Antimicrobial Preservation Overview
Jim McElroy
Composition and Structure of Microorganisms The cell surface differentiates cells from one
another The cell envelope is the rate limiting barrier
for the partitioning of : Nutrients Waste products Chemicals
Cell membranes Main components are phospholipids and
proteins Major proteins are hydrophobic The plasma structure is stabilized by
hydrogen bonding Mg2+ and Ca2+ cations stabilize the membrane
ComparisonProperty or Constituent Gram
PositiveGram Negative
1. Liposacharide Outer Layer Absent Present
2. Polysaccharide High Low
3. Lipids 2-4% 15% or more
4. Solubility of lipids in fat solvents
Resistant Less resistant
5. Amino Acids 3-4 Complete range
6. Optimal Growth pH Relatively high
Relatively low
7. Nature of toxin Exotoxin Endotoxin
Yeast and Mold Yeast: Cell wall is composed of two or more
protein polysacharide complexes held together by covalent bonds
Yeast: Mannan is a cell wall component Mold: Cell wall is composed of two or more
protein polysaccharide complexes held together by covalent bonds
Mold: Mannan is not a cell wall component
Comparison (bacteria, yeast & mold)
Property Bacteria Yeast, Mold
Cell size (μm) 0.5-5 Yeast 20-50Mold ≥100
Cell wall Teichoic acids, muramic acid peptides
Chitin, glucans, manans, and diaminopimelic acid
Cytoplasmic membrane
No sterols Sterols
Cytoplasm No mitochondria and endoplasmic reticulum
Mitochondria and endoplasmic reticulum
Nucleus Prokaryotic (no membrane) Eukaryotic (membrane)
What is a preservative?A chemical substance that is used to preserve pharmaceuticals, food or other organic material from decomposition or fermentation by preventing the growth of microorganisms.
Commonly used preservatives Ingestible & Topical
Methyl, ethyl, propyl and butyl Parabens Sorbic acid Na, K & Ca Sorbate Benzoic acid Na, K & Ca Benzoate Sodium metabisulfite Propylene glycol (15-30%) BHT, BHA Flavors w/ benzaldehyde
Topical Only Formaldehyde donors Essential Oils Monoglyceride Phenol Mercury compounds
ParabensParabens are among the most commonly used preservatives. They are relatively active against a broad spectrum of microorganisms. The methyl ester is most effective against bacteria and molds while the ethyl, propyl and butyl esters are more active against yeast and molds. Parabens are more effective against gram negative than gram positive organisms.
Parabens and pHParabens disassociate at high pH (8 or more) and are rendered ineffective.
Parabens have limited solubility in H2O
Property Methylparaben sol. g/100mg
Ethylparabensol. g/100mg
Propylparabensol. g/100mg
Butylparabensol.g/100mg
Water 18ºC 0.16 0.08 0.023 0.005
Water 25ºC 0.25 0.11 0.04 0.015
Water 80ºC 3.2 0.86 0.45 0.15
Ethanol 22 25 26 110
Propylene Glycol
1.7 0.5 0.4 0.3
Peanut Oil 0.5 1 1.4 5
Mineral Oil 0.01 0.025 0.03 0.1
Partition Coefficients and Emulsions
It is important to pay close attention to preservative partitioning in emulsion systems. Parabens prefer the oil phase thus leaving little in the water phase where it is needed.
Oil droplet
surfactant
Water phase
Partitioning of Parabens Parabens are lipophilic. Parabens will partition disproportionately into
the oil phase of an emulsion. If large amounts of the preservative partition
into the oil phase, it is not available for antimicrobial action.
Partition Coefficient Measurement Cw is the preservative left in the water phase C is overall % of preservative Kw is the partition coefficient ά is phase volume ratio
Cw=C* ά + 1 / Kw + 1
Partition Coefficients For Parabens & Other PreservativesPreservative Kw Oil
Methylparaben 7.5 Almond Oil
0.1 Mineral Oil
18.0 Isopropyl Myristate
200 Diethyl Adipate
Ethylparaben 26.0 Soya Oil
Propylparaben 87.0 Soya Oil
Butylparaben 280.0 Soya Oil
Benzoic Acid 6.10 Soya Oil
Phenol 5.60 Arachis Oil
0.07 Mineral Oil
Sorbic Acid 3.3 Almond Oil
0.21 Mineral Oil
Chelating Agents as Preservative Enhancers
Alkaline earth metals such as Ca+ and Mg+ are important for the stabilization of the outer membrane of cellular organisms. Chelating agents sequester these ions. This contributes to the partial solubilization of the cell membrane which allow preservatives a pathway into the cell. EDTA is a typical chelating agent used in formulations.
Ingredients That Enhance Preservative Efficacy Solutes (salts & high concentration of sugars) Esters Cationic and anionic surfactants Humectants (glycerin, propylene glycol) Phenolic antioxidants (BHT) Chelating agents (EDTA) Fragrances Low water activity
Ingredients That Hinder Preservative Efficacy Sugars and alcohol sugars Proteins, peptides, yeast extract Natural gums & cellulose thickeners Plant extracts (aloe vera, starch,…) Vitamins Clay compounds High water activity Surfactants (Tween 80)
Manufacturing Steps That Enhance Preservative Efficacy Sanitary condition of equipment Treatment of raw materials Heat treatment of sub-phase or final batch Sub-micron filtration Hot fill Aseptic fill
Packaging Components and Preservative Efficacy Design of opening or dispenser Susceptibility to contamination from
environment Susceptibility to contamination from human
contact
ConclusionIt is important to take into consideration all aspects of the formula before considering a preservative system. This includes type of formula, bulk handling and packaging.