Airlift Fermenters for Mushroom Liquid Spawn Fermentation

Airlift Fermenters for Mushroom Liquid Spawn Fermentation

Airlift fermenters (also known as airlift reactors) are widely used and well-established in the production of liquid spawn for edible fungi. They are particularly suitable for strains with fragile mycelia that are sensitive to shear forces and currently represent the mainstream fermentation vessel type in the industry.

The core of gas-liquid mass transfer in these fermenters lies in using the kinetic energy of the gas to drive liquid circulation, thereby achieving efficient transfer of the gas phase (e.g., oxygen) into the liquid phase (e.g., fermentation broth). Airlift fermenters lack mechanical impellers; instead, both mass transfer and mixing are driven entirely by aeration. Unlike mechanically agitated tanks, they have no moving parts, offering significant advantages such as simple construction, low energy consumption, and minimal shear damage to biological cells.

The mass transfer process centers on the mechanism of "gas lifting – circulating mixing – interfacial mass transfer."

I. Working Principle:Density-Difference-Driven "Airlift" Circulation
Simply put, the density-difference-driven "airlift" circulation is the source of the liquid's agitation and turnover:

1. Gas Injection and Breakup:
Compressed air is injected into the liquid at high velocity through nozzles or gas distributors located at the bottom of the tank (usually in the riser section). The high-speed airflow breaks the gas into a multitude of tiny bubbles, vastly increasing the gas-liquid contact area.

2. "Airlift"-Driven Circulation:
The presence of a large volume of bubbles makes the average fluid density in the riser section significantly lower than that in the downcomer section (some fermenters incorporate a draft tube to facilitate this). The pressure gradient resulting from this density difference, combined with the upward kinetic energy of the gas, drives the fluid to form a directional, circulating flow within the reactor.

3. Contact and Reaction:
As bubbles rise and the liquid circulates, oxygen from the gas phase continuously crosses the gas-liquid interface and dissolves into the liquid phase, supplying the microorganisms for growth or facilitating chemical reactions.

4. Gas Exhaust and Liquid Return:
Upon reaching the top of the tank, the gas-liquid mixture separates; the gas (such as CO₂ produced by microbial respiration) escapes from the liquid surface. As the density of the degassed liquid increases, it flows downward outside the draft tube, returning to the bottom of the tank to re-enter the circulation loop.

II. Basic Structure (Internal Circulation Type – Standard for mushroom)

1. Tank Body
Material: 316 or 304 stainless steel; Type: Jacketed tank or serpentine coil tank.
Working volume coefficient: 80%–90%; if the height-to-diameter ratio is too low, circulation is weak; if too high, circulation resistance increases.

2. Draft Tube
(Upflow tube) Currently used in small laboratory-scale tanks; rarely seen in large-scale tanks. It is a coaxial cylinder within the tank; Draft tube diameter / Tank diameter = 0.6–0.8. The interior of the tube serves as the upflow zone, while the annular space between the tube and the tank wall serves as the downflow zone.

3. Gas Distributor / Nozzle
Located at the bottom of the draft tube; breaks air into micro-bubbles, determining bubble size and the mass transfer coefficient ($k_L a$). Distributor shapes include discs, cross-types, etc.

4. Top Gas-Liquid Separation Zone
Bubbles break to release exhaust gas, while the liquid changes direction to flow downward through the annular space.

5. Auxiliary Equipment (Inspection, Temperature Control, Process Piping, etc.)
Heat exchange via jacket or serpentine coils; pH/DO/temperature probes; CIP (Clean-in-Place) ports; sampling, discharge, and feeding ports.

III. Advantages of Airlift Fermenters

1. Simple Structure and Low Cost:
Airlift fermenters lack complex mechanical agitation mechanisms, resulting in lower manufacturing costs and simpler operation and maintenance.

2. Efficient Mass Transfer and Lower Energy Consumption:
The unique "airlift" circulation enables efficient oxygen transfer. Meanwhile, energy consumption is approximately 75% lower than that of traditional mechanically agitated fermenters, offering a significant cost advantage for large-scale production.

3. Low Shear Force; Protects Mycelium:
This is a key advantage of airlift fermenters in the edible fungi industry. Relying on airflow for agitation, they cause minimal mechanical shear damage to the mycelium, making them ideal for cultivating edible fungi varieties with fragile mycelium that are sensitive to shear forces.

4. High loading factor:
Airlift fermenters can achieve a loading factor of over 80%, resulting in high equipment utilization.

5. Low risk of contamination:
The simplification of air cooling and filtration systems effectively reduces the risk of contamination by unwanted microorganisms, thereby ensuring the quality of the culture.

IV. Gas holdup and mass transfer coefficients in airlift fermenters

1. Gas holdup (ε):
This refers to the ratio of the gas phase volume to the total reactor volume. A higher gas holdup implies a greater number of bubbles per unit volume and a larger gas-liquid contact area, which forms the basis for efficient mass transfer.

2. Volumetric mass transfer coefficient (kLa):
This is the most direct and critical parameter for measuring the mass transfer rate. It integrates the effects of the mass transfer driving force and the contact area; a higher kLa value indicates a greater amount of gas transferred per unit volume per unit time.

Mass transfer pathway analysis:
Air → gas distribution to form bubbles → upward movement of bubbles; oxygen dissolves from the gas-liquid interface into the liquid phase; circulation distributes dissolved oxygen throughout the tank; CO₂ escapes in the reverse direction from the liquid phase into the bubbles for discharge.

The volumetric oxygen transfer coefficient (kLa) is the most critical indicator for airlift fermenters and is governed by four variables: aeration rate, bubble diameter, gas holdup, and liquid circulation rate.

V. Practical process flow

First-stage culture: Inoculate the shake-flask culture into the first-stage reactor for cultivation. This step is suitable for slow-growing strains or as a pre-culture stage for very large production tanks.

Second-stage scale-up culture: Inoculate a specific amount of the first-stage culture into the second-stage reactor; continued cultivation yields a large quantity of inoculum for production.

In actual production, for most varieties, the shake-flask culture is inoculated directly into the production tank.

In summary, thanks to their significant advantages—high efficiency, low energy consumption, and low shear force—airlift fermenters have become a highly valuable core technology for the liquid spawn production of edible fungi. They not only resolve the issue of low efficiency associated with traditional solid spawn production processes but also provide a reliable technical solution for achieving large-scale, low-cost production of edible fungi.