Color protection and clarification technology of PVP in juice and vegetable juice processing

Natural enemies of color and stability in fruit and vegetable juices: enzymatic and non enzymatic browning

One of the biggest quality challenges faced by fruit and vegetable juices during processing and storage is color deterioration, mainly manifested as browning. This mainly stems from two processes:

1. Enzymatic browning: When fruit and vegetable tissues are broken, polyphenol oxidase (PPO) in cells reacts with phenolic substrates (such as chlorogenic acid, catechins, etc.) in vacuoles under oxygen contact to produce quinone intermediates, which further polymerize to form dark brown pigments.

2. Non enzymatic browning: mainly includes Maillard reaction (reaction between reducing sugars and amino acids) and oxidative degradation of ascorbic acid (vitamin C). The oxidation of ascorbic acid itself produces brown substances such as furfural.

These browning reactions not only darken the color and lose attractiveness of the product, but are often accompanied by flavor deterioration and nutrient loss.

Dual color protection and clarification mechanism of PVP

PVP plays a dual role as the "color guardian" and "clarification master" in juice processing.

1. Inhibition of enzymatic browning: PVP can bind to phenolic compounds in fruit juice first, forming stable PVP phenol complexes. This prevents phenolic substances from being recognized and catalyzed by polyphenol oxidase (PPO), thereby cutting off the chain of enzymatic browning at the source of the reaction. This is an effective chemical protection measure when pasteurization cannot completely eliminate enzymes or there is a risk of subsequent aerobic exposure.

2. Delaying non enzymatic browning: PVP can indirectly protect ascorbic acid by chelating metal ions (such as Cu ²⁺, Fe ²⁺/Fe ³⁺) that catalyze the oxidation of ascorbic acid in fruit juice, slowing down the browning caused by its oxidative decomposition.

3. Collaborative clarification effect: In the production of clarified fruit juices such as apple juice and grape juice, PVP is used in conjunction with pectinase, amylase, and other enzymes. Pectinase decomposition leads to cloudy pectin substances, while PVP is responsible for removing small molecule polyphenols from it. The combination of the two can produce a more thorough and stable clarification effect, and reduce the astringency of fruit juice.

Typical Applications and Process Control

Apple juice and pear juice: Adding 0.05% -0.2% PVP immediately after juicing can effectively prevent rapid browning during initial juicing, and in the subsequent clarification process, it can be combined with enzyme preparations to produce high-quality juice with excellent light yellow, clear and transparent color.

Light colored pulp juice (such as lychee and longan): These juices are rich in easily oxidizable phenolic substances, and adding PVP can effectively protect their natural light color.

Juice rich in anthocyanins (such as blueberry and mulberry juice): The application of PVP should be cautious as it also has an adsorption effect on certain anthocyanins, which may lead to color loss. But by controlling the amount and process of addition, it can be used to remove easily polymerized phenolic precursors, which in turn helps stabilize the long-term color value of anthocyanins.

Process key points: PVP is usually added after juicing and before pasteurization. Adequate stirring is required to ensure even mixing with the juice. After a certain reaction time, it should be removed together with the formed complex through methods such as diatomaceous earth filtration, plate and frame filtration, or centrifugation.