1. Introduction

The global companion animal industry has expanded rapidly, increasing demand for nutritionally balanced, safe, and sustainable pet foods. Dogs require adequate amounts of digestible protein and essential amino acids for growth, maintenance, immune function, and overall health [1,2,3]. Although commercial dog foods traditionally rely on animal-derived ingredients such as meat, poultry, and fish because of their high nutritional quality, increasing demand for these resources has raised concerns regarding competition with the human food chain and the environmental sustainability of conventional animal protein production [4,5].

Livestock by-products represent valuable but underutilized sources of proteins, essential amino acids, minerals, lipids, and other nutrients [6,7,8,9,10]. Among edible sheep organs, heart provides high-quality protein, essential amino acids such as lysine, leucine, and methionine, and important minerals including iron and zinc [11,12,13,14,15]. Lung provides structural proteins and mineral-associated components and may complement the nutritional characteristics of heart when used in formulated diets. Because protein quality depends not only on crude protein concentration but also on essential amino acid balance and biological availability, the strategic utilization of different organ tissues may improve the nutritional value of sustainable pet foods [1,16,17,18,19,20].

Processing conditions are critical for preserving the nutritional and sensory quality of organ-based pet foods. Hot-air drying is a practical preservation method that reduces moisture, limits microbial growth, extends shelf life, and can be readily applied to dry pet food production [21,22,23,24]. However, excessive thermal exposure may cause protein denaturation, lipid oxidation, and losses of heat-sensitive nutrients [25,26], whereas controlled heating may enhance sensory characteristics through the formation of desirable aroma compounds during Maillard reactions [27]. Palatability is particularly important because food acceptance by dogs is strongly influenced by aroma, flavor, texture, and ingredient composition [28,29,30,31]. Feeding trials and two-bowl preference tests therefore provide useful approaches for evaluating voluntary intake and preference for novel formulations [28,29,32].

The use of edible livestock by-products in pet food also supports circular bioeconomy principles by converting underutilized biological resources into value-added products and improving resource-use efficiency [6,33,34,35]. Despite growing interest in alternative animal-derived ingredients, information on the use of sheep heart and lung as major protein sources in hot-air dried dog foods remains limited. In particular, studies integrating nutritional composition, essential amino acid and mineral profiles, feeding performance, and palatability of these ingredients are scarce.

Therefore, this study aimed to develop and evaluate hot-air dried dog foods formulated with sheep heart and lung, individually and in combination, as sustainable animal protein sources. We hypothesized that their incorporation would improve nutritional quality and essential amino acid and mineral composition while maintaining acceptable feeding performance and palatability compared with a conventional meat-based control diet.

2. Materials and Methods

2.1. Experimental Design

A completely randomized design was employed to evaluate the nutritional quality and feeding performance of hot-air dried dog foods formulated with sheep organ meats. Four experimental diets were prepared: (1) a control diet containing horse meat as the primary protein source, (2) a sheep lung diet, (3) a sheep heart diet, and (4) a combined sheep lung–heart diet. Each formulation was designed to compare the effects of different organ meat compositions on nutrient composition, amino acid profile, mineral content, feeding performance, and palatability.

2.2. Raw Materials and Diet Formulation

Fresh sheep heart and lung were obtained from a commercial slaughterhouse in Mongolia immediately after slaughter, while horse meat was purchased from a local meat supplier. Oat flour and corn flour were incorporated as carbohydrate sources according to the experimental formulations. All animal-derived raw materials were trimmed of visible connective tissue and external fat before processing. Four experimental diets were formulated to evaluate sheep lung and heart as individual or combined animal-protein ingredients (Table 1). The control diet was based on horse meat, the SL and SH diets contained sheep lung and sheep heart, respectively, and the SLH diet incorporated both organs in equal proportions. Oat and corn flour were included at varying levels according to the formulation requirements.

Horse meat was selected as an internally prepared meat-based reference rather than as a representation of commercial dog food. Its use provided an animal-protein reference against which the nutritional and feeding characteristics of the sheep organ-based formulations could be evaluated. Moreover, preparing the control and experimental diets under the same processing conditions minimized potential confounding effects associated with differences in commercial formulations, ingredient matrices, additives, and manufacturing processes. Therefore, the principal experimental comparison focused on animal-protein sources within similarly processed formulations, particularly horse meat versus sheep lung and/or heart.

2.3. Preparation of Dog Food

Raw materials were minced using a meat grinder equipped with a 5-mm plate and thoroughly mixed with the dry ingredients until a homogeneous mixture was obtained. The mixtures were shaped into uniform pellets and dried using a laboratory-scale hot-air dryer. Drying was continued until the moisture content of all formulations was below 10%, ensuring adequate shelf stability. After drying, samples were cooled to room temperature, packaged in airtight polyethylene bags, and stored at ambient temperature until analysis.

2.4. Proximate Composition

The proximate composition of the experimental diets was determined using standard analytical procedures described by AOAC International [AOAC International, 2019]. Moisture content was determined by oven-drying to constant weight, crude protein by the Kjeldahl method based on total nitrogen determination, crude fat by Soxhlet solvent extraction, crude ash by incineration in a muffle furnace, and crude fiber by the standard gravimetric procedure. All analyses were performed on representative samples of the dried experimental diets, and the results were expressed on a dry-matter basis where appropriate.

2.5. Dogs, feeding assessment, and palatability test

Ten clinically healthy adult dogs (five males and five females), 2–4 years of age, with a reported mean body weight of 17.8 ± 1.3 kg and body condition score of 4–5 on a 9-point scale, participated in the feeding assessment. Dogs underwent veterinary examination, routine vaccination, and deworming before the study. Feed was offered twice daily and water was available ad libitum. Daily food offered and refused were recorded. The source file reports a 30-day period comprising 23 days of adaptation and 7 days of observation.

2.6. Amino Acid Analysis

The amino acid composition of the experimental diets was determined following acid hydrolysis of representative dried samples, followed by chromatographic separation and quantification using high-performance liquid chromatography (HPLC). The analytical approach was based on established procedures for determining amino acid composition in animal-derived feed and food materials [4,13]. Essential amino acids, including lysine, methionine, leucine, isoleucine, valine, threonine, and phenylalanine, were quantified. Amino acid concentrations were normalized to the protein content of each sample and expressed as milligrams per gram of protein (mg/g protein), allowing comparison of protein amino acid composition among the experimental diets.

2.7. Mineral Analysis

Mineral concentrations in the experimental diets were determined following acid digestion of representative dried samples. The digested samples were analyzed using inductively coupled plasma–optical emission spectrometry (ICP-OES), a multi-element analytical technique commonly used for the determination of macro- and trace minerals in biological and food materials. The analytical procedure and interpretation of mineral composition were based on established approaches for animal-derived tissues and feed materials [14,25]. Iron (Fe), zinc (Zn), calcium (Ca), phosphorus (P), magnesium (Mg), potassium (K), and sodium (Na) were determined, and mineral concentrations were expressed as mg/kg dry matter.

2.8. Feeding Trial and Palatability Evaluation

Feeding performance was evaluated in healthy adult dogs during a 30-day feeding trial. Animals were offered the experimental diets twice daily, and voluntary feed intake was recorded throughout the study. Body weight was recorded at the beginning and end of the feeding period as a supportive indicator of short-term feeding performance rather than as a measure of growth promotion. Palatability was assessed using a two-bowl preference test in which each dog was simultaneously offered the control diet and one experimental diet. First choice, feed intake from each bowl, and preference ratio were recorded.

2.9. Statistical Analysis

All measurements were performed in triplicate unless otherwise stated. Data were analyzed using one-way analysis of variance (ANOVA), followed by Tukey’s multiple comparison test to determine differences among dietary treatments. Statistical significance was accepted at P < 0.05. Results are presented as mean ± standard deviation (SD).

3. Results

3.1. Proximate composition of hot-air dried experimental dog foods

The formulation and ingredient composition of the experimental diets are presented in Table 1. The four diets differed primarily in their animal-derived protein sources, with horse meat used in the control diet and sheep lung, sheep heart, or a combination of sheep lung and heart incorporated into the experimental diets. The proportions of oat and corn flour were adjusted according to the respective formulations. These differences in ingredient composition provided the basis for evaluating the effects of organ type on the nutritional composition, feeding performance, and palatability of the experimental diets.

The proximate composition of the four experimental dog food formulations is presented in Table 2. Significant differences were observed among dietary treatments in crude protein, crude fat, ash, and crude fiber contents (P < 0.05), whereas moisture content did not differ significantly among groups (P > 0.05). The crude protein content ranged from 34.8% to 42.3%. The sheep heart diet (SH) showed the highest protein concentration (42.3 ± 1.4%), followed by the combined sheep lung–heart diet (SLH; 40.5 ± 1.1%), sheep lung diet (SL; 37.6 ± 1.0%), and control diet (34.8 ± 1.2%). Statistical analysis demonstrated that SH and SLH diets had significantly higher crude protein contents than the control formulation (P < 0.001). This improvement was attributed to the high proportion of muscle-derived protein in sheep heart tissue. Crude fat content also differed significantly among treatments (P = 0.003). The SH diet showed the highest lipid concentration (15.6 ± 0.8%), while the control diet showed the lowest value (12.5 ± 0.6%). The increased lipid content in heart-containing diets may be associated with intramuscular lipid deposition and cellular membrane components present in cardiac tissue. Moisture levels remained below 10% in all dried formulations, ranging from 7.8 to 8.9%, indicating effective dehydration and improved storage stability. Ash content was significantly higher in organ meat-based diets compared with the control group (P = 0.002), suggesting increased mineral contribution from sheep tissues (Table 2).

Mineral composition results are presented in Table 3. Significant differences were observed in iron and zinc concentrations among dietary treatments (P < 0.05). The SH diet demonstrated the highest iron concentration (86.4 ± 4.2 mg/kg), followed by SLH (79.8 ± 3.9 mg/kg), SL (68.5 ± 3.5 mg/kg), and control (55.7 ± 3.1 mg/kg). The increased iron concentration in heart-containing diets was attributed to the abundance of heme-associated proteins in cardiac tissue. Zinc concentration showed a similar trend, with SH and SLH diets showing significantly higher values than the control diet. Calcium and phosphorus concentrations were relatively stable among treatments, indicating that organ meat inclusion primarily influenced trace mineral composition rather than major mineral balance.

The selected essential amino acids are summarized in Table 3.The inclusion of sheep heart significantly improved essential amino acid concentrations compared with the control and sheep lung diets. The highest lysine concentration was observed in SH (78.5 ± 2.6 mg/g protein) and SLH (76.9 ± 2.3 mg/g protein), whereas the control diet showed the lowest value (68.4 ± 2.1 mg/g protein). Methionine concentration increased significantly in heart-containing formulations. SH showed the highest methionine concentration (24.8 ± 1.1 mg/g protein), representing approximately a 27% increase compared with the control. Leucine, valine, and threonine followed similar patterns. These results indicate that sheep heart inclusion improved the essential amino acid balance of dried dog food formulations (Table 3).

3.2. Feeding trial performance

The feeding performance results are summarized in Table 4. Initial and final body weights did not differ significantly among dietary treatments (P = 0.964 and P = 0.873, respectively). Similarly, no significant between-diet differences were observed in daily feed intake (P = 0.327) or body weight gain (P = 0.118). Although mean body weight increased numerically during the feeding period in all dietary treatments, with mean body weight changes ranging from +0.5 to +0.8 kg, these values represent descriptive changes because formal within-diet comparisons of initial and final body weight were not performed. Therefore, the observed increases should not be interpreted as statistically significant within-diet changes.

3.3. Palatability evaluation

Palatability results are presented in Table 5. Significant differences were observed in preference ratio among diets (P < 0.001). The SH diet showed the highest two-bowl preference ratio (1.48 ± 0.16), followed by SLH (1.42 ± 0.15). The control diet showed a preference ratio close to 1.0. The first-choice preference test showed that 34% of dogs selected SH first, indicating enhanced attractiveness of heart-containing formulations. These findings suggest that sheep heart incorporation improved sensory acceptance, potentially due to increased concentrations of flavor-active compounds generated during thermal processing.

4. Discussion

4.1. Nutritional potential of sheep heart and lung as sustainable protein ingredients

The present study demonstrates that sheep heart and lung are promising alternative protein ingredients for hot-air dried dog food. Growing interest in sustainable pet food production has encouraged the utilization of livestock by-products that provide high nutritional value while improving resource efficiency and reducing processing waste [6,7,8,9,10,33,34,35]. Rather than being regarded as secondary products, edible organs should be considered valuable nutritional resources that support circular bioeconomy principles. Among the evaluated formulations, diets containing sheep heart exhibited superior crude protein concentrations and a more favorable selected essential amino acids than the lung-only diet. This finding agrees with previous studies reporting that muscle-derived organs contain proteins of higher biological value because of their abundance of contractile proteins such as actin and myosin [12,15,18]. Consequently, the greater protein concentration observed in heart-containing diets is consistent with the biochemical composition of cardiac muscle rather than simply reflecting a higher inclusion level of organ tissue. In contrast, sheep lung contributed less to protein quality but represented a valuable source of minerals and structural proteins. Although lung contains a greater proportion of connective tissue than skeletal or cardiac muscle, it provides micronutrients associated with vascular tissues that may enhance the overall nutritional profile of formulated diets [13,14]. The combined sheep lung–heart formulation therefore appears to provide complementary nutritional benefits by integrating the superior protein quality of heart with the mineral contribution of lung. These findings support previous reports indicating that combining animal tissues with different biochemical characteristics is an effective strategy for improving the nutritional quality of companion animal diets [36,37,38,39,40]. From a practical perspective, the utilization of sheep organs also increases the value of underused slaughter by-products while reducing waste, thereby contributing to more sustainable livestock production systems.

4.2. Effect of hot-air drying on nutritional quality

Hot-air drying effectively reduced moisture content below 10% in all formulations, indicating adequate dehydration for stable dry pet food production. Moisture reduction is essential because it limits microbial growth and slows enzymatic deterioration, thereby extending shelf life [21,22,23,24]. The consistently low moisture contents obtained in this study suggest that the selected drying conditions were appropriate for producing shelf-stable products without compromising product quality. Thermal processing inevitably influences nutrient stability, particularly proteins and lipids. Excessive heating may promote protein denaturation, lipid oxidation, and degradation of heat-sensitive compounds [25,26]. However, the relatively high protein concentrations observed after drying indicate that the applied process preserved the nutritional integrity of the formulations. Similar observations have been reported for dried meat products processed under controlled hot-air drying conditions [21,22,23,24].

In addition to preserving stability, thermal processing may improve sensory quality. Heating promotes Maillard reactions between amino acids and reducing sugars, producing volatile aroma compounds associated with roasted meat flavors that are attractive to dogs [27]. This mechanism provides a plausible explanation for the higher preference observed for organ meat-containing diets, particularly those formulated with sheep heart.

4.3. Mineral enrichment through sheep organ incorporation

Mineral composition is an important determinant of pet food quality because minerals are essential for oxygen transport, enzyme activity, immune function, antioxidant defense, and skeletal development [41,42,43,44,45]. In the present study, sheep organ incorporation, particularly sheep heart, increased dietary iron and zinc concentrations compared with the control diet. This finding is consistent with previous reports showing that edible organs contain higher concentrations of bioavailable trace minerals than skeletal muscle [14,41,42,43,44,45]. The elevated iron concentration in heart-containing diets is likely attributable to the abundance of heme-associated proteins, including myoglobin, which provide highly bioavailable iron. Similarly, the increase in zinc may reflect the naturally high concentration of zinc-binding proteins in animal tissues. These findings indicate that sheep heart can improve the micronutrient density of dry dog foods without requiring additional mineral fortification. Although sheep lung contributed lower concentrations of essential amino acids, its inclusion complemented the mineral profile, suggesting that combining different organ tissues may provide a nutritionally balanced formulation.

4.4. Improvement of selected essential amino acids

Protein quality depends on amino acid balance as well as total protein concentration. Diets containing sheep heart showed consistently greater concentrations of lysine, methionine, leucine, valine, and threonine than the control and lung-only diets, supporting previous studies demonstrating that muscle-derived tissues provide proteins with superior biological value [18,19,20,46,47].

Among these amino acids, lysine and methionine are of particular nutritional importance because they support muscle protein synthesis, immune function, collagen formation, antioxidant metabolism, and methylation reactions [1,16,17]. Likewise, branched-chain amino acids such as leucine and valine play key roles in regulating protein metabolism and maintaining skeletal muscle. Therefore, the improved amino acid profile observed in heart-containing formulations suggests that sheep heart is a nutritionally valuable ingredient for premium dry dog foods. The balanced amino acid composition of the combined lung–heart formulation further indicates that blending complementary organ tissues may represent an effective strategy for optimizing nutritional quality while maximizing the utilization of livestock by-products.

4.5. Palatability and feeding performance

Palatability is a critical determinant of the commercial success of pet foods because even nutritionally balanced diets must be readily consumed by animals [28,29,30,31,32]. In this study, all formulations were accepted by dogs, while heart-containing diets demonstrated superior preference ratios and acceptance scores. These results indicate that incorporating sheep heart improved sensory attractiveness without reducing voluntary feed intake. Improved palatability may be explained by the generation of flavor-active compounds during thermal processing. Proteins, peptides, amino acids, and lipids naturally present in organ tissues participate in Maillard reactions, producing volatile compounds that contribute to desirable roasted meat aromas [27,30]. Because dogs rely heavily on olfaction during food selection, these aroma compounds likely contributed to the greater preference observed for heart-containing formulations. Despite differences in preference, daily feed intake and body weight remained similar among treatments, demonstrating that sheep organ meats can replace conventional animal proteins without adversely affecting short-term feeding performance. However, because formal within-diet paired comparisons between initial and final body weight were not performed, the observed numerical increases in body weight should be interpreted descriptively rather than as statistically significant longitudinal changes. These findings support the practical application of sheep heart and lung as alternative ingredients in commercial dry dog foods.

4.6. Study limitations and future perspectives

Several limitations should be considered when interpreting the present findings. The feeding trial involved a relatively small number of dogs and evaluated only short-term feeding performance. Consequently, the long-term nutritional effects and health outcomes of sheep organ-based diets remain to be established. Future investigations should include measurements of apparent nutrient digestibility, metabolizable energy, fecal quality, blood biochemical indicators, oxidative stability during storage, microbiological safety, and gut microbiota responses. In addition, optimization of drying conditions should be investigated to maximize nutrient retention while minimizing lipid oxidation and quality deterioration. Evaluation of other edible sheep organs, such as liver and kidney, may further improve formulation strategies and expand opportunities for sustainable pet food production.

Overall, the present findings demonstrate that sheep heart and lung represent valuable raw materials for producing nutritionally balanced, palatable, and sustainable dry dog foods, supporting the broader application of livestock by-products within circular bioeconomy systems.

5. Conclusions

In conclusion, sheep heart and lung are promising sustainable ingredients for hot-air dried dog foods. Incorporation of sheep heart significantly improved protein quality, essential amino acid composition, mineral concentration, and palatability, whereas the combined sheep lung–heart formulation provided a balanced nutritional profile with excellent feeding acceptance. These findings support the industrial utilization of Mongolian sheep organ meats as sustainable ingredients for premium dry dog foods.