Low-Field Nuclear Magnetic Resonance (LF-NMR) Reveals the Water Migration Code for “Sterilization without Texture Damage” in Avocado Puree

01 Research Background

• The Sterilization Challenge of High-End Fruit and Vegetable Purees

Avocado, with its creamy texture and rich unsaturated fatty acids, is rapidly transforming from a “niche fruit” into a global favorite for healthy eating. However, the industrial production of avocado puree faces a core dilemma: without sterilization, safety cannot be guaranteed; with sterilization, quality is often compromised. Although traditional thermal sterilization (pasteurization) effectively inactivates microorganisms, it can cause cell wall rupture, accelerate lipid oxidation, and lead to the loss of volatile aroma compounds, ultimately depriving the product of its “freshness”. Microwave sterilization heats faster, but its uniformity is difficult to control. In recent years, non‑thermal sterilization technologies such as high‑pressure processing (HPP), gamma irradiation (GI), and electron beam irradiation (EBI) have attracted considerable attention. However, how these techniques affect the microstructure, water distribution, and flavor release in the special high‑fat, high‑moisture, high‑fiber matrix of avocado has long remained unclear.

A research team from Yunnan Agricultural University systematically compared the effects of five sterilization methods (pasteurization P, microwave sterilization MS, high‑pressure processing HPP, gamma irradiation GI, and electron beam irradiation EBI) on the overall quality of avocado puree. In this “competition of sterilization technologies”, low‑field nuclear magnetic resonance (LF-NMR) technology served as the “key referee” to reveal water migration patterns and explain texture change mechanisms.

02 Key Findings

• Sterilization Method Determines the “Fate Watershed” of Avocado Puree

The team systematically evaluated six groups of samples (including an untreated control) across physicochemical indices, nutrient retention, microstructure, texture and rheology, water distribution, and volatile flavor. The key findings are as follows:

2.1 Nutrient Retention: HPP Wins with 93.82% Vitamin C Retention

Left: Bar chart of vitamin C content; Right: Free polyphenol content.

• Non‑thermal sterilization (HPP, EBI, GI) showed a significant advantage in retaining heat‑sensitive vitamin C. The vitamin C content after HPP treatment was almost the same as that of fresh samples.
• Thermal sterilization (P, MS), although causing substantial loss of vitamin C, unexpectedly promoted the release of bound polyphenols, nearly doubling the free polyphenol content – a “by-product”of cell wall disruption caused by high temperatures.

2.2 Water Migration: The “Hidden Code” Revealed by LF-NMR

This is the most technically distinctive part of the study. Using a Niumag NMI20-060H-I low-field nuclear magnetic resonance (LF-NMR) analyzer, the team measured the transverse relaxation time (T₂) and classified the water in avocado puree into three states:

• T₂₁(0.1–10 ms): Bound water – tightly bound to proteins and polysaccharides
• T₂₂(10–100 ms): Immobilized water – trapped in the micro‑regions of the gel network
• T₂₃(100–1000 ms): Free water – movable water

Key findings:

• Thermal sterilization (P, MS) caused a significant increase in theT₂₂peak area and a decrease in the T₂₃ peak area. This indicates that after cell rupture caused by high temperature, the released water is “re‑captured” by exposed polysaccharides and denatured proteins, transforming into immobilized water. This “passive water‑locking” reduces free water, but is accompanied by structure collapse and harder texture.
• The T₂ distribution of HPP and GI treatments was the closest to that of the fresh control, indicating that these methods retain the original water distribution to the greatest extent while achieving sterilization —which is the fundamental reason why their texture and mouthfeel are much closer to fresh avocado.

03 Innovation and Value

• Establishing a “Gold Standard” for High‑End Fruit and Vegetable Processing

1. First systematic “five‑dimensional comparison”: The study integrates five dimensions – physicochemical properties, nutrition, microstructure, water distribution, and flavor – providing the most comprehensive evaluation system to date for avocado puree processing.
2. Establishment of a “structure‑water‑texture” structure‑property relationship: For the first time, LF-NMR reveals the mechanism linking water migration and texture changes under different sterilization methods. HPP maintains the original water distribution by minimizing structural disturbance, whereas thermal sterilization passively locks water through a “damage‑recombination” pathway – the two mechanisms are fundamentally different.
3. Clear preferred processing routes:

• Pursuing “freshness”: HPP is the best choice, offering the highest vitamin C retention and a flavor closest to fresh avocado.
• Pursuing “enhanced aroma”: MS and EBI can selectively enhance fruity and floral notes, suitable for developing flavor‑fortified products.
• Pursuing “polyphenol enrichment”: Thermal sterilization (P) unexpectedly becomes a “sharp tool” for increasing free polyphenols.

04 Magnetic Resonance Solution

• Discerning Water Phase States, Defining Texture Quality

As one of the key technological supports of this study, the Niumag NMI20-060H-I low‑field nuclear magnetic resonance (LF-NMR) analyzer played an irreplaceable role in revealing the quality change mechanism of avocado puree.

4.1 Why is LF-NMR an “Essential” Tool for Texture Evaluation of Fruit and Vegetable Purees?

Fruit and vegetable puree is a typical high‑moisture multiphase dispersed system. Its mouthfeel (smooth/rough, thin/thick) is essentially determined by the interaction between water and the solid matrix (cell wall fragments, proteins, lipid droplets). Traditional texture analyzers can only measure “hardness” or “elasticity”, but cannot answer “why is it hard?” or “why does water separate?”

By detecting the T₂ transverse relaxation time of hydrogen protons, LF-NMR can non‑destructively and rapidly resolve:

• Where the water is (bound / trapped / free)
• What the water interacts with (interaction intensity with polysaccharides and proteins)
• How the water migrates (dynamic changes during processing or storage)

4.2 Core Contributions of LF-NMR in this Study

Conclusion: LF-NMR proves with data that the best sterilization is one where the water “does not feel” that sterilization has occurred. HPP wins precisely because it causes the least disturbance to water distribution.

Recommended Equipment：

NMI20-060H-I

Reference

Cunchao Zhao, et al. Comparative study of thermal and non‑thermal sterilization on the physicochemical properties, microstructure, texture and flavor quality of avocado puree[J]. Food Chemistry: X (Elsevier), 2026, Volume 26, Article 100254. DOI: 10.1016/j.fochx.2026.100254.