The Kinetics of Maturation: Mastering Alcohol vs. PG Flavor Bases and Flash-Off Rates in E-Liquid Manufacturing

Author: R&D Team, CUIGUAI Flavoring

Published by: Guangdong Unique Flavor Co., Ltd.

Last Updated:  Feb 26, 2026

In the precise world of e-liquid manufacturing, consistency and depth of flavor are paramount. Producing a vape juice that tastes “good” is easy; producing one that tastes exceptional, complex, and remains stable across batches is science.

As a manufacturer of premium fragrance and flavor concentrates for the e-liquid industry, we understand that the concentrate is only the beginning. The journey from mixing vessel to the consumer’s tank involves a critical phase: steeping.

Steeping is often misunderstood as simply “letting it sit.” In reality, it is a complex series of chemical reactions, homogenization processes, and, crucially, evaporation events. The carrier solvent used in your flavor concentrates—primarily Propylene Glycol (PG) or Ethyl Alcohol (Ethanol)—dictates the mechanics of this process.

Understanding the difference in “flash-off” rates between alcohol-based and PG-based flavors is essential for manufacturers aiming to optimize production timelines, ensure flavor fidelity, and eliminate harsh off-notes. This article provides a technical analysis of how these carriers influence the steeping dynamic.

Base Lab Setup

1. The Necessity of Flavor Carriers in E-Liquids

Raw flavor molecules—aldehydes, esters, ketones, and terpenes—are incredibly potent and often exist in solid or highly viscous states. They are rarely soluble enough in vegetable glycerin (VG) on their own to create a homogeneous final product.

A carrier solvent is required to dilute these powerful aromatic compounds into a usable concentrate, ensure they disperse evenly throughout the e-liquid matrix, and stabilize them for storage.

While various carriers exist, the industry standard pivots on two primary solvents:

• Propylene Glycol (PG):The industry workhorse. It is odorless, nearly tasteless (slightly sweet), carries flavor exceptionally well, and possesses antimicrobial properties that aid in shelf stability.
• Ethyl Alcohol (Ethanol):Often used for extraction of natural flavors or for specific aromatic profiles that require a highly volatile carrier to “pop.”

The choice between these two isn’t merely about preference; it fundamentally alters the thermodynamic behavior of the final e-liquid during the maturation phase.

2. Demystifying Steeping: Homogenization and Volatility

To understand flash-off, we must first accurately define steeping. From a chemical engineering perspective, steeping is not a passive wait time. It is an active process of reaching kinetic equilibrium.

When flavor concentrates (dissolved in PG or Alcohol) are introduced to the base mix (VG/PG/Nicotine), several processes occur simultaneously:

• Homogenization (Diffusion):The flavor molecules must uniformly disperse throughout the viscous VG/PG mixture. This takes time, as VG is highly viscous, impeding rapid molecular movement.
• Oxidation and Chemical Interaction:Components react with trace amounts of oxygen present in the headspace or dissolved in the liquid. Additionally, Maillard reactions or acetal formation can occur over time, deepening the color and mellowing sharp notes.
• Volatile Evaporation (Flash-Off):The most volatile components of the mixture—specifically the carrier solvents and certain low-molecular-weight flavor compounds—escape the liquid phase into the gas phase.

It is this third point—volatile evaporation—where the distinction between alcohol and PG bases becomes critical.

According to PubChem, the open chemistry database at the National Institutes of Health (NIH), Propylene Glycol has a vapor pressure of approximately 0.13 mmHg at 25°C. In stark contrast, Ethanol has a vapor pressure of roughly 59 mmHg at roughly the same temperature. [1]

This massive discrepancy in vapor pressure means that ethanol is significantly more volatile—more prone to evaporating, or “flashing off”—than PG under standard ambient conditions.

3. The Crucial Concept: “Flash-Off” Rates Explained

“Flash-off” in the context of e-liquid manufacturing refers to the rate at which volatile substances evaporate from the mixture during steeping or processing.

Volatility is governed by the strength of intermolecular forces within the liquid. Molecules in a liquid are constantly moving. If a molecule near the surface gains enough kinetic energy to overcome the attractive forces holding it to its neighbors, it escapes into the vapor phase.

• PG’s low flash-off rateis due to its structure. It is a diol, meaning it has two hydroxyl (-OH) groups. These groups form strong hydrogen bonds with neighboring PG and VG molecules. Significant energy is required to break these bonds, leading to low volatility and slow evaporation.
• Alcohol’s high flash-off rateoccurs because ethanol is a monohydric alcohol with only one hydroxyl group. While it still participates in hydrogen bonding, the intermolecular forces are weaker overall compared to the dense hydrogen bonding network of a PG/VG matrix. Therefore, ethanol requires less energy to escape the liquid phase.

The flash-off process is not just about removing the solvent. It also carries away other highly volatile “top notes” of the flavor profile. Managing this rate is the difference between a harsh vape, a muted vape, and a perfectly balanced one.

Evaporation Test

4. Alcohol-Based Flavors: The High-Volatility Dynamic

Flavor houses often use ethanol for specific reasons. It is an excellent solvent for difficult-to-dissolve natural extracts (like certain citrus oils or complex vanillas) and it provides a distinct “throat hit” that some formulators desire.

However, when using alcohol-based flavors, the manufacturer is introducing a highly volatile component into the final mix.

4.1 The “Chemical” Taste and Breathing

Newly mixed e-liquids using alcohol-based flavors often suffer from a harsh, “perfumey,” or chemical taste immediately after mixing. This is rarely the flavor itself, but rather trapped ethanol that hasn’t yet flashed off.

If the e-liquid is immediately bottled and sealed tightly, that ethanol has nowhere to go. It remains trapped in the headspace or dissolved in the liquid, leading to an unpleasant consumer experience.

4.2 The High Flash-Off Steeping Strategy

Steeping with alcohol bases requires a strategy known as “breathing.” This involves leaving the mixing vessel or individual bottles uncapped for a specific duration (usually 12 to 24 hours) at the beginning of the steeping process.

This allows the highly volatile ethanol to flash off rapidly. Because ethanol’s vapor pressure is so high compared to most flavor molecules, it will evaporate preferentially.

The Risk: The danger of breathing is over-flashing. If left open too long, along with the ethanol, you begin to lose the desirable volatile top notes—the bright strawberry, the zesty lemon, the delicate floral notes. Once those top notes flash off, they are gone forever, resulting in a “flat” flavor profile.

Manufacturers using alcohol bases must precisely calibrate breathing times to remove the harsh ethanol while retaining the desired aromatics.

5. PG-Based Flavors: The Stable Standard

Propylene Glycol is the preferred carrier for the majority of e-liquid applications due to its stability and compatibility with VG.

PG-based flavors possess a very low flash-off rate. When a PG-based concentrate is mixed into a final product, the carrier solvent is essentially permanent. It will not evaporate to any significant degree during standard steeping conditions.

5.1 The Homogenization Challenge

Because PG doesn’t flash off to “open up” the flavor, PG-based e-liquids rely heavily on time for diffusion and homogenization. The flavor molecules must physically migrate through the thick VG matrix without the “agitation” caused by evaporating solvent.

This leads to what is known as “flavor muting” in fresh batches. The flavor is there, but it is locked into pockets of the VG and hasn’t fully integrated with the nicotine and other components.

5.2 Steeping PG Bases: Patience is Key

PG-based flavors generally require longer steeping times than alcohol-based flavors to reach peak maturity. While an alcohol-based fruit flavor might peak in 3-5 days (after the alcohol flashes off), a complex PG-based custard or tobacco might require 2-4 weeks.

The advantage here is consistency. Because you aren’t relying on the variable evaporation of a solvent, PG-based formulas are easier to replicate batch-after-batch without worrying about ambient temperature or humidity affecting the flash-off rate. The flavor profile you mix is, largely, the flavor profile that will remain, only mellowing and integrating over time.

For deeper insights into the chemical properties and industrial applications of Propylene Glycol, resources like Wikipedia offer comprehensive overviews of its diol structure and miscibility. [2]

Molecular Behavior

6. Comparative Analysis and Manufacturing Implications

For the large-scale manufacturer, choosing between alcohol and PG-based flavors (or flavors that utilize a blend) has significant implications for production scheduling and warehousing.

6.1 The Economic Impact of Steeping Time

Time is inventory capital tied up on warehouse shelves. A PG-based dessert flavor requiring a four-week steep represents a significant carrying cost compared to an alcohol-based fruit flavor ready in four days.

However, the PG base offers greater shelf stability. As noted by industry sources like the Flavour manufacturers Association of Canada (FMAC), stability in flavor delivery systems is crucial for maintaining product integrity over prolonged shelf lives, a key benefit of stable carriers like PG compared to highly volatile ones. [3]

7. Optimizing Your Process: Accelerated Steeping

Manufacturers often seek to reduce steeping times to improve cash flow. Understanding the carrier is vital when applying accelerated steeping techniques.

7.1 Heat Application

Applying gentle heat (around 40°C – 50°C) lowers the viscosity of VG, allowing for faster homogenization.

• With PG Bases:Heat is generally safe and effective for speeding up diffusion.
• With Alcohol Bases:Heat must be applied with extreme caution. Heating alcohol increases its already high vapor pressure, leading to extremely rapid flash-off. If the vessel is open, you risk losing virtually all top notes in minutes. If sealed, you increase pressure within the vessel.

7.2 Ultrasonic and Magnetic Stirring

Mechanical agitation greatly increases the surface area interaction between flavor, PG, and VG.

• Both Bases:Agitation is highly beneficial for both types. It physically forces homogenization, reducing the reliance on passive diffusion. For alcohol bases, stirring while breathing will accelerate the removal of ethanol without requiring excessive heat.

Scientific literature on emulsion technology frequently highlights high-shear mixing and ultrasonic processing as effective methods for reducing droplet size and ensuring uniform dispersion in viscous systems, which directly applies to e-liquid homogenization. [4]

Conclusion: The Balancing Act

There is no single “best” carrier for e-liquid flavors. The choice between Alcohol and PG—or more commonly, a strategic blend of both by your flavor house—depends entirely on the desired final profile and your manufacturing capabilities.

Alcohol bases offer bright, punching top notes and rapid maturation, provided the flash-off process is tightly controlled to avoid harshness or flavor loss. PG bases offer unparalleled stability, smoothness, and depth, demanding patience as they slowly homogenize into the VG matrix.

As a premier manufacturer of flavor concentrates, we do not just supply scents; we supply solutions. We understand the intricate dance of vapor pressure and molecular interaction. Whether you require high-flash natural extracts for a vibrant fruit line or ultra-stable PG-based compounds for a complex aged custard, our technical team is ready to assist you in selecting the right tool for the job and optimizing your production SOPs for the perfect steep.

Mastering the science of steeping is the final step in turning our concentrates into your customers’ all-day vape.

Apex Master Blend

Call to Action

Are you struggling with inconsistent steeping times or muted flavors in your e-liquid production?

Partner with a flavor manufacturer that understands the science behind the scent. Contact our technical team today for a consultation on optimizing your flavor selection and steeping processes, or request samples of our premium PG and Alcohol-based concentrates.

Citations

[1] National Center for Biotechnology Information (2024). PubChem Compound Summary for CID 1030, Propylene Glycol and CID 702, Ethanol. Retrieved from PubChem Database.

[2] Wikipedia contributors. (2024). Propylene glycol. In Wikipedia, The Free Encyclopedia.

[3] Flavour Manufacturers Association of Canada (FMAC). (n.d.). Industry Resources and Standards regarding flavor stability. (General reference to industry association standards).

[4] McClements, D. J. (2015). Food Emulsions: Principles, Practices, and Techniques (3rd ed.). CRC Press. (Reference to principles of homogenization in viscous systems).