The aim of the study was to optimize the conditions [inoculum size (4, 6, and 8%), fermentation temperature (31, 34, and 37 °C), and apple: tomato ratio (2:1, 1:1, and 1:2)] on the viable cell count and sensory evaluation in apple–tomato pulp by response surface methodology (RSM), and determine the physicochemical properties, antioxidant activity, and sensory properties during fermentation. The optimal treatment parameters obtained were an inoculum size of 6.5%, a temperature of 34.5 °C, and an apple: tomato ratio of 1:1. After fermentation, the viable cell count reached 9.02 lg(CFU/mL), and the sensory evaluation score was 32.50. During the fermentation period, the pH value, total sugar, and reducing sugar decreased by 16.67%, 17.15%, and 36.05%, respectively. However, the total titratable acid (TTA), viable cell count, total phenol content (TPC), and total flavone content (TFC) increased significantly by 13.64%, 9.04%, 21.28%, and 22.22%, respectively. The antioxidant activity [2,2-diphenyl-1-picrylhydrazyl (DPPH) free-radical scavenging ability, 2,2′-azino-di(2-ethyl-benzthiazoline-sulfonic acid-6) ammonium salt (ABTS) free-radical scavenging ability, and ferric-reducing antioxidant capacity power (FRAP)] also increased by 40.91%, 22.60%, and 3.65%, respectively, during fermentation. A total of 55 volatile flavour compounds were detected using HS-SPME-GC–MS among the uninoculated samples and fermented samples before and after fermentation. The results showed that fermentation increased the types and total amount of volatile components in apple–tomato pulp, and eight new alcohols and seven new esters were formed. Alcohols, esters, and acids were the main volatile components in apple–tomato pulp, accounting for 57.39%, 10.27%, and 7.40% of the total volatile substances, respectively.

The effects of different fermentation methods utilizing Torulaspora delbrueckii 1004 and Saccharomyces cerevisiae 32169 on the physicochemical properties, organic acid content, polyphenol and flavonoid concentrations, antioxidant activity, and volatile aroma compounds of Huaniu apple cider were investigated in this study. Employing methods of single inoculation, co-inoculation, and sequential inoculation, it was found that sequential fermentation exhibited strong fermentative power in the initial stages, effectively reducing the content of soluble solids and achieving a balanced composition of malic, succinic, and citric acids while maintaining a lower titratable acidity. Sequential inoculation was observed to significantly enhance the total polyphenols and flavonoids, as well as the antioxidant capacity (p < 0.05). Specifically, in the synthesis of volatile aroma compounds, sequential inoculation significantly enhanced the richness and diversity of the Huaniu apple cider’s aromas, particularly in terms of the concentration of ester compounds (p < 0.05). Principal component analysis further confirmed the superiority of sequential inoculation in terms of aroma component diversity and richness. The findings of this study suggest that sequential inoculation of fermentation with non-Saccharomyces and S. cerevisiae is an effective strategy for optimizing the flavor characteristics of Huaniu apple cider, offering valuable theoretical support and practical guidance for enhancing cider quality and fostering the development of new products.

The fermentation process and composition of volatile compounds play a crucial role in the production of Huaniu apple cider. This study aimed to optimize the fermentation conditions of Huaniu apple cider and quantify its volatile compounds using headspace solid-phase microextraction-gas chromatography–mass spectrometry (HS-SPME-GC/MS). The optimal fermentation parameters were determined using response surface methodology (RSM). The optimal fermentation temperature was 25.48 °C, initial soluble solids were 18.90 degrees Brix, inoculation amount was 8.23%, and initial pH was 3.93. The fermentation rate was determined to be 3.0, and the predicted value from the verification test was 3.014. This finding demonstrated the excellent predictability of a RSM-optimized fermentation test for Huaniu apple cider, indicating the reliability of the process conditions. Moreover, the analysis of volatile compounds in the optimized Huaniu cider identified 72 different ingredients, including 41 esters, 16 alcohols, 6 acids, and 9 other substances. Notably, the esters exhibited high levels of ethyl acetate, ethyl octanoate, and ethyl capricate. Similarly, the alcohols demonstrated higher levels of 3-methyl-1-butanol, phenethylethanol, and 2-methyl-1-propanol, while the acids displayed increased concentrations of acetic acid, caproic acid, and caprylic acid. This study provides the essential technical parameters required for the preparation of Huaniu apple cider while also serving as a valuable reference for investigating its distinct flavor profile.

High-acidity fruit wines, such as sea buckthorn wine, are valued for their nutritional benefits but often suffer from excessive tartness and limited aroma complexity, which restrict their consumer acceptance. The application of non-Saccharomyces yeasts with acid tolerance and flavor-enhancing potential offers a promising strategy to address these challenges. In this study, a highly acid-tolerant yeast strain, Pichia kudriavzevii GAAS-JG-1, was isolated from a naturally fermented apricot system and systematically characterized in terms of its taxonomy, physiological properties, and fermentation potential. The experimental results demonstrated that Pichia kudriavzevii GAAS-JG-1 maintained robust growth activity (OD₆₀₀ = 1.18 ± 0.09) even under extremely acidic conditions (pH 2.0). Furthermore, the strain exhibited a strong tolerance to high ethanol concentrations (16%), elevated sugar levels (350 g/L), and substantial sulfur dioxide exposure (500 mg/L). Optimal growth was observed at 35 °C (OD₆₀₀ = 2.21 ± 0.02). When co-fermented with Saccharomyces cerevisiae in sea buckthorn wine, the ethyl acetate content increased significantly from 303.71 μg/L to 4453.12 μg/L, while the ethyl propionate levels rose from 5.18 μg/L to 87.75 μg/L. Notably, Pichia kudriavzevii GAAS-JG-1 also produced novel flavor compounds such as methyl acetate and ethyl 3-methylthiopropionate, which were absent in the single-strain fermentation. These findings highlight the potential of Pichia kudriavzevii GAAS-JG-1 as a valuable non-Saccharomyces yeast resource with promising applications in the fermentation of high-acidity specialty fruit wines.