§ 173.20 Ion-exchange membranes.
Ion-exchange membranes may be safely used in the processing of food under the following prescribed conditions:
（a） The ion-exchange membrane is prepared by subjecting a polyethylene base conforming to §177.1520 of this chapter to polymerization with styrene until the polystyrene phase of the base is not less than 16 percent nor more than 30 percent by weight. The base is then modified by reaction with chloromethyl methyl ether, and by subsequent amination with trimethylamine, dimethylamine, diethylenetriamine, or dimethylethanolamine.
（b） The ion-exchange membrane is manufactured so as to comply with the following extraction limitations when subjected to the described procedure: Separate square-foot samples of membrane weighing approximately 14 grams each are cut into small pieces and refluxed for 4 hours in 150 cubic centimeters of the following solvents: Distilled water, 5 percent acetic acid, and 50 percent alcohol. Extraction from each sample will not exceed 0.4 percent by weight of sample.
（c） The ion-exchange membrane will be used in the production of grapefruit juice to adjust the ratio of citric acid to total solids of the grapefruit juice produced.
更多关于美国 FDA 可直接加入食品中的辅助性食品添加剂的种类法规，请点击美国FDA 21 CFR 第173部分可直接加入食品中的辅助性食品添加剂种类汇总
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