# Mixed fermentation improved apple juice quality: juice characteristics and mechanism

## TL;DR

Mixed lactic acid bacteria fermentation improves apple juice flavor and sensory quality more effectively than single-strain fermentation.

## Contribution

The study introduces a mixed LAB fermentation strategy that enhances flavor and reduces bitterness and astringency in apple juice.

## Key findings

- Mixed fermentation with L. helveticus and L. plantarum at a 1:2 ratio increased viable cell counts and sensory acceptance.
- Flavor complexity improved due to higher esters, aldehydes, and alcohols, with reduced bitterness and astringency.
- Metabolomic analysis linked amino acid metabolism and flavonoid biosynthesis to flavor enhancement.

## Abstract

Fermented apple juice demonstrates significant potential as a functional beverage with health-promoting benefits; however, optimizing its sensory quality remains challenging. This study investigated the effects of fermentation using single and mixed lactic acid bacteria (LAB) strains on the quality of ‘Red Fuji’ apple juice (AJ) from Aksu, Xinjiang.

Flavor characteristics were assessed using electronic nose, electronic tongue, and HS-SPME/GC–MS, and underlying mechanisms were investigated via non-targeted metabolomics.

The results revealed that ‘Red Fuji’ AJ is an excellent fermentation substrate, with post-fermentation total colony counts consistently exceeding 8.5 log CFU/mL. Mixed fermentation outperformed single fermentation. At the L. helveticus and L. plantarum ratio of 1:2, yielding higher viable cell counts (>9.26 log CFU/mL), superior overall sensory acceptance (p < 0.05), and a more complex and pleasant flavor profile characterized by increased levels of esters, aldehydes, and alcohols, along with significantly reduced bitterness (65.3%) and astringency (74.1%). Metabolomic analysis identified key differential metabolites and metabolic pathways (amino acid metabolism, tricarboxylic acid cycle, and flavonoid biosynthesis), providing a theoretical basis for the flavor enhancement mechanism.

This study demonstrates that specific mixed LAB fermentation can effectively address common sensory acceptance issues in fermented fruit juices by simultaneously enhancing probioticviability, enriching flavor complexity, and reducing undesirable tastes.