Precise control of temperature and humidity is not merely a »nice-to-have« in many technical and biological processes, but a fundamental prerequisite for reproducibility. Particularly in the food industry—such as in cheese production, fermentation, tobacco processing, or the drying of herbs—this consistency is crucial to the final result.

However, simple two-point controls (on/off) often fail due to system dynamics, because:

1. Constant parameter fluctuations cause stress on organic material.
2. During periods of high humidity, local microclimates (»moisture pockets«) form, which promote mold growth or decay.
3. A too abrupt drop in temperature and humidity can permanently damage the material structure.

The Challenge of Drying Plant Material

Drying plants is one of the oldest and most important processing methods. It involves far more than simply removing water: the key is to control quality, aroma, and shelf life in such a way that the product remains stable and active ingredients and aromas are preserved as much as possible.

The central challenge lies in achieving the right physical balance:

Drying too quickly:

Delicate aromas and essential oils evaporate prematurely. In addition, the cell structure on the surface »glasses over« (capillary stop), while critical residual moisture remains trapped inside.

Drying too slowly:

The risk of mold growth and microbial spoilage increases dramatically, jeopardizing product safety.

In sensitive areas, precision control is therefore the decisive factor:

• Medicinal plants: Ensuring the concentration of active ingredients.
• Tobacco & tea: Targeted control of enzymatic processes for flavor development.
• Cannabis: Protecting sensitive trichomes and volatile terpenes.

The goal: to achieve optimal storage stability without degrading the material due to thermal stress or excessive air velocities.

The Biology in Detail: Drying Cannabis

Technical precision is particularly critical when drying cannabis flowers, as the valuable compounds (cannabinoids and terpenes) are found in the trichomes. These tiny, resin-filled glandular heads are extremely sensitive to physical and thermal stress.

Water Activity and Moisture Content

Success is determined not by the relative humidity of the environment, but by the water activity (a_w value) within the flower. The a_w value describes the water available for chemical and biological processes.

Safety

When the a_w​ > 0.70, the likelihood of mold spores such as Botrytis cinerea (gray mold) germinating increases dramatically. Safe storage stability is only achieved at values between approximately 0.55 and 0.62.

Quality

At the same time, we aim for a water content (WC) of 10 – 12 %. This range has proven to be the »sweet spot« not only for cannabis but also for high-quality medicinal herbs, tobacco, and tea, ensuring the right texture and preserving the essential oils. It prevents the material from crumbling while ensuring clean further processing.

The Problem

If the temperature drops below the dew point in certain areas (e. g., on the surface of a flower), water activity rises immediately—creating an ideal breeding ground for microorganisms, even if the average values in the room appear to be within normal range.

Interpretation

The microscope images clearly show that mold is not a coincidence, but rather the result of inconsistent environmental conditions. Only a control system that actively monitors and regulates the dew point can preserve the trichomes while simultaneously eliminating the risk of mold.

VPD – The Metric for Drying Rate

During the drying process, plant material loses moisture by releasing water vapor into the surrounding environment. Physically, the rate at which this occurs depends on temperature and relative humidity. Instead of controlling both factors separately, professional drying uses the Vapor Pressure Deficit (VPD)—the difference between the maximum possible water vapor pressure and the actual water vapor pressure in the air.

Although the VPD no longer has an active biological function after harvest (such as controlling the stomata of living plants), it is the decisive indicator of how strongly the evaporative force of the environment affects the plant material.

Why the VPD Window Determines Quality

Consistent evaporation is particularly important when drying cannabis, as the valuable compounds (cannabinoids and terpenes) are found in the trichomes. Terpenes are volatile, aromatic molecules that not only determine the scent but also have pharmacological effects (e. g., anti-inflammatory or calming). They are extremely sensitive to heat, oxygen, and mechanical deformation.

During drying, the plant tissue shrinks and the trichomes also lose water. Controlling the VPD is the critical tool here:

Too high VPD (too fast drying):

The evaporation rate is too high. The trichomes lose water so abruptly that they snap, burst, or twist. Cannabinoids and terpenes are released and lost through premature vaporization or oxidation.

Too low a VPD (stagnation):

Drying comes to a near standstill. Moisture remains inside the flower for too long, keeping the water activity (a_w value) high and massively increasing the risk of mold.

The goal:

If you keep the VPD stable and dry at low temperatures, the material loses water more slowly and evenly. This keeps the trichomes largely intact and protects the valuable compounds.

In summary, VPD-controlled drying means:

• Maximum preservation of aroma and quality.
• Prevention of mold through controlled moisture removal.
• Reliable storage stability without loss of active ingredients.

Whether medicinal plants, cannabis, or tobacco—precise VPD control is the fundamental prerequisite for a final product of the highest quality, whose active ingredient profile and physical properties have been optimally preserved.

The Industrial Process: Condensation Drying in a Closed System

To preserve the physical properties and sensitive active ingredient profile described throughout the entire drying process, modern process engineering relies on the principle of condensation drying in a closed air circuit.

In industrial pharmaceutical and food production, this is the gold standard, as it completely eliminates dependence on fluctuating external conditions (temperature/humidity).

Physical decoupling through »Cool-Dry-Reheat«

The main problem with conventional dryers is the rigid coupling of temperature and humidity. An industrial system breaks this relationship through a three-phase, external air treatment process:

1. Phase – Targeted Heat Removal (Cool & Dry): The humid air is extracted from the drying chamber and passed through a heat exchanger whose surface temperature is well below the dew point. This is where the actual dehumidification takes place: water vapor condenses into liquid droplets and is continuously removed from the system. The air is now dry, but too cold for the process.
2. Phase – Thermal reconditioning (Reheat): Before the air returns to the material, it is reheated to the exact target temperature via a second heat exchanger. Only through this intermediate step can the VPD (vapor pressure deficit) be controlled independently of the cooling capacity.
3. Phase – Homogeneous Recirculation: The now perfectly conditioned air flows back into the sample chamber. Since it has a precisely defined saturation deficit, it can absorb moisture from the core of the material without damaging the sensitive surface (e. g., trichomes or leaf structure) due to excessive gradients.

Why the closed-loop system is the only option

Process stability

Because the system is completely isolated from the outside air, changing environmental conditions have no effect on the internal microclimate. This ensures consistent reproducibility of the drying parameters, regardless of atmospheric fluctuations at the installation site.

Contamination protection

Since no outside air is drawn in, no new mold spores, dust, or contaminants come into contact with the sensitive material being dried.

Precision instead of hysteresis:

Thanks to constant circulation and recirculation, there is no »stagnant moisture« or local cold bridges.

Professional technology on a compact scale: The VPD Drying Box

The challenge was to scale down the industrial principle of external air treatment in a way that makes it practical and affordable for research laboratories, pharmacies, or small manufacturing facilities. To achieve this, we rely on a hybrid approach combining industrial core components with smart infrastructure.

Precision Core Components & Logic

Thermoelectricity

Instead of slow-responding compressors, we use Peltier elements. These allow for continuous regulation of the cooling surface temperature without harmful on/off cycles.

Sensors

A network of combined T/RH precision sensors (e. g., SHT45) and dedicated temperature sensors provides the data basis for VPD calculation as well as monitoring of the cold and hot sides of the Peltier.

Control

A Raspberry Pi handles the PID control loops and data logging, enabling complex, time-controlled drying profiles.

Smart Infrastructure (PC Sector)

To cool the system efficiently and quietly, we use proven high-performance components from the PC sector: A PC water-cooling system dissipates waste heat to the outside, while high-quality ball-bearing fans and a standard PC power supply ensure stable airflow and a reliable power supply.

Enclosure Design and Zones

To make efficient use of the technical components, the enclosure design follows the principle of strict zone separation. A highly insulated polystyrene box serves as a protective shell, minimizing external thermal disturbances and thereby increasing control precision inside. The center of the box is the product chamber: Here, the material rests on special mesh layers, which protect it from direct thermal influences from the actuators. Separated from this by a physical barrier, a vertical climate shaft is located in the rear section. The actual air conditioning takes place in this channel before the conditioned air is directed back to the material in a controlled manner.

The Air Cycle (Cool-Dry-Reheat)

The airflow simulates external climate control in three steps:

1. Intake: A recirculation fan draws air from the top of the product chamber into the climate shaft.
2. Condensation: The air is directed over the cooling fins of the Peltier module, cooled below the dew point, and dehumidified. The condensate is drained to the outside and collected in a drip tray.
3. Reheat: Before the air re-enters the product chamber at the bottom, a counter-heater heats it to the exact target temperature.

The result: This combination of mechanical isolation and precise PID control results in an extremely stable VPD window.

Cascaded Control Logic

The unit employs a hierarchical cascade control system in which the vapor pressure deficit (VPD) serves as the master setpoint. This controls all subordinate variables to physically prevent drift into the »mold zone« or irreversible overdrying.

The Core: The Dew Point

The Raspberry Pi continuously calculates the dew point. based on the current humidity. This is the temperature at which water vapor condenses into liquid water—the key factor in moisture removal.

Targeted Condensation

The Peltier element is controlled so that the heat sink temperature (T_KK) drops exactly below the dew point. The difference (ΔT) between the air dew point and T_KK directly determines the dehumidification rate.

Fan dynamics

The air velocity defines the heat transfer at the heat sink and regulates the thermal coupling between the Peltier element and the airflow. In the product chamber, this control mechanism creates a steady, gentle circulation that breaks up the saturated boundary layer on the biomass. This ensures that the VPD gradient is not only detected by the sensor but also penetrates deep into the tissue of the flowers without subjecting the material to mechanical stress.

Thermal Inertia as a Physical Filter

A key feature of the box is its carefully engineered hardware design: the counter-heater is mounted on a solid aluminum heat sink.

Natural low-pass filter

This thermal capacity acts as a physical filter. It smooths out high-frequency fluctuations in the PID control and ensures extremely smooth temperature control—evident in the nearly linear measurement curves of the sensors.

Reheat logic

Since dehumidification at the Peltier inevitably extracts energy from the air, the counter-heating precisely counteracts this. Through this »Cool-Dry-Reheat« process, temperature and relative humidity (and thus the VPD) remain completely decoupled and controllable.

Which VPD for how long?

Based on experience, a timeframe of 7 to 10 days represents the optimal compromise for the duration of the drying process. Within this range, the ideal VPD range is between 1.0 kPa (start) and 0.8 kPa (end).

Quality

Anything that dries faster often results in a flat aroma and a brittle structure.

Safety

Anything that takes significantly longer keeps the moist core at a dangerously high water activity (a_w) for too long, which unnecessarily increases the risk of mold.

Diffusion Control

A stable VPD ensures that moisture migrates from the interior as quickly as the surface releases it. This prevents the dreaded »case hardening« (external glazing with internal moisture).

Equilibrium Detection

As soon as the material stops releasing water into the air, the required dehumidification capacity of the Peltier module drops to near zero. This is the physical signal: the target equilibrium has been reached. The plant material can then remain in the box without overdrying, as the humidity is maintained precisely at the target value.

The »Storage-Ready« Principle

A key feature of the box is its preparation for individual long-term storage. Since every user has different conditions (e. g., a cool basement vs. a warm living room), the box allows for precise »target matching«

• No sudden changes in humidity: The user can select the drying profile so that the plant’s water content is precisely calibrated to its future storage temperature.
• Maximum stability: When transferring from the box to the storage container, there are no surprises due to »post-sweating« or sudden spikes in water activity (a_w). The material is stable and ready for consumption from the very first minute in the jar.