A key ingredient in e-liquid composition, vegetable glycerin is greatly appreciated by vapers due to its ability to produce more vapour. However, consumers are not always aware of VG, as it is often called in the field. We asked a specialist vaping laboratory to give us a detailed explanation of the role of vegetable glycerin and provide us with a full profile of this compound which is becoming a superstar in the vaping sector. Answers from the scientific team at Laboratoire Sense (Phodé Group).
Are glycerin and glycerol the same thing?
The vegetable glycerin used in most e-liquids is made up of 99.5% glycerol. For this reason, vegetable glycerin and glycerol are often said to be identical. To be more precise, it is preferable to say that glycerol is the main molecule of glycerin.
Grade Crude glycerin Technical-grade glycerin 1st grade Distilled glycerin Distilled food-grade animal glycerin 2nd grade USP/EP grade distilled glycerin Glycerol content 50% 80% 80% 95% 99.5% Methanol 5 to 18% 0.5 to 3% 0.2% 0.1% 0.015% Food compatibility No No Yes Yes Yes Risk Highly hazardous (very high methanol content) Highly hazardous (High methanol content) Low risk Very low risk Negligible risk
The vegetable glycerin recommended for instance by the french AFNOR standard XP D-300-2 is a USP (United States Pharmacopeia) and EP (European Pharmacopoeia) pharmaceutical-grade distilled glycerin containing at least 99.5% glycerol.
The vegetable glycerin used in pharmaceutical-grade e-liquids can be food-grade if it is accompanied by a food compatibility certificate.
Is glycerin always obtained from plant sources?
Glycerin is available in the form of vegetable glycerin, used in e-liquid compositions, but also from animal sources.
Animal glycerin Source Animal carcass, fats Frying fats Vegetable glycerin Source Sunflower Rapeseed Soybean Castor Jatropha Palm and palm nut (fruit of the palm tree)
Can the vegetable glycerin contained in an e-liquid be organic?
Different organic standards apply according to the certification criteria. The cosmetic organic standard differs from the organic standard for food. As such, an organic product according to cosmetic specifications does not necessarily meet the organic criteria for food.
This is the case for the organic vegetable glycerin used in some e-liquids. In this case, the organic vegetable glycerin complies with cosmetic, nutritional or pharmaceutical organic specifications, but does not comply with organic specifications applicable to food. As vegetable glycerin is obtained using a chemical process, it cannot be considered organic according to organic criteria applicable to food.
What are the physicochemical properties of glycerol?
Glycerol (1,2,3-propanetriol) is an alcohol with the molecular formula C3H8O3
Its linear formula is: CH2OH—CHOH—CH2OH
Its fully developed formula is as follows:
Glycerol has a molecular mass of 92.11 g and a density of 1.260 g.cm-3 (at 18°C). Glycerol is therefore denser than water and PG (Propylene Glycol).
Glycerol has a boiling point of 290°C, an auto-ignition temperature of 370°C, and a flash point of 160°C.
Glycerol has a viscosity of 1.49 Pa.s at 20°C, which is equivalent to a viscosity 1500 times greater than that of water.
At room temperature, glycerol is presented in the form of a clear, viscous, colourless, odourless, non-toxic liquid with a sweet taste.
Due to the presence of the three hydroxyl groups, glycerol is very hygroscopic. It acts as a water trap. Glycerin is miscible with all proportions of water and alcohol.
How is glycerol manufactured?
Glycerol can be obtained using two main processes: fat saponification or plant oil trans-esterification.
The saponification reaction makes it possible to produce soap and glycerol from fat and soda. The glycerol obtained from saponification is of high-purity (> 99%), and is hence predominantly used in pharmaceutical and cosmetic applications.
Saponification reaction:
The biodiesel production process from oil seeds (rapeseed in particular) can also be used to obtain glycerol at the end of the process. It is the result of plant oil trans-esterification, a reaction between the plant oil and methanol.
Trans-esterification reaction:
What is glycerin used for?
Glycerin is widely used in many industrial and consumer products such as soap, detergents, medicinal products, cosmetics, foods, drinks, paints, resins, and paper.
This compound has over two thousand applications. The main ones are listed in the figure below:
Glycerin is predominantly used in the pharmaceutical industry, particularly as a substrate for antibiotics, antiseptics or in capsules. Indeed, this molecule can be found in cough syrups (expectorant) for example, and also in suppositories. It is a hydrating agent which improves the oiliness and lubrication of pharmaceutical preparations.
Glycerin is an excellent solvent for a wide range of active substances and chemical compounds.
Glycerin is also used extensively in the cosmetic industry. It is the main ingredient of toothpaste. Similarly, this compound is widely used in skin care (soaps, moisturising creams) and hair care (shampoo) products. The main qualities provided by this substance are softness, and also hydration.
Glycerin is also found in animal and human nutrition. It is frequently used as a food substrate for liquid flavours, and also as a sweetener, emulsifier, thickener, or preservative. Similarly, it is often incorporated in plant substance extraction processes.
Other applications are also listed:
- Chemical industry: for the manufacture of chemicals
- Tobacco industry: for regulating moisture in tobacco, but also removing the irritant and unpleasant after-taste
- Paper and plastic industry: as a plasticiser for example
- Textile industry: in yarn design and properties, particularly as a fibre, yarn, and fabric softener
- Paint industry: used as a binding agent
- Automotive and aeronautical industry: To protect hydraulic cylinders from freezing for example or as an antifreeze
- In e-cigarettes: the compound is present as a substrate and also to intensify the vapour released
Film industry: for simulating perspiration or smoke (special effects). [1] [2] [3] [4] [5] [6]
Where can glycerol be found naturally?
Glycerol is a substance found naturally in living beings (humans and animals). It is a precursor of glucose and glycogen. It is also involved in lipogenesis. Low concentrations (< 0.1 mmol/L) can be found in all cells (Robergs & Griffin, 1998)*
It originates from lipolysis in adipose cells, lipoprotein triglyceride hydrolysis in the blood, and, to a lesser degree, intestinal lipid digestion.
Bound to fatty acids, glycerol forms triglycerides which represent almost 10% of the lipids in living organisms.
Glycerol is not naturally found in plants. The term “vegetable glycerin” means that the glycerin has been obtained from plant oil following a catalysed chemical trans-esterification process.
How do our bodies metabolise and eliminate glycerin?
The metabolisation of a molecule by the body is defined by the sequence of chemical reactions that the compound undergoes after absorption for its utilisation or elimination.
Glycerol is primarily metabolised in the liver and kidneys.
60% of metabolised glycerol is either converted into glucose – this is referred to as gluconeogenesis – or converted into energy in cells to enable mitochondrial respiration (source of energy through incorporation in the Krebs cycle=TCA cycle).
40% of glycerol is used in free fatty acid esterification to form triglycerides (form of lipid storage in the body).
What is the toxicological rating of glycerin?
The United States Food and Drug Administration (FDA) has classified glycerol as a “GRAS” (Generally Recognized As Safe) substance. The overall toxicity risk in pharmaceuticals is low. Skin irritation is unlikely unless the skin has an open wound. Inhalation toxicity is low, due to the low volatility. On the other hand, excessive intake can cause blood glucose and lipid elevation due to metabolism. [7] [8] [9]
Respiratory exposure
Its airborne toxicity has been studied in rats. The only effect observed is tracheal mucosal thickening following high-dose exposure, which is a form of protection against chemical irritants (Renne et al, 1992).
The NOEL (no observed effect level) in rats with inhalation exposure is 167 mg/m3 (Renne et al, 1992); this type of data is used a basis for calculating occupational exposure limit values.
The blood glycerin concentration measured in rats exposed through inhalation to a PG/VG mixture does not increase, which can be explained by the greater presence of endogenous glycerol than exogenous glycerol and/or a rapid glycerol metabolism after absorption.
Glycerin supplied by the external environment does not modify the concentration of glycerin produced by the body.
Oral exposure
During gavage studies conducted on rats, mice and rabbits, no maternal toxicity and no embryotoxic or fetotoxic effects were observed (OECD SIDS 2002).
Likewise, glycerol has no carcinogenic or mutagenic effects (OECD SIUDS, 2002.)[10]
In medicinal products used as excipients according to the 2008 French Health Products Safety Agency guidelines, if the threshold is >10g, the information to be included in the special warnings and precautions for use is as follows “This medicinal product contains glycerol and can cause headaches and digestive disorders (diarrhoea). ”
Dermal exposure
There is no human or animal data indicating that glycerol is a skin sensitiser
Can vegetable glycerin cause undesirable reactions when heated?
Undesirable reactions: formation of aldehydes particularly acrolein with heat (AFNOR standard + R-VAPE 9).
When e-liquids are superheated (temperature greater than 350°C), glycerol degradation can occur, potentially inducing the formation of undesirable compounds: aldehydes, such as acrolein, acetaldehyde and formaldehyde. [11] [12 ]
The latest devices limit this “dry hit” risk with an increase in the wick supply capacity, enabling a sufficient e-liquid supply to the resistance coils. This is aimed at preventing aldehyde formation.
In any case, it is essential that you follow the manufacturers’ recommendations for use.
Study on various PG/VG mixtures at different ratios
Study conducted with the Laboratoire Sense (Phodé Group) RVAPE-9 vaping robot / AFNOR WPD90-300-3 parameters:
WORD REMOVED FROM TABLE
Loss in mass
a,b,c,d P<0.05
Regardless of the VG content, acrolein is not detected in the vapours generated by these different PG/VG mixes in these tests in compliance with the device manufacturers’ recommended conditions for use.
Similarly, the levels of the other aldehydes (formaldehyde and acetaldehyde) are below the limit levels of the XPD 90-300-3 standard.
What is the role of vegetable glycerin in vaping?
Vegetable glycerin is a moistening and hydroscopic agent that is increasingly used due to developments in vaping devices.
This hygroscopic element, which can contain up to one quarter of its volume in water, makes it possible to obtain a good aerosol density. Glycerin evaporates at 290°C, and is almost 3 times more viscous at room temperature (20°C) than mono-propylene glycol.
Disadvantages
Its relatively high boiling point and its high dynamic viscosity make it a challenging diluent for vaping systems in terms of heating power, and wick supply capacity to prevent the “dry hit” effect.
Similarly, glycerol has the disadvantage of causing resistance coil fouling. The greater the vegetable glycerin content, the higher the resistance coil replacement frequency will be. Similarly, more frequent device maintenance will be required.
Advantages
Due to both its moistening properties and its hygroscopic properties, it is a prime diluent for vapers with sensitive throats, or “cloud chasers”.
In the gaseous state, it is condensed into fine droplets (generally in the presence of an air flow), trapping some of the adjacent molecules (i.e. nicotine, aromatic compounds, water, etc.). This rapid effect produces an aerosol which visually imitates smoke.
This article was prepared with the kind participation of Laboratoire Sense (Phodé Group) coordinated by Sandrine Marckt. At the time of publication, the Vaping Post has no commercial ties with this company.