Facts Why Walnut is a Top Sustainable Plant Protein Source

In a world increasingly turning towards plant-based nutrition, walnut protein emerges as a top sustainable plant protein source.

This versatile nut not only boasts a rich nutritional profile but also exhibits unique physicochemical properties, making it a valuable ingredient in both food sustainability and pharmacological activities.

Despite concerns about allergenicity, the functional attributes of walnut protein, including its solubility, emulsification, and oil retention capabilities, highlight its potential in diverse applications.

This article delves into the multifaceted role of walnut protein, exploring its impact on health, environment, and future food innovations.

Main Findings

• Walnut proteins have distinct physicochemical properties and functional attributes beneficial for food sustainability and human health.
• They are rich in essential amino acids and exhibit properties like solubility, emulsification, and water and oil retention.
• Challenges include allergenicity and some physicochemical limitations, but strategies for future research and processing are suggested.
• Walnut proteins and peptides show potential in pharmacological activities and food applications.

Keywords: walnut; protein; sustainability, sustainable plant protein source

Introduction to Walnut Proteins

In an era marked by population growth and climate challenges, global food systems face sustainability threats due to increased greenhouse gas emissions from food production.

To address this, the spotlight is on sustainable protein sources, and one contender is walnut protein (Juglans regia L.).

Recognized by the FDA for its health benefits, walnuts are celebrated for their nutritional value and are often incorporated into various foods ¹.

Walnuts have an oil content of 62–68%, rich in polyunsaturated fatty acids.

However, the production process generates substantial walnut meal, with over half of it being walnut protein.

Sadly, this protein is underutilized and often relegated to animal feed due to its poor solubility and functional properties ².

Yet, if we can harness walnut meal protein effectively, it could revolutionize the walnut industry and health food development ³.

This article provides a comprehensive exploration of walnut protein, emphasizing its physicochemical properties, global distribution, and nutritional attributes.

It also addresses current challenges in its application and offers strategies that may benefit emerging plant protein sources.

Walnut protein holds promise not only for health-conscious individuals but also for a sustainable food future.

Walnut Varieties Around the World

The Global Landscape of Walnut Protein: A Sustainable Plant Protein Source

In 2018, the world dedicated a vast 1,186,398 hectares to walnut cultivation, yielding over 3.6 million metric tons annually ⁴.

Half of these walnuts hail from China, followed by the USA, Iran, and Turkey ⁵.

Figure 1 offers a visual summary of key walnut-producing countries and their production volumes, influenced by factors like species, environmental conditions, and historical traditions.

Walnuts belong to the Juglandaceae family (Juglans), with over 20 species distributed across Asia, Europe, and the Americas.

Asia

China, the leading global walnut producer, cultivates over 20 walnut species.

Juglans sigillata, prominent in Yunnan Province, boasts a protein content of approximately 15%.

Shanxi, the second-largest walnut-producing province, offers Fenyang walnuts with over 57% fat and 15% protein.

Xinjiang, with its ample daylight and temperature fluctuations, nurtures Aksu walnuts containing 19.35% protein, meeting human nutritional needs.

Iran ranks third globally in walnut production, contributing 9.1% to the world’s total ⁶.

Iran also stands as the fifth-largest in terms of harvested walnut orchards, covering 4.9% of global walnut cultivation ⁷.

Modern markets are also keen on cloned varieties, such as the “Qingxiang” walnut from Nagano, Japan, with an average protein content of 23.1%.

Europe

Turkey cultivates walnuts across the country, with the Aegean region leading with 7696.3 hectares and Yalova No.

1 walnuts boasting 23% protein content ⁸.

Romania, in the decade from 2000 to 2010, saw walnuts, wild or semi-wild, account for 3% of all fruit production.

Notable varieties like Franquette, originating in France, have found their way into European and American walnut-producing regions, with economic advantages.

America

In the United States, walnut cultivation spanned 107,600 hectares in 2010, with 99% of the output concentrated in California.

Varieties like Chandler, with 37% of the walnut acreage, lead the pack in yield and protein potential ⁹.

What Makes Walnut Protein Special

Structural and Physiochemical Properties of Walnut Proteins

Composition and Molecular Structure of Walnut Protein

Walnut proteins possess distinct properties determined by their molecular structure.

These proteins can be categorized into four types: water-soluble albumin (7.54%), salt-soluble globulin (15.67%), alcohol-soluble prolamins (4.73%), and alkaline-soluble glutens (72.06%) of the total walnut protein content.

Gluten, the dominant storage protein in walnuts, encompasses gliadins and glutenins, contributing to its solubility in aqueous alcohol.

Glutenins form glutenin macropolymer through disulphide bonds, while gliadins remain monomeric, impacting gluten’s elasticity.

Additionally, albumin and globulin fractions contain multiple polypeptides, ranging from 12,000 to 85,000 kDa.

Amino acid analysis reveals balanced essential amino acid content in gluten, albumin, and globulin, meeting FAO recommendations for adults.

These proteins are also rich in Glu and Arg, hydrophilic amino acids, with uniform distribution among the fractions.

Circular binary chromatography shows that walnut protein’s secondary structure mainly comprises α-helix, β-fold, β-rotation, and random coil, with α-helix and random coil being predominant.

7S vicilin, a typical globulin, stands as one of the few walnut proteins with a crystal structure.

Walnut vicilin is unique for being a copper-binding protein.

2S albumin, without a reported crystal structure, exhibits α-helix domains linked by loops, stabilized by three disulphide bonds.

In contrast, gluten and prolamin exhibit high structural heterogeneity due to their complex composition, resulting in low solubility ^{10 11 12}.

Properties of Walnut Protein

Solubility

Protein solubility is a critical quality parameter affected by factors like solvent, temperature, and pH.

Walnut protein’s solubility is pH-dependent, with minimal solubility at pH 4 and maximal at pH above 8.

Heat sensitivity impacts solubility, peaking at 55°C and decreasing thereafter until denaturation at 67.05°C.

Surface hydrophobicity and surface charge balance influence solubility.

0.1 M NaOH enhances solubility, while 70% ethanol aqueous solution decreases protein mass.

High-speed shearing, high-intensity ultrasound, and high-pressure treatment improve solubility, with ultrasonic treatment increasing water solubility by 22%.

Chemical modification using sodium trimetaphosphate and sodium tripolyphosphate enhances solubility, and enzymatic modification alters properties and functional properties while increasing digestibility ^{13 14 15 16 17}.

Emulsification

Globulin exhibits the highest emulsification stability among walnut proteins, influenced by pH, salt ion concentration, concentration, and temperature.

High hydrostatic pressure enhances emulsion activity but decreases stability.

Ultrasound is a promising technique to prepare protein emulsions, with high-power ultrasound improving emulsifying properties.

Polyphenols, including catechin and chlorogenic acid, enhance emulsification by obstructing protein-lipid interaction, stabilizing emulsions, and acting as antioxidants ^{18 19 20}.

Water and Oil Retention

Water retention of walnut protein is influenced by factors like pH, salt concentration, temperature, and ionization.

The water-holding ability increases with pH above 4 and salt concentration.

Temperature variations also impact water retention.

Oil retention is influenced by enzymatic hydrolysis, which exposes hydrophobic amino acids, increasing oil-binding capacity.

Structural changes during processing can also affect oil-holding capacity, as demonstrated in modified walnut glutelin improving various functional properties ^{21 22 23}.

Understanding these properties and functionalities of walnut proteins is essential for utilizing them effectively in various food applications, supporting sustainable plant-based nutrition.

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Health Benefits of Walnut Protein

Walnuts have long been recognized as exceptional among functional foods due to their impressive nutritional and functional attributes, including anticancer, anti-inflammatory, and antioxidant properties.

Additionally, they offer various pharmacological benefits such as brain and cardiovascular protection and diabetes management ²⁴.

Understanding Bioactive Peptides in Walnut Protein

Walnut proteins are not only rich in essential amino acids but also house bioactive peptides that have gained attention for their health-promoting properties.

Bioactive peptides, composed of 2 to 20 amino acid residues, possess unique physiological functions and nutritional significance ²⁵.

These peptides, with their small molecular weight, can be directly absorbed by the human body without the need for digestion.

Notably, they exhibit protective effects against neuroinflammation and oxidative stress.

For instance, the tripeptide Leu-Pro-Phe (LPF) has demonstrated robust anti-neuroinflammatory effects, attributed to its high hydrophobicity, which enables it to traverse cell membranes effectively ²⁶.

Peptides rich in leucine or proline have shown anti-inflammatory activity by inhibiting specific signaling pathways.

Several studies have highlighted the ability of walnut peptides to combat oxidative stress and reduce neuroinflammation associated with conditions like neurodegenerative diseases ²⁷.

Some walnut peptides have also displayed antioxidant activity and cytoprotective effects ²⁸.

Walnut peptides have the potential to regulate lipid metabolism and even inhibit angiotensin l-converting enzyme (ACE) ²⁹.

These peptides are gaining recognition not only as nutritional supplements but also as components of clinical therapies.

Methods to Extract Peptides from Walnut Protein

To extract these valuable peptides from walnut proteins, various methods are employed.

Enzymatic hydrolysis, fermentation, and in vivo or in vitro digestion are among the primary techniques used to release peptides from proteins.

Enzymatic hydrolysis, known for its specificity, efficiency, and gentleness, is the most widely utilized method.

Enzymes like alkaline protease, papain, trypsin, and pepsin have been reported for the hydrolysis of walnut protein.

Physical methods such as ultrasound, high pressure, and microwaves are also employed to enhance hydrolysis efficiency ^{30 31}.

Walnut Protein as a Nanocarrier

Walnut protein’s unique properties make it an ideal candidate for use as a nanocarrier for various applications.

These plant-based proteins are considered safe and stable in vivo, eliminating the risks associated with animal proteins carrying pathogens ^{32 33}.

Walnut protein exhibits a natural affinity for non-polar molecules like fat and offers excellent emulsification and solubility under alkaline conditions ³⁴.

One promising application involves encapsulating lipophilic compounds, such as curcumin, to enhance their bioavailability.

In one study, walnut protein nanoparticles successfully encapsulated curcumin, improving its solubility and stability.

The protective effect of walnut protein allowed curcumin to remain intact in the stomach and release in the intestine, enhancing its bioavailability.

These nanoparticles also exhibited increased antioxidant activity due to bioactive peptides released during in vitro digestion ^{35 36}.

Microencapsulation for Food Preservation

Microencapsulation using walnut protein has significant potential in food preservation.

For example, carvacrol, a natural phenolic compound with antibacterial and antioxidant properties, was successfully encapsulated using walnut protein and gum Arabic.

This approach improved carvacrol’s thermal stability and allowed for its gradual release in the intestine, enhancing bioavailability.

Furthermore, these microcapsules displayed antibacterial activities, highlighting their potential for food preservation ³⁷.

Walnut Protein in Functional Food Products

Walnuts, often hailed for their health benefits, are emerging as a functional food that can combat inflammation when incorporated into one’s diet ³⁸.

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Traditional Walnut Milk: A Nutrient-Rich Beverage

Traditional walnut milk is a popular and nutritious beverage, boasting ample protein, vitamin B, nicotinic acid, and an appealing walnut aroma.

Factors like the water-to-oil ratio have been identified as key influencers, impacting characteristics like turbidity, size index, and opacity ³⁹.

Investigations into its physicochemical stability have examined the effects of pH, freeze-thaw cycles, and thermal sterilization.

The choice of emulsifier and the duration of thermal sterilization were found to significantly affect both the physical and oxidative stability of the beverage, possibly impacting the interfacial layer formed by walnut protein and xanthan gum ⁴⁰.

Fermentation has also gained popularity as a processing method for walnut-based beverages, enhancing both taste and nutritional attributes.

Research has explored the use of kefir grains as an inoculum, suggesting optimal conditions for sensory quality enhancement, including temperature, duration, inoculum size, and sucrose concentration ⁴¹.

To address concerns related to the safety of plant-based beverages, a strategy based on DNA barcoding has been developed to accurately identify raw ingredients in walnut milk beverages ⁴².

Walnut Protein Elevating Meat Products: A Heart-Healthy Option

Walnut protein has proven to be a versatile component, acting as an effective adhesive, filler, and flavor enhancer.

It finds wide application in meat products such as sausages and ham, preserving the products’ exquisite texture and flavor.

Studies have assessed the physicochemical and sensory characteristics of beef steak enriched with varying proportions of walnuts.

The findings indicate that walnuts can influence the formation of protein network structures, resulting in improved properties ⁴³.

Additionally, incorporating walnuts into meat products has been observed to significantly enhance frozen stability without any adverse effects ⁴⁴.

This suggests that the inclusion of nuts in meat products can offer potential heart health benefits.

Furthermore, the consumption of meat enriched with walnuts has been shown to boost the antioxidant status of individuals at risk of coronary heart disease ⁴⁵.

Consequently, walnuts emerge as a promising raw material for functional foods, providing adequate energy for overweight and obese individuals without negative impacts on body weight.

Understanding Challenges with Walnut Protein

Addressing Walnut Allergenicity

Walnuts, although celebrated for their health benefits, can pose allergenic risks, with English walnut (Juglans regia) being a common culprit ⁴⁶.

Severe allergic reactions to walnut proteins have been documented in various studies ^{47 48 49}.

English walnuts contain four major allergens, namely Jug r 1 (a 2S albumin), Jug r 2 (a 7S vicilin-like globulin), Jug r 3 (a non-specific lipid transfer protein), and Jug r 4 (an 11S legumin-like globulin) ^{50 51 52 53}.

Sordet and colleagues identified four linear IgE epitopes on Jug r 1, contributing to its allergenicity ⁵⁴.

Interestingly, despite structural similarities between Jug r 2 and peanut allergen Ara h 1, cross-reactivity between them is low ⁵⁵.

Notably, Jug r 2 maintains its allergenicity even after various food processing methods, such as irradiation, microwaving, frying, and grilling ⁵⁶.

Moist heat treatments, compared to dry heat, tend to reduce the antigenicity of walnut protein, potentially due to differing chemical reactions induced by water content ⁵⁷.

Challenges in Detecting Allergenic Proteins

Detecting allergenic walnut proteins can be challenging due to their low abundance and entwinement within food matrices.

The reduced solubility of walnut proteins further complicates detection ⁵⁸.

To ensure consumer safety and effective allergen management, the development of sensitive methods for detecting walnut allergens is imperative ⁵⁹.

Enhancing Walnut Protein’s Functional Properties

Walnut protein primarily comprises glutenin (about 70%), which has limited water solubility, hindering its application in water-based foods.

Therefore, improving the physicochemical properties of walnut protein is crucial.

Innovative Protein Extraction Techniques

Traditional alkali solution extraction, while simple, consumes substantial solvents and energy ⁶⁰.

High-intensity ultrasound (HIU) emerges as an eco-friendly alternative, boasting high extraction rates, reduced solvent use, and enhanced protein properties ^{61 62 63}.

HIU achieves this through acoustic cavitation, which disrupts protein interactions, increasing solubility ⁶⁴.

Ultrasonic treatment optimally enhances water solubility and functional properties of walnut protein ⁶⁵.

Reverse Micellar Extraction (RME)

Reverse micellar extraction (RME) stands as another promising method, facilitating simultaneous separation of oils and proteins using nanoscale reverse micelles ⁶⁶.

RME is efficient, energy-friendly, and minimizes protein denaturation, making it an eco-conscious choice ⁶⁷.

The process involves dissolving biomolecules in a reverse micelle system, followed by reverse extraction into the aqueous phase.

The Future of Walnut Proteins

Walnut proteins offer an emerging sustainable dietary protein source.

This review compared various walnut species, emphasizing protein quality based on solubility, emulsifying ability, and water and oil-holding capacities.

However, structural information on walnut proteins is limited, highlighting a need for advanced structural determination technologies and AI-driven molecular docking.

Exploring walnut protein properties under different conditions and its potential as nanocarriers for unstable functional materials was discussed.

Allergies are a concern, requiring precise identification and processing methods.

Overcoming physicochemical limitations in plant proteins, including walnuts, necessitates green, safe, and cost-effective protein extraction techniques.

Extensive research is vital for walnut’s promising protein-rich future.

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Review date not set.