Evaluation of Nutritional Content of the Larvae of Tenebrio Molitor, and Formulation of Broiler Stockfeed

INTRODUCTION

Edible insects have been claimed to serve as a source of protein as they have been reported as effective feed converters and have a high nutritional value [1-4]. According to reports, the caloric value of insects is 50 % greater than soybeans, 87 % greater than corn, and 63 % greater than beef [5]. Among the edible insects, the larvae of Tenebrio molitor (yellow mealworm) have been reported as one of the most used insects as a protein source for both humans and animals [6]. The larvae are most famous for being used as bait for fish and as food for fish, amphibians, reptiles, turtles, birds, crows, and some small animals kept as pets [7]. In some countries, Tenebrio molitor larvae are being produced industrially as feed and feed supplements for pets, zoo animals, and even livestock [8]. They are raised in vast numbers and are referred to as the best source of animal protein. Numerous scientists demonstrated that freshly produced yellow mealworm larvae have a 15 % fat and 20 % protein content. It has also been reported that mealworms do not only consume existing food supplies more effectively than other animals, but they can also quickly transform low-nutrient by-products of popular crops like maize, wheat, millet, and peanuts into high-quality food [9-11].

Some studies have also been conducted on the nutritional makeup of mealworms but it has been reported that values vary by species and developmental [12-15]. The yellow mealworm can be fed to animals in its live form, but more commonly they are dried and may be ground into powder. Currently, the production of soya bean (a major source of protein in Zimbabwe), has been reported to have declined by 46 % due to the harsh economic conditions. Therefore, the decrease in productivity means that the gap has to be met so researchers have decided to look for other sources of protein, particularly the larvae of Tenebrio molitor. Mealworm breeding has been reported to be cheap and fast from an economical point of view as its life cycle is very short. Thus, this study aimed to determine the nutritional value of Tenebrio molitor mealworms reared in Zimbabwe for the first time and to investigate whether they can serve as a sustainable alternative source of protein for livestock in Zimbabwe. In this particular study, feeding trials were carried out on broiler chickens using the Tenebrio molitor stock feed that was formulated in accordance with the standard nutritional quantities of commercial broiler stockfeed.

MATERIALS AND METHODS

The yellow mealworms were reared in plastic containers (35 × 25 × 20 cm) at 28 ±1 °C, on the feed of wheat bran with the addition of carrots as a source of water. The nutritional evaluations of these mealworms were carried out at 10 weeks of age. Approximately 1000 larvae were used for nutrient analysis. The mealworms were fasted for three days before the analysis. After 3 days of fasting, the mealworms were washed 3 times with tap water. The mealworms were blanched in boiling water for 3 minutes. Blanched mealworms were kept in a colander for cooling and removing excess water. After 30 min, the remaining water was removed with a paper towel. The mealworms were then dried using a hot-air dry oven at 60 °C for 12 hours. Dried mealworms were pulverized with a blender (EM-BL-1056-2IN1- Electro Master) until the resulting powder could pass a 30-mesh sieve (535 µm). The mealworm powder was then weighed and stored at room temperature.

Nutritional analysis of the mealworm powder

The methods used in the determination of moisture content, crude fat, crude protein, ash content, mineral analysis, crude fiber, and carbohydrates were based on Standard methods by the Association of Analytical Chemists (AOAC). All of the analyses were done in duplicate for all the nutrients under investigation.

Determination of moisture content

The moisture content was determined by drying the wet sample of Tenebrio molitor to a constant weight in an air-circulating oven at 60 °C to ascertain the moisture content of the samples.

Determination crude fat

Extraction of crude fat was carried out using the Soxhlet method. The glassware was rinsed with petroleum spirit and dried in an oven at 102 ºC for 30 minutes. The Soxhlet extraction unit was assembled over an electric heating mantle. A total of 5 g of dried samples of mealworm powder (from moisture determination) were put into thimbles and these thimbles were transferred into the Soxhlet extraction unit. Cotton wool was plugged into the top of each thimble to prevent the sample from spilling. The extraction was done for 6 hours in a fume hood. The extraction unit was removed from the heat source and the extractor and condenser were detached. The flasks were removed from the heat source and the solvent was evaporated off on a rotavapor. The flasks and their contents were placed in an oven at 102 ºC for approximately 1-2 hours until a constant weight was reached.

Determination crude protein

The defatted mealworm powder was used for protein determination. A total of 1.0 g of mealworm powdered sample, 5 g of Kjeldahl catalyst, and 25 ml of concentrated H₂SO₄ were added into the digestion flask. The blank was prepared by adding 5 g of Kjeldahl catalyst and 25 ml of sulphuric acid but no sample was added. The flasks were placed in an inclined position and gently heated in a fume hood until unit frothing ceased. The heating continued until the solution cleared. The flasks were connected and then immediately started to digest the bulb on the condenser, with the tip of the condenser immersed in boric acid. Approximately 5-7 drops of mix indicator (100 ml of 0.1 % methyl red mixed in 95 % ethanol with 200 ml of 0.2 % bromocresol green in 95 % ethanol) were added to a receiver. The receiver was removed as well as the tip of the condenser. Then the solution was titrated with 0.1N hydrochloric acid.

Determination of crude fiber

An amount of 2 g of the mealworm powder was added to 1L flasks. Boiling diluted Sulphuric acid was added to the flask. Each flask was connected to water following the heating of the contents to boiling for 30 minutes. The sample was then filtered through a Whatman No: 54-grade filter paper held in a funnel. The insoluble matter was washed with boiling water until the washings were free from acid. The residue on the filter paper was put back in the original flask using a wash bottle containing boiling sodium hydroxide solution. Immediately each flask was connected to the reflux condenser and boiled for exactly 30 minutes. The flasks were removed and allowed to stand for 1 minute. The sample was then filtered under a suction pump through a Whatman No: 54-grade filter paper. The whole of the insoluble material from the filter paper was transferred using boiling water. It was washed with boiling water and then with hydrochloric acid and finally with boiling water. The insoluble material was washed twice with ethanol and three times with diethyl ether. The insoluble matter was transferred to the porcelain crucible using a wash bottle containing boiling water and evaporated in a water bath. The crucible and the residue were ovens dried at 105 ºC to constant weight for at least an hour. The contents were cooled in a desiccator to room temperature and weighed. The residue was incinerated in the electric muffle furnace at 500 ºC until all the carbonaceous matter was burnt The crucible was allowed to cool to 200 ºC then samples were transferred to a desiccator using tongs for further cooling, followed by weighing.

Determination of ash content

Mealworm powder amounting to 5 g was weighed into the crucibles. The crucibles were heated over a low Bunsen flame with the lid half covered in a fume hood. The crucibles were placed in the furnace at 550 ºC overnight to ensure that impurities on the surface of the crucibles were burnt off. The crucibles were cooled in the desiccator for 2 hours, followed by weighing the crucibles and lids.

Determination of minerals

The metals were measured using atomic absorption spectrophotometry (AAS). The sample was ground into powder using a blender. Concentrated HNO₃ was added to the homogenized samples and left to digest overnight. The samples were heated in a sand bath until a clear solution was obtained. They were cooled to room temperature and filtered. Each sample was transferred to a volumetric flask and diluted to the mark with distilled water. Standard stock solutions with concentrations of 1000 mg/L of Cu (NO₂)₄, Mg(NO₂)₄, Na(NO)₂and KNO₂ were used. Working standards for the metals were prepared by serial dilutions of the standard solutions. Concentrations were determined through linear calibration obtained from absorbance measurements of, at least, five different concentrations of the working standard solutions. Phosphorus was determined using molecular absorption spectrophotometry (UV/VIS).

Determination carbohydrates content

Carbohydrates were determined using the difference method. The percentages of all the other nutrients obtained in the investigation were added and this total was subtracted from 100 %.

Formulation of stockfeed

The Tenebrio molitor stock feed was formulated according to the following formula using the feed calculator application in accordance with the standard nutritional quantities of commercial broiler stockfeed. The starter broiler feed contained 59 % carbohydrates (ground maize), 20 % proteins (ground T. molitor powder), 5 % Fat (fish meal), 15.6 % minerals (premix), and 0.4 % salt. The grower's broiler feed contained 61 % carbohydrates (ground maize), 18 % proteins (ground T. molitor powder), 5 % Fat (fish meal), 15.6 % minerals (premix), and 0.4 % salt. The finisher broiler feed constituted 63 % carbohydrates (ground maize), 16 % proteins (ground T. molitor powder), 5 % Fat (fish meal), 15.6 % minerals (premix), and 0.4 % salt.

Broiler feeding trial

 Fifteen, one-day-old chicks were purchased at a local poultry store. Upon arrival, the chicks were randomly divided into 3 groups and were used in the feeding trial. Each group consisted of 5 chickens. The control group (A) was fed with a basal diet of commercially available broiler stock feed. Group (B) was fed with the known brand of stock feed, supplemented with 3 % dried Tenebrio molitor mealworms, and group (C) was fed with the formulated Tenebrio molitor stock feed. Each group was fed on 2 kg of starter feed for the first 2 weeks, 5 kg of growers feed throughout the third and fourth weeks, and 10 kg of broiler finisher for the last 2 weeks. All the rearing conditions, besides feed, were kept constant. The conditions kept constant included ventilation, lighting, feeding times, amount of feed, amount of water, and frequency of cage cleaning. All of the chickens from each group were weighed and mass was recorded after 7 days using a standard scale.

Statistical analysis

Results are the mean of determinations, and the standard deviation (SD) is reported. Results were analyzed by the ANOVA test.

RESULTS AND DISCUSSION

Mealworms were washed and dried, followed by oven drying of the mealworms and crushing of the mealworms to produce the mealworm powder as shown in (Figure 1).

The proximal contents of Tenebrio molitor mealworm powder were determined and results are shown in (Figure 2). The results obtained in this study are in line with those reported by Makkar [16].

The proximate composition of yellow mealworm larvae shown in (Figure 2) is similar to the results reported by other authors. The protein content was in the range of values of (41.0 –53.2 %) which has been reported by other authors [12]. The moisture content of 4.75 % obtained in this particular study was slightly lower than what has been reported by other researchers and the moisture is believed to be related to the size of the analyzed mealworms such that a lower moisture content indicates the bigger the mealworms [17]. It is significant to outline that in this particular study a nitrogen-protein conversion factor of 6.25 was used. The fat content that has been obtained in this study also falls in the range of (24.7–50.2 %) previously reported [18]. The ash content was also in the range of values of (2.36–4.74 %) published by quite several researchers [12, 19].

The mineral content of Tenebrio molitor larvae powder was also determined and results are shown in (Table 1). These results are in line with those obtained by Aguilar [20] and Kirk [21] who confirmed that Tenebrio molitor mealworm contains a considerable amount of vitamins and minerals.

Table 1. The mineral content of T. molitor larvae powder (mg of mineral per kg of the sample)

Note: Mineral content determination was done in duplicate

The mineral composition of Tenebrio molitor mealworm powder is shown in Table 1 and the obtained values for each element in this particular study fall in the range of values that have been published by other authors [16]. The levels of Na (2100 mg/kg) and Mg (2450 mg/kg) were slightly higher as well as the sodium content which was (2100 mg/kg). It is important to mention that the amount of Na in 100 g of dried larvae is extremely below the maximum recommended daily uptake, which is advantageous given that high sodium intake increases blood pressure [22].

Carbohydrate content in mealworm powder

Carbohydrates were determined using the difference method. The percentages of all the other nutrients obtained in the investigation were added and this total was subtracted from 100 %.

Growth of broilers over six weeks

The chickens were first kept in a container for the first two weeks as it is crucial at this stage to take good care of the chicks for maximum growth and better results. The chicks upon arrival are shown in (Figure 3). The chicks in all three groups had an average weight ranging from 0.033 – 0.034 kg

After 6 weeks of feeding the chickens, the chickens were found to have an average weight of between 2.3-2.5 kg in all groups. However, group B of chickens fed with commercially available broiler stock feed, supplemented with 3 % dried Tenebrio molitor had the highest average weight. The average weights of chickens in groups A and C were almost similar without any discernible difference. The image of the chickens is shown in (Figure 4).