All the animals were treated following the Reference ethical framework for biomedical research: ethical principles for research with laboratory, farm and wild animals (CONICET 2005) and all the experiments were approved by the Animal Ethics Committee of the School of Biochemistry (Universidad Nacional del Litoral).
Animal used in this work are part of the harvest of a sustainable management program that uses wild Caiman populations, approved by Provincia de Santa Fe (Law 11820), registered in Dirección de Fauna Silvestre de la Nación (following resolutions No 283/00 y 03/04), fulfils CITES normative and follow recommendations of the Crocodile Specialist Group (CSG/SSC/IUCN). These caiman populations are not endangered.
2.1. Animal hatching and breeding
Caiman latirostris individuals used for this study were provided by Proyecto Yacaré (Larriera et al., 2008). Eggs collected from wild sources were incubated and hatched in an artificial incubator and kept under controlled captive conditions in a heated room (30 °C), in a cement pool (4.5 m wide x 0.8 m high x 0.7 m deep). Inside the pool, each individual had access to water and dry areas; pools were cleaned and the water was renewed every other day. The animals were fed ad libitum six days a week, with a base diet composed of crushed chicken heads and dry feed formulated for reptiles (Avigan ®; Table 1) (70/30; FA profile of diet in Table 2; Larriera et al., 2008, Simoncini et al., 2020).
2.2. Animal testing
The specimens were raised on average for two years to reach the weight and length suitable for being marketed (X ± SD; total length 96.7 ± 4.9 cm, snout-vent length 47.8 ± 3 cm, weight 4.2 ± 0.6 kg). Thirty individuals were randomly selected from the six pools (five individuals per each pool) and moved to individual plastic containers with a capacity of 150 liters. Animals were under conditions suitable for raising caimans, having one-part dry and the other with water, at an average temperature of 31°C. Each caiman was randomly assigned to an experimental treatment. Their weight was recorded with a digital scale (0.1 kg precision) and total length and snout-vent length were measured with a tape measure (0.5 cm precision). After a 7-day habituation, during which the condition of the animals and their normal feeding activity were checked the dietary experiment was initiated.
For 50 days’ caimans were fed according to the assigned treatment: 1. Control Diet (C, n=10): crushed chicken head and a dry food formulated for these reptiles in a 70/30 ratio, 2. Flax Seed Diet (FS, n=10): Control diet with 10% of mass in crushed flax seed, 3. Flax Oil Diet (FO, n=10): Control diet with 10% flaxseed oil (Table 2). We used 30 individual plastic containers: 10 replicates per treatment.
The supplemented diets were given three a week during the entire experimental period, while the remaining days animals were fed the control diet. While previous results obtained from caiman meat enhancement with dietary enrichments were supplemented six days a week (Piña et al., 2016), prior data (Valli, 2020), evidenced modification in caiman tissues with three days a week of supplementation. Therefore, this work attempts to evaluate the possibilities of the most efficient supplementation in terms of time and cost, and that is why we propose three times a day and the incorporation of flaxseed oil.
The control diet as well as the treatments were offered in amounts of 150 g per individual per day (calculated ration of what is offered per capita, in captivity in breeding pools). This portion of food was offered for 24 h, and was withdrawn if there were food leftovers, at the moment we cleaned the enclosure.
The feed conversion rate evaluated in captivity for the species, under similar farming conditions, varies between 5.4 ±0.35 (Parachu Marco et al., 2009). In our study, although we did not measure feed conversion, we ensured that the caimans were fed the portions offered and had food left over (ad libitum), to incorporate the necessary amount of food and energy. After 50 days, animals were slaughtered and tissue samples were obtained (meat cuts from tail and neck).
2.3. Physico-chemical characterization of diet and meat
Diets were freshly prepared, gassed with nitrogen and stored at -10 °C for subsequent FA profile analysis. The animals were sacrificed following the protocol of the Proyecto Yacaré (Yacarés Santafesinos/MUPCN, approved slaughterhouse N° 4081) and according to the sanitary conditions established by SENASA (National Service of Agrifood Health and Quality) for meat production for human consumption. Two samples of 5 g were taken from the tail (Ilio-ischio-caudalis muscle) and neck (Occipito-cervicalis-medialis muscle) of each animal and preserved at -18 °C until further processing.
The FA composition of the tissues and experimental diets was determined by gas chromatography with a Shimadzu (GC 2014) chromatograph equipped with an automatic injector (AOC-20i auto injector Shimadzu) and a flame ionization detector (SFID1). Analyses were carried out with a capillary column CP Sil 88 (100 m length, 0.25 mm i.d., 0.25mm film thickness) (Varian, Walnut Creek, CA, USA, Part N°CP7489). The column temperature was held a 75°C for 2 min after injection, then 5°C/min to 170°C, held for 40 min, 5°C/min to 220°C and held 40 min. Nitrogen was the carrier gas with an inlet pressure set at 100 kPa and a split ratio of 1:20. The injector and detector temperatures were maintained at 220 and 250°C, respectively. Injection volume was 0.5 mL and the column flow was 0.8 mL/min. Total fat in tissues, serum, and diets were extracted using the method described by Bligh and Dyer (1959). Briefly, samples were homogenized in trichloromethane:methanol 1:2 (vol:vol). After drying under nitrogen, the samples were dissolved in 1 mL of hexane for methylation. The fatty acids methyl esters (FAME) were formed by transesterification with methanolic potassium hydroxide solution as an interim stage before saponification (ISO 5509:2000, Point 5 IUPAC method 2.301). FAME were identified by comparison of their retention times relative to those of commercial standards using GC Solution Postrun software (Version 2.30 00 SU6). Standards GLC-463 Reference Standard containing 52 FAME mixture (purity > 99%) and trans-mix GLC 481 (purity > 99%) were purchased from Nu-Chek (Nu-Chek Prep, Inc., Elysian, MN, USA). Linoleic acid methyl esters, cis/trans mix (Catalog #47791) was obtained from Supelco (Bellefonte, PA, USA). Conjugated linoleic acid, cis/trans mix (Catalog #05507) was purchased from Sigma Chemical Co. Others FAME standards were provided by the International CYTED Net (208RT0343). All solvents and reagents used for the FA quantification were of chromatography grade, and all the other chemicals used were at least American Chemical Society (ACS) degree. Values of FA content were expressed as percentage of total FAME.
We expressed grouped as ΣSFA (saturated fatty acids), ΣUFA (unsaturated fatty acids), ΣPUFA (polyunsaturated fatty acids), Σn-3 FA (C18:3+C20:5+C22:5+C22:6), Σn-6 FA (C18:2+C18:3 6c9c12c+C20:3+C20:4+C22:4+C22:5+C20:2), and the ratio of ΣSFA/ΣPUFA. Also, we determinate the ratio of C20:4/C18:2 n-6 (ARA/LA), C22:6/C18:3 n-3 (DHA/ALA), and n-6/n-3 ratio.
We estimated the index of D9- and D6-desaturase activities and elongase according to Kazala et al. (1999), Malau-Aduli, Siebert, Bottema and Pitchford, W (1997), and Pitchford et al. (2002), as follows: Index of C16 desaturase activity = 100 [(C16:1 cis-9)/ (C16:1 cis-9+C16:0)]; Index of C18 desaturase activity = 100 [(C18:1 cis-9)/ (C18:1 cis-9 + C18:0)]; and Index of C16–C18 elongase activity = 100 [(C18:0+C18:1 cis-9)/ (C16:0+C16:1 cis-9+C18:0+C18:1 cis-9)]. The atherogenic index, considered as a health indicator related to the risk of cardiovascular disease, was computed according to Ulbricht and Southgate (1991) as: [4(C14:0) +C16:0]/(ΣMUFA+ΣPUFA).
2.4. Statistical analysis
To compare the proportion of FA in two cuts of meat from animals fed different diets we analyzed the data with General Linear Models (GLM). In all cases the assumptions of normality and homogeneity of variances were tested (analyzed graphically and with Shapiro Wilk's test). In all GLMs we analyzed each fatty acid according to the type of cut (two levels: tail, neck) and the diet (three levels: C, FS, and FO) and we evaluated the interactions between the cut and diet variables. If the interaction was not significant (P > 0.05), it was dropped from the model, and the data re-analyzed with a model including only the main effects of cut and diet. In cases where we detect differences (p<0.05) we make a posteriori comparisons using the DGC test. Data were analyzed using the InfoStat software, version2018 (Di Rienzo et al., 2018).