A meta-analysis: Protein level of the substrate affect nutrient, macromineral, and amino acid profiles of maggot

Protein content of the substrate affects the nutritional composition of maggot (Hermetia illucens). This study aimed to summarize and confirm a wide range of findings about the effect of substrate protein on the nutritional composition, macrominerals, and amino acids of maggot. This meta-analysis data was acquired from papers indexed by Scopus throughout the past decade. The substrate’s protein concentration was used to define the fixed factor, whereas several studies were incorporated as the random factor. The selection and compilation of data followed the PRISMA-P. The high protein content of the substrate resulted in a significant (p<0.05) increase in dry matter and a decrease in neutral detergent fiber, but had no effect on macrominerals. The predominant of characteristics of essential and non-essential amino acids increase significantly (p<0.05) whenever protein quantities are added to the substrate. This finding implies that the protein content of the substrate had improvement on the nutrient composition (DM and NDF) and amino acid profiles (alanine, aspartic, arginine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine) of the maggot.


Introduction
The consumption of food protein from animal products in 2035 in Indonesia is predicted to increase by 41% from 2019 if the projected population increases to 304 million people, accompanied by an increase in protein consumption demands [1].One type of meat is poultry or white meat.This type of meat is classified as cheap when compared to red meat (cow meat).Nevertheless, the consumption of poultry meat in Indonesia is only 9 kg per capita compared to Malaysia which reaches 36 kg per capita [2].One factor that leads to this phenomenon is the price of poultry meat in Indonesia which is more expensive than in Malaysia [3].It is presumably caused by feed price because the feed price itself covers 60%-80% of total production cost [4,5].
On the other hand, livestock production in over the world is increasingly affected by external factors.These include surging demands for products of animals and effort to meet the demand for feed raw materials needed that result from the competition for natural resources [6].The main environmental issues associated with livestock production are water, soil, climate change, air pollution, water use, and biodiversity.Nevertheless, the issue related to biodiversity has become one of the major factors required to be solved [6].A study carried out by Dunn et al. revealed that shifts in livestock production were blamed as the major impact on biodiversity losses in rangeland ecosystems.The intensity of grazing is not mainly practiced to meet livestock needs.Yet, the transition into the crop-land ecosystem also led to the decline of biodiversity [7,8].Hence, it is important to explore other alternative feeds that are economic and environmentally friendly.
One important factor in rearing black soldier flies is the substrates.In order to support government regulation and the spirit of zero waste, most maggot breeders used a variety of organic wastes as primary substrates, such as chicken manure, household wastes, and tofu pulp [20,21].Nonetheless, the waste used as maggot feed has protein variations that might effect on reared-maggot`s protein content.Furthermore, it is crucial to determine the protein levels of the substrate in maggot production.Hence, to estimate this problem, the study metadata using meta-analysis is critically required to quantify the effect of the protein level of the substrate on the nutrient, macromineral, and amino acid composition of maggots so that the objective of this study was to determine the protein composition of maggot substrate.

Experimental details
The keywords used to search the data were maggot, crude protein, and substrate.The search was carried out in June 2022, the research time limitation of the article was the last ten years, and it was conducted using the Science Direct search engine.The total number of articles from the search results is 86.The selection criteria for selecting relevant articles are having a digital object identifier (DOI), reporting the amount of protein from the substrate, and the type of substrate (Preferred Reporting Items for Systematic Review and Meta-analysis Protocols or PRISMA-P) [22,23].Collected 55 rows of data and 29 columns of observational variables obtained from 10 scientific articles (Table 1).Parameters observed were (i) nutrient composition (dry matter, ash, organic matter (OM), crude protein (CP), ether extract (EE), neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), and chitin), (ii) macrominerals (calcium and phosphorus), (iii) essential amino acids (arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine), and (iv) non-essential amino acids (alanine, aspartic, cysteine, glutamine, glycine, proline, serine, and tyrosine).All input and observed variables have uniform units, i.e., % on a dry matter basis.Study differences were grouped as random effects, and protein levels in the substrate were grouped as fixed effects.The size of the test statistic used p-values and root mean square errors (RMSE).It is significant if the p-value is less than or equal to (0.05).An RMSE value close to zero is a good model.The R software version 4.2.0, with the addition of the nlme package, was used for data analysis [32][33][34].This meta-analysis uses a linear mixed model with the following equation [35,36].

Result and discussion
Protein from the substrate significantly affected the nutrient composition, and essential and non-essential amino acids of maggots, as summarized in Table 2.The high protein level in the substrate caused an increase (P=0.019) in content of dry matter maggots, with an increase of 0.308% for every 1% increase in protein.The NDF content of maggot decreased (P=0.044) by 0.798% when the substrate protein level increased by 1% on a dry matter basis.Macrominerals were not observed to have a causal relationship with the protein level of the substrate.The protein substrate significantly increased histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, and tryptophan (P<0.05).The relationship between protein substrate levels and non-essential amino acids such as alanine, aspartic, cysteine, glutamine, glycine, proline, serine, and tyrosine was significant (P<0.05).
Substrate protein does not have a close relationship with maggot protein content, but variations in maggot amino acids does.It is suspected that this is caused by the level and type of substrate protein, which mostly comes from food industry waste with relatively high crude fiber content.Maggot amino acid synthesis is influenced by the substrate's protein level and the type of substrate related to fiber content or amino acid balance.In addition, the presence of ligands or minerals that make up the side groups of amino acids also has a significant role [37,38].
Table 2.The linear mixed model equation for the protein levels of the substrate (% of dry matter basis) on nutrient composition, macromineral, and amino acid from maggot.Acid detergent fiber (ADF), lignin acid detergent (ADL), neutral detergent fiber (NDF), average residual error (RMSE), and standard error (SE).No.
Response The synthesis of methionine and lysine was influenced by the availability of sulfur in the substrate; based on the results of this meta-analysis, it was stated that the two amino acids were significantly increased.It is suspected that the availability of sulfur in the substrate from which this organic waste originates has sufficient sulfur for synthesizing the two types of amino acids [37].If the sulfur in the organic waste is immobilized, then the second assumption is the presence of degrading microbes in symbiosis with maggots [39].On the other hand, it is important to remember that other factors such as temperature, humidity, and the availability of other nutrients can also affect the nutrient composition of maggots.Therefore, the source of protein substrate may be an important factor, but it is not the sole determinant that influences the nutrient composition of maggots.

Summary
The nutrient composition of maggots is strongly influenced by the protein level of the substrate, especially the composition of essential amino acids such as methionine and cysteine.The solubility and degradation of protein from the substrate can be affected by other nutrients, including components such as chitin and crude fiber (ADF, ADL, and NDF).However, the type of amino acid substrate on the nutrient composition of maggots cannot be estimated due to limited data; therefore, this gap can be used as a foundation for future research.

Table 1 .
Literature sources were used in a meta-analysis.1)Age of harvested maggot (days) and2)crude protein level of the substrate (% of dry matter basis).