Juice is extracted from fresh squeezed fruits. This liquid comes from the vacuole or cytoplasm and is rich in vitamin, sugar, protein, organic acid, mineral and polyphenol. These substances in the juice often precipitate or be oxidized during storage, making the juice turbid and losing its delicious flavor.
The precipitate and turbidity take time to form, so even the initially clear juice requires clarifying agent. Gelatin is used as a fining agent in the production of fruit wine, beer, juice and tea. It can form flocculent suspension colloid with negatively charged polyphenol and pectin. These flocculent colloids adsorb suspended matters, ions and proteins in the beverage, and sink to the bottom of container. After filtration, flavor and appearance of the beverage are improved by increased transparency, reduced precipitate and removal of bitterness. According to the beverage, gelatin (1~2g/L) can be added together with diatomaceous earth, silica gel or albumen powder to achieve a better result.
Why do fruit beverage and wine become cloudy after long-term storage?
Tiny fragments in the juice
Tiny cell fragments will be produced during juicing, and they may pass through the filter during filtration. The suspended small fragments collide with each other in the juice to form larger insoluble granules that will make juice cloudy. These granules will precipitate after a long time storage.
Metal ions
Fruit juices also usually contain metal ions, such as iron ions that come from fruit and production facilities. These divalent ions are oxidized by air to trivalent ions. They react with phosphate in the juice to get iron phosphate, which produces a white precipitate at the bottom of the container. These cations also combine with negatively charged molecules such as tannins to produce blue insoluble compounds. Organic acid tends to form salt with minerals. For example, tartaric acid, citric acid, and calcium ions will produce insoluble salt.
Pectin and starch
Negatively charged pectin adsorbs to protein and cell fragments to form colloid. Soluble starch in fruit juice cannot be separated by filtration devices. When the juice is cooled and concentrated, the starch rearranges itself into water-insoluble macromolecules owing to starch retrogradation.
Protein and polyphenol
The proteins in fruit juice are soluble in water, but these amphoteric molecules adsorb positively charged metal ions and negatively charged polyphenol and pectin; protein molecules also flocculate to form colloid due to the change of PH and attraction of functional groups. The colloid will slowly become larger and sink to the bottom after a long time. Fruit juice is also rich in polyphenol. It is a strong reductant and is easily oxidized to produce brown compound. The oxidized polyphenol gives darker color, poor transparency and a bitter taste to juice.
The reasons for precipitation vary from juice to juice and may only match a few of these factors. This is why clear juice become cloudy again after filtration. Fermented beer, wine is similar to fruit juice. They also contain a large number of yeasts whose death causes the cloudy wine and precipitate.