April 22, 2016
A major component of the food ingredients market involves gums. As described in Pressing News #249 Hydrocolloids, the key types are pectin, xanthan gum, and carrageenan. Alginate is a fourth key material in the gum category. All four of these are polysaccharides.
Both carrageenan and alginate are extracted from seaweed. Xanthan gum is produced in a fermentation process. And pectin is extracted from citrus peel, apple pomace, and sugar beets.
Vincent Corporation continues to gain ground in screw press applications involving these food ingredients. We have worked with pectin for over twenty years, and we are now on firm footing with xanthan gum and alginate. (Carrageenan remains a challenge.)
Our experience with alginate has been very encouraging. Key alginate producers we have interfaced with include Cargill in France, FMC in Norway, and DuPont in France. Japan and China are major sources of alginate, although we have not had contact with any of them.
To start with, here is a good description of alginate: "Alginic acid, also called algin or alginate, is an anionic polysaccharide distributed widely in the cell walls of brown algae, where through binding with water it forms a viscous gum. In extracted form it absorbs water quickly; it is capable of absorbing 200-300 times its own weight in water. Its colour ranges from white to yellowish-brown. It is sold in filamentous, granular or powdered forms."
We think of alginate as both a thickener and dispersant, an odd combination. It is used in ice cream, welding rods, cakes, and pharmaceutical applications. It keeps gas bubbles in suspension. Going into a screw press, it is like large clumps of wet cotton, although with shorter fibers.
Alginate is made from brown seaweed. Depending on the facility location, during the summer harvest seaweed may be brought in fresh. Through the winter processing facilities can run imported washed, dried, and shredded seaweed; Chile is a key exporter of this raw material. This seaweed looks much like ground corn cob, with pea sized pieces.
Pectin and xanthan gum require vapor-tight screw presses because these are produced through precipitation with aqueous alcohol. In contrast, alginate is produced in a water-based process. As with pectin and xanthan gum, there are intermediate and final pressing steps. We offer different screw press configurations for each of these applications.
In producing alginate, the seaweed is treated mechanically, thermally, and with chemistry. The mechanical part is to break through the cell wall. It can involve grinding the seaweed into an emulsion, with less than 100 micron particles. With wet processing they can start with a shredder with a 1/8" perf screen. That becomes an extrusion process, similar to using a Wyler grinder. The Urschel 1700 Comitrol will get it down to under 100 microns, with the consistency of snot.
Some chemistry and heat reduces the seaweed's alginate into a basic solution. To achieve this, the seaweed can be cooked with alkali, taking the pH of the solution up to 13.
The alginic solution is then mixed with acid, which neutralizes the solution's pH and precipitates the alginate. Chlorine and other bleaching agents are added to this flow to give the alginate a white color. Environmental considerations and food regulations are driving efforts to use less noxious forms of chlorine and fewer amounts of them.
The alginate in the flow from the bleaching tank is somewhat like mashed potatoes when it comes to dewatering, difficult. Hydrocyclones and centrifuges are seen. We, of course, prefer that screw presses be used to separate the water and bleaching agents.
A range of 15% to 25% press cake solids can be achieved in this first dewatering step. Potassium chloride, KCl, may be sprayed on the flow to act as a press aid. However, the trend is away from using potassium chloride.
The cake from first pressing is conveyed to a tank where it is washed with water. This is to remove bleaching agents, including formaldehyde. The flow then goes to a draining table, with the solids going into final pressing ahead of the dryer.
Direct fire drum dryers can be used. The final product is sold in powder form.
Using wedgewire screens in the presses, with slots which are only 250 microns wide, has proven to be a key technical improvement. Presses with this feature have reduced the loss of alginate in the press liquor.
There was good response adjustment of the discharge cone pressure. In one test, at 2 bar half a dozen drops of water could be squeezed from a fist full. But at 5 bar not even a film would form between fingers.
Our presses have worked with production rates from 50 to 600 kg/hr of dry solids. Our best results were achieved running the screws of the presses at very low rpm. For final pressing we recommend the Series VP presses because of their longer L/D (screen length divided by screw diameter) design. The lower torque Series KP presses are more suitable for intermediate pressing.
Because of the presence of chlorine, the presses are built in 316L stainless.
Producers offer a range of alginate products. These perform differently in the screw press. A thickening alginate may be specified to have 27% to 28% dry matter coming from the final pressing, while a jellifying alginate could be 21% to 25%. (The jellifying is more hydroscopic.) We have seen both varieties come out of our presses at over 30% DM.
The residual waste from producing alginate is a powdered cellulose fiber. Generally this is dewatered with a filter press before being landspread. So far we have not tried making this material run in a screw press.
There are many hydrocolloids besides the four mentioned above: Agar, Gum Arabic, Cassia Gum, CarboxyMethyl Cellulose, Gelatin, Gellan Gum, Guar Gum, Locust Bean Gum, MC/HPMC, Microcrystalline Cellulose, Starch, and Tara Gum.