Drying and defatting insect meals: Effects on ruminant nutrition and fermentation

Introduction: Why are insect meals gaining attention in animal nutrition?

With the rising need for sustainable protein sources, researchers are looking into alternative feed ingredients, and insect meals are showing great promise [1,2]. The black soldier fly (Hermetia illucens) and yellow mealworm (Tenebrio molitor) are particularly interesting because they have high protein and fat content, similar to traditional feeds like soybean meal and fishmeal [2,3]. However, the quality of insect-based feeds depends a lot on how they are processed, especially the drying temperature and whether they are defatted. These factors can change how well animals digest them and how they affect the animals' metabolism [4].

How drying temperature impacts full-fat insect meal quality

Studies show that drying insect meals at higher temperatures (up to 70°C) reduces moisture and keeps more fat, but it can also make the protein less soluble and increase chitin, which might affect digestion in the rumen [3,4]. On the other hand, defatting insect meals improves their digestibility in the rumen by increasing the production of beneficial fatty acids and reducing the breakdown of polyunsaturated fats, making nutrients more available [4].

Even with these findings, we still do not fully understand how drying temperature affects rumen fermentation. Also, the impact of defatting on methane production and microbial activity needs more research to make the best use of insect meals in ruminant diets. This mini review aims to (1) evaluate how low drying temperatures affect the breakdown of full-fat insect meals in the rumen and (2) understand how defatting changes fatty acid breakdown and microbial fermentation. By doing this, we hope to give farmers and feed makers useful information to ensure insect-based feeds are both nutritious and environmentally friendly.

Effect of Drying Temperature on Full-Fat Insect Meals

1. Nutritional Composition and Digestibility 

Drying temperature significantly impacts the composition of insect meals. Higher drying temperatures (e.g., 70°C) reduce moisture content more efficiently [5], but this can decrease crude protein and fat content. For example, high temperature drying (HTD) reduced crude protein by 17.85% and fat by 22.55%, while increasing carbohydrate (mainly chitin) content by 191.21% [6]. Freeze-drying (FD) and low-temperature air-drying (LTD) better preserve protein integrity but take longer [3]. Additionally, drying temperature affects protein digestibility. Higher temperatures (≥70°C) slightly reduce ammonia production in in vitro rumen studies, suggesting lower protein degradation [2]. This implies that moderate drying temperatures may enhance rumen-undegradable protein (RUP), therefore, improving post-ruminal amino acid availability.

2. Fatty Acid Stability and Lipid Oxidation 

The lipid composition of insect meals is also sensitive to temperature. Moderate drying (≤70°C) preserves polyunsaturated fatty acids (PUFAs), but excessive heat can accelerate lipid oxidation [7]. Microwave-drying (MWD) retains higher unsaturated FA levels compared to oven-drying, which may benefit animal health [7]. Lauric acid (C12:0), a beneficial medium-chain FA abundant in HI larvae, remains stable under controlled drying conditions [8].

3. Structural and Microbial Considerations 

Higher drying temperatures cause structural damage, including increased porosity and shrinkage [5]. However, they also reduce microbial load, enhancing food safety by eliminating pathogens like Salmonella and E. coli [9]. Pre-treatments such as blanching before drying could further improve microbial safety without significantly altering nutritional quality [10].

Impact of Defatting on Insect Meal Digestibility

1. Enhanced Fermentation and Gas Production

Defatting significantly increases in vitro rumen digestibility. Studies show that defatted insect meals produce more total gas, volatile fatty acids (VFAs), and methane (CH₄) compared to full-fat meals [4]. This suggests that fat inhibits microbial fermentation, and its removal enhances substrate accessibility for rumen microbes.

2. Changes in Fatty Acid Biohydrogenation

Defatting reduces PUFAs in rumen digesta while increasing saturated (SFAs) and branched-chain fatty acids (BCFAs) [4]. This shift indicates higher biohydrogenation rates, likely due to reduced lipid inhibition on microbial activity. Consequently, defatted meals may be more suitable for ruminant diets, though the increased CH₄ production warrants mitigation strategies.

3. Species-Specific Responses

 HI and TM meals respond differently to defatting due to their distinct FA profiles. HI is richer in lauric acid (C12:0), whereas TM contains more oleic (C18:1) and linoleic (C18:2) acids [4]. Both species exhibit improved digestibility upon defatting, making them viable protein sources for livestock.

Practical Implications for Farmers and Feed Producers

1. Optimal Drying Methods

• Hot-air drying (50–60°C) balances efficiency and nutrient retention [11].
• Microwave drying preserves unsaturated FAs but may require energy optimization [7].
• Freeze-drying maintains protein quality but is cost-prohibitive for large-scale use [3].

2. Defatting for Improved Digestibility

• Defatted insect meals enhance rumen fermentation but increase CH₄ emissions, necessitating dietary adjustments [4].
• Farmers should consider partial defatting to balance lipid content and digestibility.

3. Economic and Environmental Considerations

• Hybrid solar-electric drying (HSED) systems reduce energy costs and CO₂ emissions, making them economically viable for small-scale producers [12].

Conclusion 

Drying temperature and defatting greatly influence the nutritional quality and rumen digestibility of insect meals. Higher drying temperatures reduce processing time but may compromise protein and fat content. Defatting enhances digestibility but alters FA metabolism. Farmers should adopt moderate drying techniques (50–70°C) and consider defatted insect meals for ruminant diets, while monitoring methane emissions. Future research should explore cost-effective drying technologies and strategies to mitigate CH₄ production from defatted insect-based feeds.

References

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