A unique character of MFC is that it normally comes as a water suspension and at very low concentrations, in some cases as low as 2% of active matter in water. This is a positive feature in the sense that non-dry MFC is readily activated and easy to introduce into various formulations.
However, the high water content can also cause some challenges. As we all know, water will freeze at 0°C and evaporate at 100°C at atmospheric pressure. Water is also a good medium for growth of microbes. Temperature conditions and product contamination are therefore two important points to consider in order to ensure storage stability of non-dry MFC.
Let me try to walk you through some recommendations for handling non-dry MFC.
Stability against temperature
MFC is a robust rheology modifier and performance enhancer which can be used in various chemical and physical environment. This article covers its proven PH stability. Moreover, MFC is pretty robust to temperature. However, due to the freezing and boiling of water, the temperature limits of an MFC suspension in water are above 0°C and below 100°C at atmospheric pressure. Within these temperature limits, the viscosity of MFC in water is rather constant, and MFC is stable.
On the other hand, out of this temperature range, if the water of the MFC suspension is evaporated or the suspension frozen, the three dimensional structure of the microfibrils collapse, forming very strong bonds between the fibrils. A decreased surface area and a permanent change of surface character is the result. We call it hornification and it occurs on all cellulosic surfaces.
The solvent in wich the MFC is suspended must be stored in closed containers. The temperature must be kept above the freezing point and below the evaporation point of the solvent to retain the material's performance.
In a formulated mixture of ingredients, the MFC may be much more robust to various temperatures. The other components of a formulation might interact with the surface of the fibrils, shielding them from interacting with each other chemically and avoiding hornification. Thereby allowing freeze-thaw stability of the end product containing MFC; a requirement for many consumer products.
Stability against micro-organism
If MFC is a suspension in water, it is important also to consider stability against micro-organisms since water is a great medium for growth. It is, however, not only water that allows growth; like all organisms, micro-organisms need nutrients to remain and multiply. Two exclusive sources of nutrients and contamination are the cellulose and the water used in preparing the MFC suspension. That's why the purity of these two is a key to limiting the growth of the microbes.
Contamination from the water can easily be avoided by using highly purified versions such as UV treated water. However, preventing the introduction of nutrients, i.e. different organic impurities, to the MFC suspensions can be challenging. As well as reducing growth risk, the purity of the cellulose source determines the amount of residues from other ingredients that may act as nutrients to micro-organisms. But it does not exclude the fact that growth could occur if the product is contaminated with nutrients and micro-organisms at temperatures and pH conditions that favor the growth of these unwanted organisms. It is therefore imperative to avoid contamination in a microbe-free MFC suspension and in some cases consider using a preservative system.
The two following precautions should ensure storage stability of non-dry MFC and keep the material’s performance and safety during shelf-life:
- Control storage and shipping temperatures of MFC, especially MFC suspended in water. No freezing and no drying should occur before the use of the material to avoid the collapse of the three-dimensional network of fibrils and the corresponding drop in performance.
- Start from a clean MFC and keep it contamination free during handling and packaging. Sterilized or clean processes, as well as equipment, during production, packaging and use are recommended. If the storage conditions may lead to product contamination, consider optimizing the production process such that all the MFC is used after opening the packaging. If not, add a convenient preservative that fits the application in which the MFC will be used.