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December 20, 2018
FREE vs. BOUND AND ENTRAPPED WATER
OR A POSSIBLE WAY TO DEWATER MORE
Materials have a “natural” moisture content – this is the limit below which a screw press cannot dewater. This limit varies from material to material, and there is no “magic press” which will squeeze below it. This newsletter explains the chemistry behind these limits.
Many models exist to describe water in a material, but the simplest way to visualize it is to think of water in two categories – (a) “bound water” and “entrapped water” which is held physically or chemically (or both) onto and within the material to be dewatered, and (b) “free water”, or the rest of the water. Free water is easily removed by mechanical processes, but bound and entrapped water cannot be removed without some help.
Water is polar, like a magnet. Think of a water molecule as looking like Mickey Mouse’s head, with Mickey’s chin having a slightly negative charge while his ears each having a slightly positive charge. Water molecules can loosely “stick” to one another because of this polarity (positive to negative – ear-to-chin-to-ear-to-chin etc.). They can also “stick” to plant and meat molecules because these molecules also have polarity regions.
In addition, plant and meat molecules that are “long chain” molecules can fold and self-associate. Think of an actual chain, with some links having positive and negative polarities (and charges) … and even other smaller links coming off the main chain with positive and negative polarities (and charges), too. These chains and side links can fold up, because their charges can self-attract. Water molecules can associate with them because of water’s polarity. Water molecules can also be “stuck” or entrapped within the folded molecules of plants and meats.
For the more chemically inclined – this 3D folding occurs due to the long chain molecules twisting and bending to form hydrogen bonds, salt bridges, disulfide bonds and other covalent bonds. Hydrophilic (polar) interactions with the surrounding solvent, and hydrophobic (non-polar) interactions between amino acid or polysaccharide side chains also play a part in 3D folding. Water can associate and loosely structure in layers at the hydrophilic portions of these molecules, and water can also become entrapped in macrostructures and superstructures.
Most of Vincent’s work is with plant material, which consists of polysaccharides such as cellulose, hemicelluloses, and pectins, as well as lignin and smaller amounts of proteins and enzymes. We also work with meat and fish proteins. These contain muscle fibers and connective tissue which are made primarily of polypeptides such as actin, myosin, and collagen plus fat. Both plant material and protein systems have bound, entrapped, and free water, and their total moisture contents range from about 70% to 95%.
Bound and entrapped water comes up when we look at the limits of moisture which can be removed by any press. Mechanical pressing removes free water, but bound and entrapped water will almost never be removed. For example, if you run fresh orange peel at 80% moisture or fish fillet at 86% moisture through a screw press, the resulting cake will still have 80% and 86% moisture respectively. You cannot squeeze even a drop of liquid out of either cake with your fist.
But if you add steam to these materials and heat them to 180°F or more, those plant and meat molecules change. Heat adds energy to the system, which unfolds and breaks some molecules’ water associations. System disorganization increases, macrostructures and superstructures break down, other bonds may be formed, and some molecule refolding and reorganization can occur. This causes some of the bound and entrapped water to be released, and it allows a screw press to take orange peel down to 73% moisture and fish down to 55%.
An alternative to heating a material is causing a chemical reaction to change molecules’ structures and release some of the bound and entrapped water. This is notably true with citrus waste. Mixing hydrated lime with citrus waste results in a chemical reaction which allows a screw press to reduce 80% moisture orange peel down to the same 73% moisture content that steam heating accomplishes.
The take-home lesson here is that, if you change the structure of molecules, then you change the association of the bound water to these molecules and entrapped water in the macrostructures and superstructures can also be released. These changes can make for easier mechanical dewatering.
Heat, organic solvents, acids and bases, inorganic salts, and radiation can all be used to change the structure of molecules and release bound and entrapped water. This could help the performance of a screw press. The method you use depends on the plant material or protein you have and what energy source or chemicals you choose.
Vincent accumulates performance data on the limits to which materials can be dewatered in a screw press. Since there is no charge for trials and pilot plant services, many hundreds of materials have been tested. Familiarity with this data prevents us from promising unachievable moisture removal with our screw presses.
Contributed by Elizabeth Webb
REFERENCES
Water – (https://www.jstor.org/stable/25044558?seq=1#page_scan_tab_contents)
Water Forms – (https://www.vincentcorp.com/content/four-kinds-of-water/)
Plant material – (https://www.ncbi.nlm.nih.gov/pubmed/7732730)
Meats – (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4789028/)
Issue 310