Proteins and Amino Acids

;

Use of a Four-Parameter Logistic Model to Evaluate theGluality of the Protein from Three Insect Species whenFed to Ratsl

MARK D. FINKE,f* GENE R. DIFOLIART* exp NORLIN J. BENEI/ENGAI|

Depaftments of fNutritional Science, *Entomology, and iheat and Animal Science, University ofWisconsin, lvladison, Wl 53706

ABSTRACT The quality of three insect protein sources

[Mormon cricket meal (MClvl), house cricket meal (HCM)and Eastern tent caterpillar meal (TCM)I was evaluatedrelative to that of lactalbumin (lA) and soy protein (SP)by using both amino acid analysis and a rat bioassay. Theamino acid pattern of the three insect meals indicated thatmethionine should be the first limiting amino acid for grow-ing rats. In the rat bioassay, weanling Sprague'Dawley ratswere fed graded levels of the five proteins in purified dietsand the response (weight or nitrogen gain) evaluated as afunction of nitrogen intake. The individual nitrogen intake-animal response results could be described by a series ofcurves using a four-parameter logistic model. The use ofparameter sharing permitted the full range of responses tobe described so that statistical differences between thedose-response curves could be identified. When used foreither weight maintenance, nitrogen equilibrium, maximumweight gain or maximum nitrogen retention, the five proteinsources could be ranked in the following order: I-A > HClvl> Mcivl = SP > TClvl. Relative to lactalbumin, the valueof all four protein sources decreased with increasing nitro'gen intake. The low values obtained for TClvl may havebeen related to factors other than protein quality. The re-sults of this study indicate that some insect proteins are

equivalent or superior to soy protein as a source of aminoacids for growing rats. J. Nutr. 119: 864-871. 1989.

INDEXING KEY WORDS:

. Iactalbumin ' soy protein ' insect protein

. protein quahty . nonlinear models ' togisticmodel . parameter sharing

Since the 1960s, results from a number of studiesdesigned to investigate the nutritional value of insectprotein have been published. House fIy {Musca do-mestica L.) pupae (l-3), tace fly lMusca autumnalisDeGeer)pupae (4), Mormon crickets lAnabrus simplexHaldeman) (5, 6), house crickets lAcheta domesticusL.) (7) and avariety of lepidoptera species (8-10) havebeen evaluated as a source of protein in poultry diets.

0022-3166/89 $3.00 O 1989 American Institute of Nutrition. Received

864

In general, these studies indicated that insect proteinwas a suitable substitute for soybean meal in practicalpoultry diets. Amino acid analyses of insect protein

' indicated that the sulfur amino acids, methionine andcysteine, are probably the first limiting amino acids forpoultry. However, when Mormon crickets or housecrickets were used as a source of protein in purifieddiets for poultry, methionine and arginine were foundto be co-limiting {5, 71.

We know of only three studies designed to evaluatethe value of an insect species when fed to animals otherthan poultry. In the first study, the quality of proteinfrom the termite species, Mauotermes falciget, wasevaluated in a rat bioassay using the PER method, anda value o{ I.7 was obtained (ll). The low valud fortermite protein was attributed to its low digestibility(sl"/"|trelative to that of casein (84%1. The high tem-peratures used in the processing of the insect materialmay have altered its protein quality by decreasing aminoacid availability. In another study, soldier fIy larvae,Hetmetia illucens, were incorporated into a practical-type diet and fed to pigs. Dry matter digestibility wassignificantly lower for this diet than for a corn-soybeanmeal control diet (12).In the third study, dried, groundether-extracted Mormon crickets were used as a proteinsource in a multi-dose rat bioassay (13). When the Mor-mon cricket meal diets were supplemented with 0.4%

, r-methionine, the amount of crude protein required toattain 95o/o of the maximal growth rate or nitrogenretention was reduced by 45%. When the Mormoncricket meal was mixed with a complementary proteinsource/ corn gluten meal, the amount of crude proteinrequired to attain 95% oI the maximal growth or ni-trogen retention was reduced by approximately 30%relative to Mormon cricket meal alone.

Because of the encouraging results obtained with

rThis work was supported by the College of Agricultural and Life

Sciences, University of Wisconsin, Madison, WI'

3 )anuary 1989. Accepted 3l fanuary 1989.

USE OF A LOGISTIC MODEL TO EVALUATE PROTEINS 865

Mormon cricket meal (13), the present investigationwas conducted to compare the quality of protein ofthree insect species, the Mormon cricket, the housecricket and the Eastern tent caterpillar (Mallacosomaamericanus Fab.),,with that of lactalbumin and isolatedsoy protein. The five protein sources were comparedusing both amino acid analyses and a rat bioassay pro-cedure described earlier (14). A nonlinear model wasused to describe the dose-response curves/ and the rel-ative values of the proteins were evaluated by com-paring the amount of nitrogen required to achieve iden-tical levels of performance (13, 14).

MATERI,ALS AND METHODS

Protein sources. The iactalbumin and isolated. soyprotein were purchased commercially (Teklad Test Diets,Madison, WI). Adult Mormon crickets were collectedand processed as described earlier (13). Adutt housecrickets were obtained from a colony maintained at theUniversity of Wisconsin, Department of Entomology,and killed by fteeztng. Final and penultimate instarEastern tent caterpillar larvae were obtained from mul-tiple locations in Dane County, WI and killed by freez-ing. All insects used in these experiments were kept

,

frozen until needed.The lactalbumin and isolated soy protein were used

as purchased. The Mormon crickets and house cricketswere dried by lyophilization. The Eastern tent cater-pillar larvae were dried in a radiant heat oven at 45'Cf.or 72 h to singe off the body hairs. After drying, eachinsect preparation was ground through a l-mm (ap-proximately) screen in a Wiley mill. The lipid materialwas removed by refluxing with diethyl ether for 12 h,and the dry powder was redried at 45"C for 6 h to re-move any residual ether. All protein sources were passedthrough a 40-mesh screen prior to use.

Each of the protein sources was analyzed for ash,water and acid detergent fiber using procedures estab-lished by the A.O.A.C. (15). Ether extract was deter-mined by a24-h Soxhlet extraction using diethyl ether.Nitrogen was determined by a micro-Kjeidahl digestionprocedure with ammonia determined by a phenol-hy-pochlorite reaction ( 15).

The amino acid concentration was determined in a40-pg sample. The sample was placed in an acid-washedignition tube containing 200 p,l of 6 N HCI with}.z%phenol, the tube evacuated to 25 mTorr, hydrolized for20 h at 110"C, evaporated to dryness and then redis-solved in a 0.15 Nlithium citrate buffer solution at pH2.2 (I7lr. The sample was then centrifuged [12,000 x gfor 10 min at room temperature (21'C)] and a sampleof the supernatant used for analysis (Beckman model119 amino acid analyzer, Palo Alto, CA).

Diets. All diets contained a vitamin mix, 0.5% (lB);a mineral mix (Tekiad Test Diets, Madison, WI), 5.0%(18); corn oil (Archer Daniels Midland, Decatur, IL),5.0%, choline chloride (U.S. Biochemical, Cleveland,

OI{l, 0.2%, cellulose (U.S. Biochemical), 0.2% (used as

a carrier for the choline choloride); and equal weightsof cornstarch (Staley, Decatur, IL) and glucose mono-

.hydrate (Cerelose, Corn Products (CPC) International,Englewood Cliffs, N|). The protein sources were addedto the diet at the expense of the cornstarch-glucosemixture. While the mineral mix used has been shownto contain an inadequate level of. zinc when used withamino acid diets, the possibility of zinc limiting growthis unlikely in thisexperiment, since both lactalbuminand soy protein contain adequate levels olzinc (52 and39 mg/kg, respectively). In general, the dried, groundinsect species used in these experiments have beenshown to contain high levels of zinclT,19, unpublisheddata); Mormon cricket, 130 mg/kg; house cricket, 250mg/kg; and Eastern tent caterpillar, 80 mg/kg.

The five protein sources were evaluated using a ratbioassay in a 3-wk experiment {13, 14). Six dietary lev-els of lactalbumin were used, while seven levels wereused for the other four protein sources. A diet contain-ing no added protein was formulated to serve as a base-line control. The dietary protein concentrations se-lected were not the same for all protein sourses testedand were based on the amino acid composition of thetest proteins. These levels were selected in an attemptto provide diets which would support a full range ofanimal responses/ from maximum weight loss to,max-imum weight gain. The diets were fed in powdered formin metal feed cups. Diet and water were available adlibitum.

Animals. Weanling male Sprague-Dawley rats (21-d-old, Sprague-Dawley Farms, Madison, WI) were dividedinto 36 groups such that the mean weights of the groupswere similar (mean 51.5 g; range 47-58 g). Groups offive rats were assigned to each of the 35 dietary treat-ments. In addition, one group was killed at the begin-ning of the experiment to obtain an estimate of bodynitrogen at the time the rats were assigned to the treat-ment groups. Rats were kept in a room maintained at25'C, with controlled lighting (12-h light/dark cycle|and housed individually in stainless steel wire-bot-tomed cages. Rats were weighed daily, and food intakewas determined three times per week over the 2I-dexperiment. At the end of the experiment, the rats werekilled by ether inhalation, the gastrointestinal tract ofeach rat was removed, washed free of its contents andreturned to the carcass. The carcasses were placed inindividual, airtight plastic bags and kept frozen untilneeded for nitrogen analysis.

To prepare each rat carcass for nitrogen analysis, itwas chopped into 5-10 small pieces, lyophilized andrefrozen. The pieces were immersed in 500 ml of liquidnitrogen for 5 min, and then ground in a Wiley mill at-20"C. The resulting powder was used for nitrogenanalysis. A micro-Kjeldahl procedure with ammoniadetermination via a phenol-hypochlorite reaction wasused to determine carcass and dietary nitrogen content(16). AII nitrogen determinations were done in tripli:

866

cate. Nitrogen gain was calculated for each rat by sub-tracting the average calculated nitrogen content of fiverats at the start of the experiment from that obtainedfrom analysis of each rat at the end of the experiment.

Logistic model. The data from this experiment wereanalyzed using a four-parameter logistic equation (13,

14,20ir. The model2 and a description of the parametersused are shown below.

R-*+[b(t+c) -R-*]dIl+cdlr€SPOnS€ =

where:

| : predicted response of rats fed a protein-free

diet (g/21 dl

c : shaping parameter

d : scaling parameter

R-"* : predicted maximum response (S/2I dl

I : nitrogen intake (S/21 dl

AII dose-response curves (weight or nitrogen gain vs.nitrogen intake) were fitted using a simultaneous curve-fitting routine previously described (13, 14,21). AI-though the values for individual rats were used in thejcurve-fitting routines, only the mean values for eachtreatment are shown in the figures.

RESULTS

The composition of the five protein sources testedin these experiments is shown in Table 1. The highcontent of acid detergent fiber (ADF) in the insect mealsis of interest because some of the nitrogen is probablyin the form of chitin (N-acetylglucosamine) (221. Thenitrogen content of chitin (6.ly"l is similar to the ni-trogen content of the ADF residue of the three insectmeals (5.0,7.6 and 5.2% for MCM, HCM and TCM,respectively). If this nitrogen represents nonprotein ni-trogen, then 5.3, 5.5 and 7.0"/o of. the total nitrogen inMCM, HCM and TCM, respectively, is nonprotein ni-trogen. Thus, animal response per unit of nitrogen con-sumed should be less for rats fed diets containing MCNI,HCM or TCM based solely on dilution.

The indispensable amino acid content of lactalbuminwas higher than that of all of the other protein sources.The low sulfur amino acid content of the other proteinsources indicates that their value should be substan-tially less than that of lactalbumin.

The dietary nitrogen content/ food intake, weight gainand nitrogen gain of each treatment group are shownin Table 2. Rats fed HCM clearly reached a plateau forweight gain, although none of the animals clearly reacheda plateau for nitrogen retention. Diets containing TCMappeared to be less palatable than the other diets, es-pecially at high dietary nitrogen concentrations.

FINKE, DEFOLIART AND BENEVENGA

The final parameter estimates for the fit of the modelto the data when weight gain was the dependent var-iable are shown in Table 3. All curves were able to

- share a eommon parameter estimate for both b andR*"*. lJnique estimates for parameters c and d wereobtained for the curves describing the response of ratsfed lactalbumin and TCM. This indicates that both theshape and magnitude of the response of rats fed dietscontaining these two protein sources differed from thatof rats fed diets. containing the other three proteinsources. The curves describing the response of rats feddiets containing soy protein, MCM or HCM shared acommon value for parameter c, while the curves de-scribing the response of rats fed diets containing soyprotein and MCM shared a common estimate for allfour parameters. Figure I shows the relationship of thefive curves to the data when the body weight gain wasthe dependent variable. The response of rats fed dietscontaining HCM could be described by a curve with anidentical shape (parameter c) to that of rats fed dietscontaining soy protein or MCM, although the slope ofthe curve (parameter d)was slightly steeper. The curvedescribing the response of rats fed diets containing TCMhad the least slope, while that for rats fed lactalbuminhad the steepest slope. The fit of the curves to the datawhen nitrogen gain was the dependent variable is shownin Figure 2. As was seen when weight gain was theresponse variable, the curve describing the nitrogen in-take-nitrogen gain relationship of rats fed diets con-taining lactalbumin had the steepest slope, while thecurve describing the response of rats fed diets contain-ing TCM had the least slope. The shape {parameter c)

of the curves describing the response of rats fed dietscontaining HCM was identical to that for the curvesdescribing the response of rats fed diets containing soyprotein and MCM, but the slope (lower value for pa-rameter d) was slightly steeper.

Final parameter estimates for the fit of the model tothe data when weight gain or nitrogen retention wasthe dependent variable are shown in Tables 3 and 4,respectively. Although the parameter estimates ob-tained were dependent upon the response criteria se-'lected, the patterns observed were identical.

Table 5 shows the relative values of the four test

2The original model (equation l) can be modified to allow param-eter I to retain the appropriate units (g nitrogen consumed/2l d). Theconversion to the final model (equation 3) and a description of theparameters are shown below.

R*."+[b(l+cl -R-".ld'response= l+.d,since

dI : exp[(ln d)I]

The final equation becomes

R-". + [b(l + C) - R-..] expl(ln d)IlresPonse: I+."l<pl(lnd[lWe thank Steve Denham for this modification of the original equa-tion.

(equation l)

(equation 2)

{equation 3)

USE OF A LOGISTIC MODEL TO EVALUATE PROTEINS 867

TABLE 1

Composition of the protein soutces used in the feeiling expertments

Protein source

SPLA MCM HCM TCM

Crude protein (6.25 x N)rEther extractrAshrWaterrAcid detergent fiberr

Alanine2ArginineAspartic acidCystineGlycineGlutamic acidHistidineIsoleucineLeucineLysineMethioninePhenylalanineProlineSerineThreonineTyrosineTryptophan3ValineAmmoniaTotal indispensable

amino acidsTotal dispensable

amino acidsTotal amino acids

First limitingamino acida

Chemical scorea

7.r1.79.00.5

5441

1005422

r522046

r2585254044496l41

ND54t6

s92

4211013

88.02.1o.c9.10.4

34688516

35155

3346852

8

4438473530,ND3414

412

39s807

Met-Cys48

78.02.65.46.69.1

857874

9

s9rt2254076591/3446434353ND57l8

49r

4209lI

Met-Cys52

6l.s2.28.57.2

13.2

534072

7

408918

28484412

2928JI

OL

34ND3614

328

319647

Met-Cys38

His80

o//o

69.93.07.77.4

tr.7mg/g uude protein

81

635413

5410422J/685415

JO

39484347ND4920

415

380795

Met-Cys58

rEach value represents the mean of three analyses. Protein source of each diet is as follows: LA, Iactalbumin; SP, soy protein; MCM, Mormoncricket meal; HCM, house cricket meal; TCM, Eastern tent caterpillar meal.

2All amino acid values are expressed as mg/g crude protein. For lactalbumin and soy protein, the values shown represent one analysis. For

the three insect species, the values shown represent the mean of two analyses.3Not determined.aUsing NRC requirements for growth (25), chemical score was calculated as: ([First limiting amino acid (mg/g protein)] + [NRC requirement

(mg/g protein)l) x 100.

protein sources when lactalbumin was used as thestandard. The relative values are calculated by dividingthe nitrogen intake requfued to achieve a speciJic weightor nitrogen gain {or rats fed diets containing lactalbu-min by the nitrogen intake required to achieve an iden-tical response for rats fed diets containing the test pro-tein. When weight gain was the dependent variable, therelative value of soy protein and MCM decreased from73% at maintenance to 50% at 95o/o of the maximumpredicted weight gain. The relative value of HCM andTCM also declined from highs of.89o/o and 560A at main-tenance to lows of 6L% and 25o/o, respectively, at 95o/o

of the maximum predicted weight gain. When nitrogengain was the dependent variable, the relative values ofall {our protein sources were less than those seen when

weight gain was the dependent variable, although thepatterns observed were identical.

DISCqSSION

Amino acid analysis indicated that the sulfur aminoacids, methionine and cystine, are probably the firstlimiting amino acids in protein from MCM, HCM andTCM (Table l). These results are similar to amino acidanalyses of other insect species |L,3, 6,7, 10, l2). Feed-ing studies with rats have confirmed that methioninewas the first limiting amino acid for growth in Mormoncricket meal (13) and that further additions of lysineand histidine had no effect (unpublished results). No

868 FINKE, DEFOLIART AND BENEVENGA

TABLE 2

Dietary nitrogen contetlt and 2I-iI food intake, weight gain and nitrogen gain of rats fed diets containing the test protein sources'

Protein source Dietary N Food intake Wt gain N gain

LA

SP

MCM

HCM

TCM

Protein-free

276.4 + 9.8269.5 + 0.9227.8 ! 2O.4

204.4 ! 15.8

164.6 + 7.4131.4 + 9.327t.0 ! 6.6274.6 + 4.2298.6 + 6.9281.0 + 10.6

194.8 t 1.9

144.2 + 5.4135.4 ! 7.8286.0 + 5.3298.4 + 5.4314.8 + 8.4277.8 ! 5.9

226.5 + 7.7145.4 ! 4.9103.4 t 5.8290.6 + 4.5297.2 + 7.7300.4 * 7.6309.6't 13.0

254.4 + l4.l145.5 + 2.7115.6 t 4.02ll.O + 7.9229.8 t 19.6188.2 t 15.2194.2 + 1.8L69.2 + 7.7121.4 + 3.2103.4 + 3.793.6 + 3.2

8124.8 ! 6.5108.0 + 4.771.0 r 8.045.6 + 4.9

.19.8 + 1.62.2 + 2.O

124.0 t 1.7

123.4 ! 3.5107.0 + 5.282.6 + 4.638.0 I 0.8

6.6 + 1.3

- 1.8 + 1.3120.8 + 1.3117.8 t 3.0110.8 + 2.679.6 ! 2.741.0 + 4.26.0 + 1.8

-3.0 + 1.6135.6 + 2.8137.2 ! 4.6124.2 + 4.1IO7.4 + 4.855.8 + 4.411.2 + 0.7

-2.6 ! 0.467.0 + 6.075.8 ! 9.445.5 ! 7.441.4 + 1.32l.O t 2.8

-3.2 + 0.7

-7.4 + I.2-12.2 + 0.4

3.77 ! 0.2r3.10 + 0.142.27 ! 0.361.35 r 0.120.68 + 0.050.16 + 0.043.72 ! O.1O

3.54 i 0.123.12 t 0.132.38 + 0.161.02 a 0.030.26 I 0.030.02 + 0.033.75 + O.l23.56 + O.l23.17 + 0.102.28 1 0.05l.l9 I 0.090.23 + 0.05

-0.01 + 0.034.20 t 0.103.96 = 0.183.55 r 0.102.88 + 0.141.47 + 0.110.30 + 0.010.03 i 0.012.04 t O.I52.01 + 0.211.36 r 0.17l.I4 ! O.O7

0.58 a 0.070.00 a 0.02

-0.16 t 0.02

-0.28 + 0.02

o//o

2.422.011.581.220.800.404.734.033.232.37t.520.770.484.763.913.r22.451.590.740.364.693.973.182.50r.640.82o.4s4.794.003.182.471.58

0.780.510.00

rAbbreviations for protein sourses are described in footnote to Table 1. The values for rats {ed the diet containing TCM at 4.79% N represent

the means + sEM, n = 3. The values for rats fed diets containing TCM at 4.oO% N, and MCM at 1.59% N, represent the means + stt''t,

n = 4. All other values represent the means + sEM/ n = 5.

feeding studies have yet been conducted to confirm thatthe sulfur amino acids are first limiting in HCM orTCM when fed to rats. When MCM or HCM was fedto young broiler chicks in purified diets, methiohineand arginine appeared to be co-limiting (6, 7).

When compared to lactalbumin, the other four pro-tein sources were in{erior at all levels of animal per-

formance (Table 5|. Two of the three insect meals, MCMand HCM, were equal to and slightly superior to soyprotein in the promotion of maximum growth or ni-trogen gain. In contrast, the ability of TCM protein topromote maximum growth or nitrogen retention wasonly half that of soy protein. These results suggest thatthe nutritional quality of insect meal may depend uponthe insect species. The reasons for the poor nutritionalquality of TCM is unknown, but may be related tofactors other than protein quality. Rats fed diets con-taining the two highest levels of TCM developed a pro-nounced diarrhea during the study. When these rats

were killed at the end of the experiment and the gas-

trointestinal contents removed, it was noted that manyof these rats had abnormally distended stomachs and,colons. Two rats fed diets containing the highest levelof TCM, and one rat fed the diet containing the secondhighest level of TCM, died after I wk.

In an attempt to coripare the relative value of theseproteins by utilizing the amino acid content of the pro-

teins, a decision must be made on whether or not me-thionine or total sulfur amino acid content is used forevaluating proteins first limiting in the sulfur aminoacids. Calculations based on total sulfur amino acidswere used here because diets containing the sameamount of sulfur amino acids (mmol sulphur/lOO g diet)supported identical gowth rates even though the amountof cystine varied from one half to over two thirds ofthe total (23, 24ll. Although not measured in this ex-periment, it was assumed that tryptophan was not thefirst limiting amino acid (7, I3l.

USE OF A LOGISTIC MODEL TO EVALUATE PROTEINS

TABLE 3

Final parametet estimates of the weight gain-niuogen intake rcsponse curves of rats Ieil diets containing the test prctein soutces

Parametert

869

Protein source R_,,

LASP

MCMHCMTCM

-11.6 10.9-11.6 + 0.9

-11.6 t 0.9

-11.6 r 0.9

-11.6 10.9

Using the amino acid requirements for growth (25),

the chemical score was determined for each of the fiveprotein sources (Table 1). A comparison of these valueswith the ranking of the four protein sources at 0, 50

and 95% of the maximum nitrogen gain (Table 5) re-

vealed that the values based on amino acid'analysescompared most favorably with those from the multi-dose assay when the values at 0o/o of R*o were used.

The relative values based on amino acid analysis (60,

soy protein i 73, MCMi 65, HCM, and 48, TCM) varied{rom 86 to LO9% of that of the measured values at,

nitrogen equilibrium, and 116 to 229% at 95o/" of themaximum nitrogen gain. Thus, it appears that the qual-ity of insect protein is as good as soy protein and, Iikesoy protein, the values relative to lactalbumin decline

t/tr

/n

F

A

6

trtr-./ ./t_J,. A A .-./L a

,t//

oa'/a

/Aa,

Nitrogen lntake (G)

FIGURE I Body weight gain of rats fed graded levels ofprotein from one of five different protein sources. Lactalbu-mln 111, house cricket meal (nl, soy protein (A), Mormoncricket meal (A), Eastem tent caterpillar meal (O) and proteinfree (+ l. The lines are the best fits using the logistic model.

160

140

120

100

oc(!o

Et6)

3

-20--l 151311

2.567 + O.64L1.005 t 0.3461.006 + 0.3461.006 + 0.3460.019 + 0.400

0.645 + 0.0320.826 + 0.0210.826 t 0.0210.792 ! 0.0260.926 ! 0.022

152.6 + 5.6152.6 ! 5.6152.6 + 5.6152.6 ! 5.6152.6 + 5.6

rAbbreviations for protein sources are described in footnote to Table l; b : estimated weight gain of rats fed the protein-free diet for 21 d;

R-"- : estimated maximum 21-d weight gain. All values represent means t sE.

with increasing concentration in the diet. With the ex-

ception of TCM, the relative value of all of these pro-

teins declined at similar rates relative to lactalbumin.One wonders if this is a coincidence, or if the value ofproteins limited by the same indispensable amino aciddecline at the same rate.

One o{ the advantages of the multi-dose assay is theability to compare the relative values of proteins across

animal growth rates from 0 (maintenance) to 95"/o of.

the estimated maximal growth rate {Table 5). The valueof the four test proteins, relative to that of lactalbumin,fell as nitrogen intake and nitrogen gain increased (Table

5). At maintenance lzero gain), the relative values ofthe test proteins {soy protein, MCM, HCM and TCM}varied from 50 to 76"/o that of lactalbumin. At 95% of

otr'a(,coctto.=z

-1 151311-1

Nitrogen lntake (G)

FIGURE 2 Nitrogen gain o{ rats fed graded levels of proteinfrom one of five different protein sources. Lactalbumin (l),house cricket meal {n}, solprotein {A}, Mormon cricket meal(Al, Eastern tent caterpillar meal (O) and protein free (+ ). Thelines are the best fits using the logistic model.

*4Kt.v4,'.

tr

//^

/ru .'t,t',' -

870 FINKE, DpFOLIART AND BENEVENGA

TABLE 4

Final parametet estimates oI the nitrogen gain-nitrogen intake rcsponse cuwes of nts Ied diets containing the test protein sources

Parameterr

Protein source R-""

LASP

MCMHCMTCM

-0.25 t 0.03

-0.26 t 0.03

-0.26 ! 0.03

-0.26 t 0.03

-0.26 t 0.03

1.733 + 0.5560.765 + 0.3950.765 ! 0,3950.765 a 0.395

-O.27O + 0.015

0.732 ! 0.0420.872 ! 0.0270.872 ! 0.0270.855 t 0.031

" 0.957 + 0.025

5.29 + 0.425.29 + 0.425.29 + 0.425.29 + 0.425.29 + 0.42

rAbbreviations for protein sources are describedd; R-- = estimated maximum 21-d nitrogen gain'

in footnote to Table 1; b : estimated nitrogen gain of rats fed the protein-free diet for 2lAll values represent means + sE.

TABLE 5

Proiected ninogen intake (g/21 il) anil relative values rcqufued for 0, 50 and 95% of the maximum predicted weight and nitrogen gain

Percent of themaximum body wt gain

Percent of themaximum N gain

Protein source 95509s50

LA: N intakerelative value

SP: N intakerelative value

MCM: N intakerelative value

HCM: N intakerelative value

TCM: N intakerelative value

0.55100

0.74730.74

IJ

0.5189

0.9755

3.73100

6.2759

6.27s9

5.147210.1037

9.84100

19.6150t9.6150t6.076I40.1725

0.40100

0.6167

0.6167

0.53760.80

50

4.45r00

7.8856

7.8856

6.896513.38Ji1

12.8910026.244926.244922.955652.4r2l

the maximum nitrogen gain, these values fell to 2l-56"/o of. the value o{ lactalbumin. The relative declinein the value of insect proteins cannot be assessed withsingle-point bioassays such as NPU or PER (14). Whilethe chemical score method was capable of accuratelypredicting the relative values of the four protein sourceswhen used to maintain nitrogen balance, it could notaccurately predict the relative values of the proteinsources at 95% of the maximum nitrogen gain.

When evaluating protein quality using a multi-dosebioassay, an attempt should also be made to formulatediets so that for each protein source tested, the responseof at least one group of animals is at or near the responseplateau. These data should support a more accurateassessment of both the "diminishing returns'/ area ofthe response curve and the maximum response lI3, 14,261. An accurate estimate of the "diminishing returns//area of. the response curve is critical in assessing theefficiency of incremental increases of dietary nutrientconcentration as the response approaches R*.*. This"area of diminishing returns/' has been shown to varywidely between rats fed diets containing different di-etary protein sources (14).

ACKNOWLEDGIVIENTS

We would like to thank fim and Penny Creasy ofBrown's Park National Wildlife Refuge for kindly pro-viding lodging while the Mormon crickets used in thisstudy were collected. We would also like to acknowl-edge the assistance of |im Rozencranz for help in col-lecting crickets, and Larry Cohen and Ron Smith forhelp during the rat feeding study.

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