To provide scientific reference for the low-temperature storage of cold-pressed walnut oil（nut oil press）, this study explored the effects of temperature on the quality and key aroma components of walnut oil by simulating different low-temperature storage environments. Cold-pressed walnut oil was stored at 25. 4. -10. and -20 ℃ for 8 weeks, and changes in acid value and peroxide value during storage were monitored. In addition, electronic nose and solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) were used to analyze flavor compounds and volatile components, and key aroma components were screened based on relative odor activity values. The results showed that low-temperature storage could effectively delay the deterioration of walnut oil quality, with -20 ℃ storage demonstrating excellent performance in retaining the fresh flavor and key aroma components of walnut oil to the greatest extent, providing data support for walnut storage practices.

1. Materials and Methods

1.1 Test materials

Test sample: Cold-pressed walnut oil (freshly prepared, without added preservatives).

Main instruments: Electronic nose (for qualitative analysis of flavor compounds), Gas Chromatography-Mass Spectrometry (GC-MS) (for quantitative detection of volatile components), Solid-phase microextraction fiber head (for extraction of volatile components), Titrimeter (for determination of acid value and peroxide value).

Test reagents: hydrochloric acid, sodium thiosulfate, phenolphthalein indicator, etc. (all of which are analytically pure).

1.2 Test method

1.2.1 Sample processing

The fresh low-temperature pressed walnut oil was separately packaged in transparent sealed containers and stored in constant temperature environments at 25. 4. -10. and -20 ℃ for a storage period of 8 weeks. Samples were taken weekly to measure relevant indicators.

1.2.2 Determination of quality indicators

Acid value: Refer to GB 5009.229 - 2016 "National Food Safety Standard - Determination of Acid Value in Foods", and use titration method to measure, reflecting the degree of hydrolysis of oils and fats.

Peroxide value: Referring to GB 5009.227-2016 "National Food Safety Standard - Determination of Peroxide Value in Foods", the sodium thiosulfate titration method is used to reflect the initial degree of oil oxidation.

1.2.3 Analysis of flavor and volatile components

Electronic nose analysis: Take an appropriate amount of sample and place it in a dedicated sample vial. The electronic nose sensor array detects flavor compounds and analyzes the differences in the types of flavor compounds (such as nitrogen oxides, hydrocarbons, etc.) under different storage conditions.

SPME-GC-MS analysis: Solid-phase microextraction (SPME) fiber heads are used to adsorb volatile components in the sample. After gas chromatography separation, the types and contents of components are determined by mass spectrometry detection, and the categories and quantities of aroma components are statistically analyzed.

1.2.4 Identification of key aroma components

Key aroma components are screened by calculating the Relative Odor Activity Value (ROAV). Components with ROAV ≥ 1 are key aroma components that primarily contribute to the overall flavor, while those with ROAV between 0.1 and 1 are secondary aroma components.

II. Results and Analysis

2.1 Changes in acid value and peroxide value of walnut oil under different storage temperatures

With the extension of storage time and the increase in storage temperature, both the acid value and peroxide value of cold-pressed walnut oil showed an upward trend. However, the indicators for all temperature groups did not exceed the national standard limits (GB/T 22327-2019 "Walnut Oil").

25℃ group: The acid value and peroxide value increased rapidly, and both indicators reached their high values after 8 weeks, indicating that the hydrolysis and oxidation reactions of oils and fats were more intense under normal temperature storage.

-20℃ group: The increase rates of the two indicators were slow, and the difference from the initial value after 8 weeks was small, indicating that low temperature can significantly inhibit oil deterioration.

2.2 Analysis results of electronic nose

The detection results from the electronic nose sensor array indicate that the flavor compounds of low-temperature pressed walnut oil primarily consist of nitrogen oxides, hydrocarbons, terpenes, sulfides, alcohols, aldehydes, and ketones.

Flavor similarity: The flavor response values of walnut oil stored at -20℃ are close to those of fresh walnut oil, indicating good flavor retention. However, the flavor response values of walnut oil stored at 25℃ differ significantly from those of fresh samples, indicating noticeable flavor deterioration.

Temperature influence: As the storage temperature decreases, the variation range of the types and contents of flavor compounds diminishes, indicating a significant protective effect of low temperature on flavor stability.

2.3 Analysis of aroma component types and contents

A total of 39 aroma components were detected through SPME-GC-MS, with the specific classification as follows:

Aldehydes: 17 types, representing the aroma category with the highest content, and are important contributors to the flavor of walnut oil.

Hydrocarbons: 11 types, accounting for the second largest proportion, and having a certain auxiliary effect on flavor.

Other categories: including 4 acids, 2 alcohols, 1 ketone, and 4 other compounds, which collectively constitute the characteristic flavor of walnut oil.

2.4 Changes in key aroma components

Fresh walnut oil: A total of 17 key aroma components were detected, determining its fresh flavor characteristics.

Changes after storage: After 8 weeks of storage, there were differences in both the number and types of key aroma components among different temperature groups:

25℃ group: 12 key aroma components were detected, with 7 shared with fresh samples.

4℃ group: 13 key aroma components were detected, with 7 components shared with fresh samples.

-10 ℃ group: 12 key aroma components were detected, with 7 shared with fresh samples.

-20℃ group: 13 key aroma components were detected, with 10 components shared with fresh samples, making it the group with the most key aroma components shared with fresh samples among all temperature groups.

III. Conclusion

Through analyzing the quality and flavor of cold-pressed walnut oil stored under different low-temperature conditions, this study draws the following conclusions:

Low-temperature storage can effectively delay the hydrolysis and oxidation reactions of walnut oil, and the lower the storage temperature, the more significant the effect. The acid value and peroxide value of all temperature groups meet the national standard requirements.

Electronic nose analysis indicates that storage at -20 ℃ can largely retain the initial flavor of walnut oil, while storage at 25 ℃ has a significant impact on flavor.

Analysis of key aroma components reveals that walnut oil stored at -20℃ shares a high number of key aroma components (10 types) with fresh samples, better preserving the fresh flavor characteristics.

In summary, low temperature is the preferred condition for walnut oil storage. Storage at -20℃ can maintain the superior quality and flavor of walnut oil for 8 weeks, which is close to that of fresh walnut oil. This result can provide a scientific reference for the optimization of storage processes in walnut oil production enterprises and preservation methods for household users.

Source: "The Impact of Different Storage Temperatures on Key Aroma Components of Cold-pressed Walnut Oil"