Microbiology: A practical approach.

13. j. Soc. Cosmet. Chem., 46, 199-220 (July/August 1995) Cosmetic preservation DANIEL K. BRANNAN, Department of Biology, Abilene Christian University, Abilene, TX 79699. Received February I995.

14. Geis, 2006, Preservations Strategies in Cosmetic Microbiology: A practical approach

15. http://www.creative-developments.co.uk/papers/Preservatives%20SPC%202000.htm

16. http://www.creative-developments.co.uk/papers/Preservatives%20SPC%202000.htm

17. Belgium Michel J. Devleeschouwer, Free University of Brussels, Brussels, Belgium, 2001

18. W. Siegert* Boosting the Antimicrobial Efficiency of Multifunctional Additives by Chelating Agents) Typically EDTA is used for this purpose in cosmetics however there are other alternatives.

19. Kabara, J.J Preservative-Free and Self-Preserving Cosmetics and Drugs: Principles and…, Handbook of Cosmetic Microbiology).

20. http://personalcaretruth.com/2010/12/mild-preservatives-fact-or-fiction/

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