PHA Production: The Biological and Industrial Process

Polyhydroxyalkanoates, or PHAs, are a family of natural polyesters produced by microorganisms like bacteria and archaea. They serve as a form of stored energy and carbon, similar to how animals store fat. This class of biopolymers has gained attention because they are both biodegradable and biocompatible, meaning they can break down in natural environments and are not toxic to living tissue. These characteristics position PHAs as a potential substitute for conventional plastics derived from petroleum, which persist in the environment for centuries.

The Biological Mechanism of PHA Synthesis

The production of PHA by microorganisms is a survival strategy triggered by specific stress conditions. This process occurs when microbes are starved of an element like nitrogen or phosphorus while having access to an overabundance of a carbon source. This nutrient imbalance prevents the microbial cells from multiplying at their normal rate.

Unable to use the excess carbon for cell growth, the organism redirects its metabolic pathways. It converts the surplus carbon into long polyester chains, which are stored as small granules within the cell’s cytoplasm. The yield of these granules can be substantial, sometimes accounting for up to 80% of the organism’s total dry weight.

This process is managed by specific enzymes, with PHA synthase responsible for linking monomer units together to form the polymer chain. The microbe can later break down these stored polymers for energy, allowing it to survive periods of famine.

Carbon Sources and Feedstocks

The type of carbon provided to the microorganisms influences the PHA production process and the resulting polymer. Initially, production focused on pure feedstocks like sugars (glucose and sucrose) or vegetable oils. While these substrates are efficient, their cost is a significant barrier to making PHAs economically competitive, as they often compete with food production.

To address costs and improve sustainability, the focus has shifted to second-generation feedstocks. These are often waste products from other industries, turning a disposal problem into a resource. Examples include agricultural residues like sugarcane bagasse, molasses, cheese whey, and food waste from processing plants. Using these materials can lower the raw material cost, which can account for up to 50% of the total production expense.

Some specialized microorganisms can also use gaseous carbon sources. Certain bacteria synthesize PHAs using methane, a potent greenhouse gas, as their food. Others, like cyanobacteria, use carbon dioxide from the atmosphere through photosynthesis. This approach utilizes a waste stream and offers a pathway for carbon capture.

Industrial Scale Production Process

Industrial PHA production is a multi-step process centered on fermentation in large, sterilized steel containers called bioreactors. The process is managed as a two-stage batch. In the first stage, microorganisms are given a balanced diet of nutrients to encourage rapid cell growth and maximize the microbial population.

Once a high concentration of cells is achieved, the second stage begins. The feed is switched to a medium that is rich in carbon but limited in a nutrient like nitrogen. This change induces the metabolic shift, causing the bacteria to stop dividing and start accumulating PHA granules inside their cells.

Following fermentation, the downstream recovery process begins. First, microbial cells are harvested from the fermentation broth through centrifugation. The next step is to break open the cells to release the polymer granules, which is accomplished through high-pressure homogenization, enzymatic treatments, or chemical methods.

After cell disruption, the PHA is separated from cellular debris like proteins and lipids. This is done using solvent extraction, where a solvent dissolves the PHA but not other components. The PHA is then precipitated out of the solvent, and the final step involves washing and drying the recovered PHA into a pure resin or powder.

Controlling Final Polymer Properties

The material properties of the final PHA plastic can be tailored by manipulating the biological and chemical inputs. The PHA family is vast, with over 150 different monomer units identified, allowing for polymers with a wide spectrum of characteristics, from rigid to flexible. This versatility is a direct result of the specific microbial strain used and its diet.

The choice of carbon feedstock is a primary way to control these properties. The simplest form of PHA is poly(3-hydroxybutyrate) (PHB), which is produced when microbes are fed glucose. While strong, PHB can also be quite stiff. By introducing additional carbon sources, known as co-substrates, producers can prompt bacteria to create copolymers with altered characteristics.

For instance, feeding bacteria a substrate like propionic acid alongside the main carbon source can lead to the creation of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). The inclusion of these hydroxyvalerate (HV) units disrupts the polymer’s crystalline structure, making the resulting plastic less stiff and more ductile than pure PHB. This ability to fine-tune properties allows manufacturers to design specific PHAs for applications ranging from medical implants to food packaging.