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A Cold-Active GH42 β-Galactosidase from Klebsiella variicola for Aroma- Preserving, Energy-Efficient Processing of Reconstituted Tobacco

This study characterizes a novel cold-active GH42 β-galactosidase (β-Gal 1527) from *Klebsiella variicola* that, when applied at 10°C, efficiently hydrolyzes glycosidically bound precursors to significantly enhance the aroma profile of reconstituted tobacco while offering an energy-efficient, low-temperature processing alternative that minimizes the loss of thermolabile volatile compounds.

Original authors: Lei Xue, Julong Huang, Kaleem Imdad, Shixi Song, Ying Wang, Yonghong Wang, Caiping Chang, Duobin Mao, Shen Huan, Baoan Deng

Published 2026-10-19

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CC BY 4.0

Original authors: Lei Xue, Julong Huang, Kaleem Imdad, Shixi Song, Ying Wang, Yonghong Wang, Caiping Chang, Duobin Mao, Shen Huan, Baoan Deng

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine a world where the most delicate flavors in food and drink are trapped inside invisible cages, locked away from our senses until the right key turns. In the natural world, many of the compounds that give fruits, flowers, and even tobacco their distinctive scents exist in a bound, non-volatile form. They are attached to sugar molecules, rendering them odorless and stable. To release these hidden aromas, nature often relies on enzymes, which are biological tools that act like specialized scissors, cutting the bond between the sugar and the scent molecule. However, traditional industrial methods often use heat to speed up these processes. While heat is efficient, it is also a blunt instrument; it can cook away the very delicate, heat-sensitive scents it is meant to preserve, leaving behind a product that tastes flat or burnt. This creates a dilemma for manufacturers who want to enhance flavor without destroying it: how can they unlock these aromas without turning up the heat?

A team of researchers from China and Pakistan has found a solution by looking for a tool that works in the cold. They discovered a specific enzyme, a type of biological cutter called a beta-galactosidase, that thrives at temperatures near freezing. This enzyme was isolated from a bacterium called Klebsiella variicola, a microbe found in soil and plant environments. Unlike the enzymes used in most factories, which require temperatures around fifty degrees Celsius to function, this new discovery works best at just ten degrees Celsius. The researchers, led by Lei Xue and Baoan Deng, successfully copied the gene for this enzyme into a common laboratory bacterium, E. coli, allowing them to produce it in large quantities. They then purified the enzyme, creating a clear, potent liquid ready for testing.

The team put this cold-active enzyme through a series of rigorous tests to understand its behavior. They found that the enzyme is incredibly sensitive to temperature; it works with peak efficiency at ten degrees Celsius but loses most of its power if the temperature rises above thirty degrees. This confirms its nature as a cold-loving, heat-sensitive tool. The researchers also discovered that the enzyme's performance is heavily influenced by the minerals it encounters. While calcium and other metals tended to slow it down, a small amount of iron actually boosted its activity by nearly forty-five percent. In terms of speed and efficiency, the enzyme proved to be a capable worker, capable of processing its target molecules rapidly even at low temperatures. Its ability to function in a cool, energy-saving environment without losing its effectiveness was the first major breakthrough.

To see if this enzyme could solve the real-world problem of preserving flavor, the team applied it to tobacco, a crop where aroma is everything. They treated both raw tobacco leaves and a processed tobacco concentrate with the enzyme at ten degrees Celsius, keeping the mixture cool for several hours. They compared this to a control group treated with plain water. The results were striking. When the treated tobacco was analyzed, the researchers found that sixteen different aroma compounds had increased significantly. Some of these increases were dramatic, with certain floral and fruity scents rising by more than ninety percent. The specific compounds that grew included benzyl alcohol, which adds a floral note, and solanone, a key component of tobacco's characteristic scent. The enzyme had successfully cut the sugar bonds, releasing the trapped aromas without the need for high heat that would have destroyed them.

The impact of this discovery extends beyond just the smell of the final product. Because the process happens at ten degrees Celsius, it requires no energy to heat the reaction, and the product does not need to be cooled down afterward. The researchers calculated that for a standard industrial batch, this approach could save significant amounts of electricity and reduce carbon emissions compared to traditional hot methods. Furthermore, the sensory evaluation of the treated tobacco, conducted by a trained panel of experts, confirmed that the cold-enzyme treatment improved the overall quality, making the smoke smoother and the aroma more intense. The study suggests that this enzyme is not just a laboratory curiosity but a practical tool for the food and flavor industries, offering a way to unlock nature's hidden scents gently, efficiently, and without the waste of energy. By using a biological tool designed by nature for the cold, the researchers have provided a new path for preserving the delicate essence of our favorite products.

Technical Summary: A Cold-Active GH42 β-Galactosidase from Klebsiella variicola for Aroma-Preserving, Energy-Efficient Processing of Reconstituted Tobacco

Problem Statement
Conventional processing of reconstituted tobacco often relies on high-temperature treatments to release aroma compounds from cell-wall polysaccharides. However, these thermal conditions cause significant volatilization and degradation of heat-sensitive aroma precursors, leading to a loss of intrinsic flavor and reduced smoking quality. While cold-active enzymes offer a potential solution for energy-efficient, low-temperature biocatalysis that preserves thermolabile compounds, reports on cold-active β-galactosidases (β-Gal) from the genus Klebsiella are scarce. Furthermore, the specific applicability of such enzymes to tobacco reconstitution, particularly regarding the release of bound aroma compounds under mild conditions, remains largely unexplored.

Methodology
The study focused on the cloning, expression, purification, and characterization of a β-galactosidase gene (GenBank accession PZ320841) derived from the pectinase-producing strain Klebsiella variicola GB3.

Cloning and Expression: The 685-amino-acid gene was cloned into the E. coli BL21(DE3) expression vector pET-28a(+). Recombinant protein expression was induced at a low temperature (16 °C) to favor the production of a cold-active enzyme.
Purification: The enzyme was purified using Ni-NTA affinity chromatography followed by size-exclusion chromatography (SEC).
Characterization: The purified enzyme (designated β-Gal 1527) underwent comprehensive biochemical profiling, including:
- Structural Analysis: Phylogenetic tree construction, homology modeling based on the thermostable GH42 template Gan42B (PDB 4OIF), and AlphaFold prediction.
- Enzymatic Properties: Determination of optimal temperature and pH, thermostability, pH stability, and the effects of various metal ions.
- Kinetics: Steady-state kinetic parameters ( $K_m$ , $V_{max}$ , $k_{cat}$ , $k_{cat}/K_m$ ) were determined using o-nitrophenyl-β-D-galactopyranoside (ONPG) as a substrate at 10 °C.
Application: The enzyme was applied to Zhongyan-100 tobacco strands and reconstituted-tobacco concentrate at 10 °C. Effects were evaluated through:
- Sensory Evaluation: A trained panel assessed aroma quality, intensity, and smoothness.
- Chemical Analysis: Headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS) quantified 16 specific aroma compounds.
- Time-Course Analysis: Volatile compound classes were monitored over five days to track the evolution of aroma profiles.

Key Results

Enzyme Properties: β-Gal 1527 is a 77.5 kDa protein belonging to glycoside hydrolase family 42 (GH42), featuring conserved catalytic residues E157 and E314. Structural analysis revealed extended surface loops compared to thermophilic homologs, a feature associated with cold adaptation.
Optimal Conditions: The enzyme exhibits a sharp temperature optimum at 10 °C and retains >90% activity after 30 minutes at 0–20 °C, but loses >80% activity above 30 °C, confirming its thermolabile, cold-active nature. The optimal pH is 6.5, with a broad activity plateau between pH 6.0 and 7.0.
Metal Ion Effects: The enzyme is strongly activated by Fe²⁺ (increasing activity to ~145%) but is significantly inhibited by Ca²⁺, Mg²⁺, Mn²⁺, and Cu²⁺.
Kinetics: At 10 °C, the enzyme displays a $K_m$ of 3.05 mM, a $V_{max}$ of 222.2 µmol·min⁻¹·mg⁻¹, a $k_{cat}$ of ~287 s⁻¹, and a catalytic efficiency ( $k_{cat}/K_m$ ) of ~9.4 × 10⁴ M⁻¹·s⁻¹.
Tobacco Application:
- Sensory Improvement: Treatment of tobacco strands at 10 °C resulted in statistically significant improvements in aroma quality, intensity, smoothness, and sweetness compared to water controls.
- Aroma Release: HS-SPME-GC-MS analysis of reconstituted-tobacco concentrate showed a 16.6% to 91.0% increase in all 16 monitored aroma compounds. Notable increases included benzyl alcohol (+90.9%), 9-hydroxy-megastigma-dien-3-one (+84.8%), theaspirone (+73.1%), and solanone (+33.0%).
- Mechanism: The increases in norisoprenoids (e.g., β-damascenone, tabanone) and other volatiles suggest the enzymatic hydrolysis of glycosidically bound aroma precursors. The release of galactose and polysaccharide fragments may also contribute to Maillard reactions during subsequent processing.
Economic and Environmental Impact: The authors estimate that operating at 10 °C instead of a conventional 50 °C for a 10 m³ batch could save approximately 342 kWh of heating energy, reducing costs by ~$34 and CO₂ emissions by ~195 kg per batch.

Significance and Claims
The paper claims that β-Gal 1527 represents a promising cold-active GH42 biocatalyst specifically suited for the low-temperature processing of reconstituted tobacco. Its primary significance lies in its ability to release glycosidically bound aroma compounds at 10 °C, thereby minimizing the thermal degradation and volatilization of valuable flavor precursors that typically occurs in high-temperature processing.

The study positions this enzyme as a tool for "aroma-preserving, energy-efficient processing." By enabling effective biocatalysis at low temperatures, the enzyme offers a pathway to reduce energy consumption (by eliminating the need for heating) and lower the carbon footprint of tobacco production. The authors suggest that this approach has broader potential applications in the flavor, fragrance, and food biotechnology sectors for the gentle release of bound volatiles. The paper modestly notes that while the application experiments demonstrated clear improvements, further scale-up and mechanistic studies (such as direct measurement of released aglycones) are warranted to fully confirm causal attribution and optimize industrial implementation.

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