J. Mollus. Stud.-1998-GOMOT-173-81

J. Moll Stud. (1998), 64,173-181 © The Malacological Society of London 1998

BIOCHEMICAL COMPOSITION OF HELIX SNAILS:INFLUENCE OF GENETIC AND PHYSIOLOGICAL FACTORS

ANNETTE GOMOTLaboratoire de Biologie des Organismes et Ecosystimes, U.F.R. des Sciences et Techniques, place Marichal

Leclerc, 25030 Besancon Cedex, France(Received 16 September 1996; accepted 24 May 1997)

ABSTRACT

This paper describes the biochemical composition ofdifferent species (Helix lucorum. Helix pomatia) andsub-species of snails (Helix aspersa aspersa, Helixaspersa maxima) reared in the same conditions with afeed ('Helixal') specially designed for edible snails.In addition, the composition of wild H. pomatia andH. lucorum is presented to allow comparisonbetween snails of different origins. Analyses deter-mined the percentages of proteins, lipids andminerals. They reveal both similarities and differ-ences in composition according to the species and thepart analysed (whole body, pedal mass, and visceralmass). H. pomatia contains the highest percentage ofmineral matter and the lowest percentage of lipids.Surprisingly, protein contents are slightly differentbetween artificially reared H. aspersa maxima of3 months old and wild H. pomatia. The results makeit possible to evaluate nutritional quality of snailswith the composition of the body of four edible snailspecies.

INTRODUCTION

While detailed analyses are available for manyfoods (Alais & Linden, 1991), there are fewdata on the composition and nutritional valueof edible snails. The essential amino-acidcomposition of the molluscan pest Achatinafulica Bowdich was determined by Mead &Kemmerer (1953) to determine the possibilityof using snail meat as a source of animalprotein in feed for poultry and livestock.

For Helix pomatia L. and Helix aspersaMuller, the information available up to 1975 ontheir composition of proteins, lipids, carbo-hydrates and minerals was summarized byCadart (1975), who remarked on their richnessin mineral salts. Since that date some informa-tion has appeared on species that are notalways well defined, following various treat-ments of the flesh (Table 1). Thus, in spite ofthe fact that there is a significant internationaltrade in snails and prices vary greatly according

to the species, there is no information on theircomparative chemical composition. Only thenutritive value of farmed Helix aspersa ofrelatively low bodyweight has been evaluated(Clayes & Demeyer, 1986).

Many snail farms are being established at thepresent time, in part to compensate for thedecrease in natural populations in certaincountries and in part in order to produce good-quality snails for consumption. Thus it seemeduseful to make a comparative study of differ-ences in biochemical composition between themost commonly eaten species. With that end inview, analyses of snails of clearly identified ori-gin were made. In addition, these snails weresubject to genetic studies in our laboratory(Borgo, Souty-Grosset & Gomot, 1995; Borgo,Souty-Grosset, Bouchon & Gomot, 1996)which allowed an up-to-date and accurateidentification. Some were collected from wildpopulations (which are still the main source ofsupply for human consumption), while otherswere reared in a controlled environment thatallowed us to study the effect of differentfactors (species, age and environmental param-eters) on the biochemical composition of theirtissues.

MATERIALS AND METHODS

The animals

The study was conducted on four species or sub-species of the genus Helix: Helix aspersa aspersa,Helix aspersa maxima Taylor, Helix pomatia andHelix lucorum L. The majority of the analyses wereof animals reared in off-the-ground cages speciallydesigned for snail rearing and in optimal environ-mental conditions for growth (photoperiod 18L : 6D;temperature 20°C; relative humidity 90%) (Gomot &Deray, 1987). The H. a. aspersa came from a strainfrom the Cavaillon region of France. The H. aspersamaxima, originally imported from Algeria, had beenraised in France for 30 generations. The H. pomatiaparent snails were collected in the Choye forest (70-

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174

Table 1. Data availablerelated species.

A. GOMOT

in the literature on the biochemical composition of snails for the same or

Species

Whole Raw SnailHelix pomatia

Raw Snailspecies not defined

Raw Giant African SnailAchatina

H. pomatiaRaw Foot

H. pomatiaEdible Part

H. pomatiaWhole Snail

H. aspersaWhole Raw Snail

—small snails (2.5 g)—large snails (3.5 g)

H. pomatia

H. aspersaReared Snail

Raw Snail

Oreohelix- strigosaWhole Snail

Water(%)

79.2

89.2

72.8 to80.7

79

84.9

83.387.6

84.9

81.6

79

78 to81

gperProteins

16.1

9.9

16

12.3

129.9

12.6

16.3

16

10.4

100 g fresh matterLipids Carbohydrates

0.79

1.4

1.4

1

0.7

0.70.5

0.5

0.8

1

1.5

2

4.4

2

0.50.4

2

4.6

Ash

1.3

2.1

1

1.9

2.71.2

1.8

1.3

1

Authors

Oudejans & Vander Host, 1974

Watt & Merril,1975

Watt & Merril,1975

Wieser & Schuster,1975

Souci etal., 1981

Avagnina, 1983

Claeys & Demeyer,1986

Fontanillas, 1989

Bonnet etal., 1990

Feinberg etal., 1991

Rees & Hand, 1993

France) and the H. lucorum came from a populationestablished at Caluire in the Lyon area (69-France).The rearing method was as follows: Following matingof the parents, the eggs laid were collected and theyoung that hatched were reared in cages aboveground on a meal (E3-2) prepared specially for snails(Trademark Helixal: Ets Lupine Alivor - 39 Clair-vaux les Lacs - France). The analyses of the snailfood was: proteins 13.4%, crude fat 4.3%, cellulose2.5%, ash 31.4%, calcium 10.8%, vitamins A, D3, Erespectively 15 000, 2 000 IU.kg"1 and 20 mg.kg"1.

Sample preparation

This was performed on a series of artificially rearedanimals from 3 to 5 months of age and also on adultsof the species H. pomatia and H. lucorum collectedfrom wild populations in order to determine whetherthere are differences in these species that are afunction of their origin and age.

The snails for analysis were starved for 24 hours,then decapitated and separated from their shell. Thewhole body or its different parts were put into plasticbeakers kept in ice. Analyses were performed bothon the whole body and on the two principal parts: thepedal mass, with the head and mantle edge; and thevisceral mass, containing the digestive gland, gonad,

albumen gland, genital ducts, kidney and heart. Themucus and hemolymph leaking during separation ofthe two parts was combined with the foot. Thenumber of animals sampled per lot was chosen togive 130-200 g of organs before dehydration, thequantity necessary to perform the various analyseson the same pool of snails.

The soft tissues were rapidly frozen at -70°C andstored at that temperature. They were then freeze-dried to constant weight to determine the moisturecontent. The freeze-dried tissues were then ground ina knife mill (Janke & Kundel A10) and reduced topowder by passing through a sieve with holes 0.25mm in diameter. The shells were dried at 60°C toconstant weight and the percentage shell to totalliveweight of the animal was calculated. The crudeash content of the samples was determined by calci-nation of 2 g of powder in a muffle furnace (PyrectonKY type KY 2C4 Prolabo) at 550 ± 5°C for 6-7hours.

Analysis of the organic matter

Protein content was determined by the colorimetricmethod of Lowry, Rosebrough, Farr & Randall(1951). Total lipids were determined by extractionwith a 2:1 V/V chloroform:methanol mixture and

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BIOCHEMICAL COMPOSITION OF HELIX 175

titrated according to Folch, Lees & Stanley (1957).Total monosaccharides were determined at Biologi-cal Chemistry Laboratory of Lille University (DrMichalsky and F. Delplace) by gas chromatographyafter methanolysis and trimethylsilylation followingKemerling, Gerwig, Vligenthart & Clamp (1975).

Determination of minerals (Ca, Mg, P) and traceelements (Cu, Fe, Zn)

Following ashing of the samples, Ca, Mg, Fe, Cuand Zn were determined by atomic absorptionspectrometer (Perkin-Elmer 1100). Phosphorus wasdetermined using a Technicon autoanalyser withnitro-vanado-molybdic reagent.

Initially, determinations were made on 5 snailsanalysed individually to calculate the standard devia-tion and on pools of from 5 to 10 snails. The resultsbeing very close, the figures in the tables are thevalues obtained from analyses of pools of animals,which are mean values.

RESULTS

Composition of the body of four species ofHelix snails

The composition of the whole body wasstudied on artificially reared snails of the sameage (3 months) of each of the 4 species (Table2). Determinations were also made on H.pomatia at 4 months and 5 months, since thisspecies, like H. lucorum, grows more slowlythan H. a. aspersa or H. aspersa maxima. Thispermitted identification of changes in composi-tion with age.

The weight of the shell as a percentage of thetotal liveweight was between 10.3% and 11.5%for snails of the same age of the 4 species fedon the same feed E3-2 (Table 2). The shell as apercentage of liveweight is much higher in thesnails (H. pomatia and H. lucorum) collectedfrom wild populations (17.5% and 22%),largely due to the animals being older (over 2years of age) but also to other factors like thenature of the soil and vegetation, as well as theinfluence of the seasons (interrupted growth,hibernation, etc.).

The moisture content of snails varies withthe temperature and humidity of their environ-ment. However, in the identical conditions inwhich sampling was performed, artificiallyreared H. aspersa maxima has a moisturecontent very close to that of wild H. pomatia,whereas H. a. aspersa has a slightly lowermoisture content, close to that of H. lucorum.To prevent differences in moisture contentaffecting the percentages of the different con-

stituents of the tissue, the latter are expressedas percentage dry matter. They are also givenas a ratio to fresh weight to provide a compari-son with the flesh of other animals.

The ash content was highest in the wild H.pomatia and H. lucorum. Among the artificiallyreared snails, H. pomatia had the highest ashcontent (11.7% at 3 months and 12.6% at 5months). The other three species are similar intheir ash content (8.7 to 9.4%). With H. poma-tia the influence of age can be seen on the ashcontent, dry matter and proportion of the shellto liveweight as the animal goes from 3 to 5months of age with the same feed (Table 2).

The percentage of protein, like that of mois-ture, differs little between artificially reared H.aspersa maxima and wild H. pomatia; these arethe lowest percentages (55 to 59.3%). Artifi-cially reared H. pomatia has a higher percent-age protein, close to that of artificially rearedH. a. aspersa and H. lucorum (65.2 to 68.2% at3 months and 72.5% for H. pomatia at 5months). For H. lucorum one notes a differ-ence between the two lots collected from wildpopulations, the cause of which might be adifference in the development of the reproduc-tive apparatus—larger or smaller albumengland according to whether the snails werecollected before or after egg-laying.

The percentage of lipids is lowest in wild H.lucorum and H. pomatia, with differences of1.9% and 3.1% between the two lots analysedfor these species. The percentage lipids in theartificially reared snails of all the species ishigher than in the wild ones, in spite of the factthat the latter are older. Of the 4 species, H.pomatia contains the least lipids and one alsonotes a decrease between the ages of 3 and 5months.

The percentage of total sugars is commonlyobtained as a difference between 100% andthe sum of protein + lipids + ash. With thiscalculation one obtains 23.7% of total sugarsfor H. aspersa maxima. In contrast, an accuratedetermination of sugars by gas chromatog-raphy gives 13.8% for this species (Table 2),which shows that the calculation by differenceis unsatisfactory.

Comparison of the biochemical composition ofthe foot and viscera in the four species

Snails are prepared for human consumption indifferent ways, depending on the species andthe region. Petits Gris (//. a. aspersa) aregenerally eaten whole. With the larger species(H. pomatia, H. lucorum and H. aspersa

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Tab

le 2

. A

naly

ses

of t

he

wh

ole

bo

dy

of

fou

r sn

ail

spec

ies.

Spe

cies

H.a

.asp

ersa

H.a

sper

sam

axim

a

H.

luco

rum

H.

pom

atia

Ag

e(m

onth

s)

3 3 3 ? 3 5 ?

Sna

ilfe

eds

E3-

2

E3-

2

E3-

2[N

atu

ral

mid

Ju

ly]

E3-

2

[Na

tura

len

d Ju

ne

]

Nu

mb

er

of

an

ima

ls

16 8 10 Mea

n

36 8 6 24 17 6 6

Mea

nw

eig

ht

<g> 9.1

16.3

15.7

16 4.8

15.9

19.6 5.6

8.9

24.2

26.9

# She

ll

11.2

11.5

10.9

11.2

10.8

17.5

19 10.3

12.7

22.2

18.1

** Wat

er

83.7

84.3

87.4

85.8

84 82.4

82.1

84.2

82.3

85.3

86.8

#•

Dry

ma

tte

r

16.2

15.6

12.5

14.1

16 17.5

17.8

15.8

17.7

14.6

13.1

Ash

/FM

1.5

1.3

1.1

1.2

1.4

2.3

1.8

2.2

1.8

/DM 9.4

8.7

9 8.8

9.2

13.4

11.7

12.6

12.6

Res

ults

: g

Pro

tein

s(L

ow

ry)

/FM

/D

M

10.5

9.1

7.2

8.1

10.8

11.9

10.7

10.7

12.8 8.6

7.3

65.2

58.5

57.6

58 67.8

68.2

60 68.1

72.5

59.3

55

per

100

g

Lipi

ds

/FM

/D

M

1.6

1.8

1.2

1.5

1.5

1.1

0.6

1.1

0.9

0.7

0.4

10 12 9.7

10.8

9.7

6.5

3.4

7 5.5

5 3.1

Car

bo-

hyd

rate

s/F

M

/DM

>

1.7

13.8

g o

*: %

in w

eig

ht

of

he

alth

y sn

ails

; **

: %

in w

eig

ht

of d

esh

elle

d sn

ails

.; F

M: F

resh

mat

ter.

; D

M:

Dry

ma

tte

r.

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maxima) the visceral hump is removed, since ithas a bitter taste, and only the fleshy part—foot, head and mantle edge—is eaten. To takeaccount of these differences in culinarypractice, the foot and viscera of snails fed withthe same food were always analysed separately.Table 3 reveals common features but also somespecific differences.

The common features include: a relativelyconstant foot/viscera fresh-weight ratio of 72.8± 1.2% for the foot, 27.1 ± 1.2% for theviscera; the dry matter of the viscera exceedsthat of the foot, with different ratios accordingto the species (x 1.9 on average): 1.64:1 (H.lucorum); 1.82:1 (//. pomatia); 1.96:1 (H. aspersamaxima); 2.36:1 (H. a. aspersa); the lipids arealways at a higher concentration in the viscerathan in the foot (x 1.8 on average), but as forthe dry matter the ratio varies with the species:1.2:1 (H. pomatia); 1.8-1.9:1 (H. lucorum, H. a.aspersa); 2.5:1 (H. aspersa maxima).

The differences include: the protein propor-tion, which varies with the species and part ofthe body. The percentage of proteins is similarin the foot and viscera of H. a. aspersa and H.lucorum, the latter being richer in proteins thanH. a. aspersa in both parts. In H. pomatia thevisceral mass contains a higher protein levelthat the foot, while for H. aspersa maxima thereverse is true. In addition, the percentage ashis similar in the viscera and foot of H. aspersamaxima, while in the other 3 species, mineralmatter is more abundant in the foot than in theviscera.

Overall, for both parts of the body, one notesthat the dry matter of artificially reared H.pomatia and H. lucorum is richer in proteinsthan that of H. a. aspersa and H. aspersamaxima, while the opposite is true for lipids.

Analysis of the mineral matter of the foot andviscera of snails fed with the same feed (E3-2)

H. pomatia has the highest concentration of ash(Table 2) and in general the ash concentrationis higher in the foot than in the viscera, with theexception of H. aspersa maxima for which thetwo tissues are similar in concentration. Analy-sis of minerals reveals which elements areresponsible for the differences (Table 4).

Macro-elements: Ca, Mg, P

In the foot (the edible part) the most abundantelement is Ca. The percentage of dry matteris: Ca (1.4-^.6) > P (0.4-1) > Mg (0.3-0.5).Comparing the species with each other, for Ca,

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//. pomatia > H lucorum > H. a. aspersa > H.aspersa maxima. The concentration of Ca in H.pomatia is respectively 2.4, 2.8 and 3.2 timesthat in H. lucorum, H. a. aspersa and H. aspersamaxima.

In the case of Mg, the situation is almost thereverse of that of Ca: H. aspersa maxima hasthe highest concentration and H. pomatia thelowest (H. aspersa maxima > H. lucorum > H.a. aspersa > H. pomatia).

For P, the differences are less marked thanfor Ca, since the concentration in H. pomatia isrespectively 1.6, 1.7 and 2.3 times that in H.aspersa maxima., H. a. aspersa and H. lucorum(H. pomatia > H. aspersa maxima 3= H. a.aspersa > H. lucorum).

In the viscera, concentrations of Ca and P(% dry matter) are relatively close: P (1.3-1.9)-» Ca (1.1-1.3) > Mg (0.3-0.7). The differencesbetween the species in Ca and P concentrationsof the viscera are much less marked than forthe foot.

Measurement of the three minerals (Ca, Mg,P) in the foot and viscera shows that the higherconcentration of ash in H. pomatia (Table 2) isdue to higher concentration of Ca and P in thefoot of this species.

Trace elements: Cu, Fe, Zn

In the foot (to which the haemolymph wasadded), Cu is the most important of these met-als: Cu (0.1-0.3%o) > Fe (0.06-0.17oo) - Zn(0.06-0.08%o). If one compares the concentra-tions in the 4 species:

For Cu: H. pomatia > H. a. aspersa <« H.lucorum > H. aspersa maxima

For Fe: H. pomatia > H. aspersa maxima >H. lucorum ~ H. a. aspersa

For Zn: the interspecific variation is muchless marked.

In the viscera, Zn is the major metal:Zn (1.4-2.4%o) > Fe (0.2-0.7%o) > Cu (0.04-0.06%o). The order of the species by concentra-tion in the viscera is different for each metal:

For Zn: H. pomatia > H. a. aspersa ~ H.lucorum ~ H. aspersa maxima

For Fe: H. lucorum > H. pomatia -> H.aspersa maxima > H. a. aspersa

For Cu: H. a. aspersa •*• H. lucorum > H.aspersa maxima «• H. pomatia

DISCUSSION

The comparative analysis of the biochemicalcomposition of raw snails presented here is thefirst complete study on these animals. It reveals

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both similarities and differences betweenspecies.

Until now only fragmentary data have beenavailable, sometimes without clear identifica-tion of the species (Table 1). In addition,heterogeneous sampling methods and analysisof animals of differing origins have led to majordifferences in reported protein content for thesame species (from 9.9% to 16.3% for H.aspersa). All this provides justification for thepresent analysis of different species reared inthe same conditions.

The results are much more homogeneous foreach species (Tables 2, 3, 4) and clearly showthe role of genetic and physiological factors onthe composition of the snail. Account must betaken of these factors in interpretation ofbiochemical analyses and in the developmentof a snail food that promotes the nutritionalqualities of their meat. The principal geneticand physiological characteristics of the fourspecies of snail studied are summarized anddiscussed below.

The genetic factor that should be mostemphasised is growth rate. This allows us todistinguish two groups of species: H. a. aspersaand H. aspersa maxima with a rapid growthrate and adult weight attained at the age of 3months in favourable environmental condi-tions; and H. pomatia and H. lucorum with aslower growth rate and more than 5 monthsrequired to attain adult weight.

Among the most significant differences, wecan underline that when fed on the same feed(E3-2) H. pomatia and H. lucorum were therichest in protein (•» 68% of dry matter)followed by H. a. aspersa (65%) and H. aspersamaxima (58%) (Table 2) whereas for lipids, H.pomatia contains the least. Lastly, H. pomatiahas the highest content of Ca, P, Cu and Fe(Table 4). In fact, this species often presentsthe clearest differences.

Among the physiological factors, age andenvironmental factors are those that appearmost clearly. This can be seen in the relativeweight of the shell in H. pomatia, which risesfrom 10.3% of liveweight at 3 months to 12.7%at 5 months and 24.2% in the adults collectedfrom wild populations at an age of from 3 to 5or 6 years.

On the other hand, the lipid content of H.pomatia decreases with age and the samephenomenon has been observed with H. a.aspersa and H. aspersa maxima. This finding isnew and specific to snails, since in other farmedanimals ageing is accompanied by an increasein percentage of lipids (Beitz, 1985).

The influence of environmental factorsrequires further study, especially of snailscollected from wild populations in differentbiotopes, to analyse the influence of the vegeta-tion consumed and the nature of the soil, sincewe have previously shown that this last plays avery important role on the growth of H. a.aspersa (Gomot, Gomot, Boukraa & Bruckert,1989). In previous studies we have also deter-mined the optimum conditions of photoperiod,temperature and stocking which induce theendocrine factors necessary for fast and bal-anced development of edible snails (Gomot &Deray, 1987; Gomot & Gomot, 1995). Theresults presented in this paper illustrate theway in which genetic differences—revealed byelectrophoresis of the muscle proteins of thefoot, to distinguish between the meat of H.pomatia and Achatina fulica (Bracchi, 1988) orby study of mitochondrial DNA (Borgo et al.,1995)—translate into differences in the propor-tions of the essential biochemical constituentsof the tissues of these species.

In order to position the snail relative to otherfoodstuffs, we report (Table 5) the averagecomposition of various, types of meat and fish(from Kayser, 1963) compared with our analy-ses of the flesh of H. aspersa maxima ready forconsumption (the majority of the digestivegland being removed and the remaining fleshboiled in a court-bouillon). The snail is anadvantageous foodstuff from a dietary point ofview in that it is a source of proteins whileremaining low in calories, its energy value(calculated from the conversion coefficientsproposed by Dupin, Cuq, Malewiak, Leynaud-Rouaud & Berthier, 1992) being lower thanthat of the leanest meat or fish.

In conclusion, the value of the complete snailrearing system developed in our laboratory forrational study of the influence of factors affect-ing development and biochemical compositionmust be emphasised. Control of the variousparameters makes it possible on the one handto influence the quality of the animals rearedfor human consumption without the possibleeffects of pollutants which may occur in thefield such as bioaccumulation of heavy metals(Martin & Coughtrey, 1982; Dallinger &Wieser, 1984; Campani, Bracchi, Guizzardi,Madarena & Del Bono, 1992).

ACKNOWLEDGEMENTS

This work has been performed in the framework of aCIFRE convention (n° 607-90) of the Association

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Nationale de la Recherche Technique between theZoology-Embryology Laboratory of the Universityof Franche-Comte' and the Institut de Rechercheset d'Innovation Scientifiques of Paris. We havebenefited from the collaboration of specialists, whomwe warmly thank, for analysis of sugars (Dr J.C.Michalsky and F. Delplace: Biological ChemistryLaboratory, Lille University), for the analysis ofproteins and minerals (M. Regnier, UCAAB,Chateau-Thierry) and metals (Prof. J.C. Pihan,Ecotoxicology Laboratory of the University ofMetz).

We thank Pr L. Gomot and Pr C.R. Marchand formany fruitful discussions during the course of thesestudies. We are also very grateful to Brigitte Joliboisfor the preparation of the manuscript and verymuch indebted to Dr LJ. Elmslie for his help with itstranslation.

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J. Mollus. Stud.-1998-GOMOT-173-81