1. Introduction
Goat milk and cheese have nutritional qualities relevant to human health due to the similarity between this and human milk, as well as smaller fat globules and a lower proportion of allergenic proteins when compared with cow milk [
1]. However, goat farming is mainly practiced in rural areas, in extensive production systems [
2]. In these systems, these animals are fed diets based on highly fibrous pastures of low nutritional quality, which limits their production performance [
3].
Feedlotting is an important strategy to provide the nutrients necessary for the maintenance and production of animals. This practice allows increases in yield [
4] and prioritizes the use of concentrate diets. Nonetheless, feeding is the costliest factor in animal production, accounting for up to 70% of total costs [
5]. As a strategy to reduce production costs, unconventional ingredients are used to replace ingredients of high commercial value, such as soybean meal and ground maize.
Among unconventional feed ingredients, byproducts have successfully been used to reduce the disposal of these materials into the environment. In addition, the efficient use of by-products in ruminant diets can result in the production of foods of high biological value for humans (e.g., milk and meat) [
6]. The oil palm (
Elaeis guineensis) is a plant of South African origin that stands out for its high bioenergetic potential and ease of cultivation, as it adapts well to different soil–climatic conditions [
7]. Palm kernel cake (PKC) is obtained after the oil is extracted through pressing. The chemical composition of the cake includes 96.6–105.6 g kg
−1 ether extract (EE), 143.4–169 g kg
−1 crude protein (CP), and 599–656.3 g kg
−1 neutral detergent fiber (NDF) [
8,
9,
10]. These nutritional characteristics are promising for the formulation of animal diets. In a study including the sensory evaluation of meat from goats fed diets containing PKC, greater sensory acceptance was achieved with the highest inclusion levels (240 and 360 g kg
−1 dry matter (DM)) [
11].
Thus, the use of PKC in the feeding of lactating goats is not expected to adversely affect the sensory aspects of milk and cheese, but rather to improve the quality of these products.
In this scenario, this study was developed to examine the effect of including PKC in the diet of dairy goats, at the levels of 0, 80, 160 and 240 g kg−1 DM, on the dry matter intake, fatty acid profile, milk yield and composition, and the sensory quality of their milk and Minas Frescal cheese, made from this milk.
2. Materials and Methods
2.1. Ethics Committee and Experiment Location
The experiment followed animal welfare rules; hence, the project was approved (approval no. 73/2018) by the Ethics Committee on the Use of Animals (CEUA) at the Federal University of Bahia (UFBA). The experiment was conducted in the goat-farming section of UFBA, located in the municipality of Entre Rios—BA, Brazil (11°56′31″ S, 38°05′04″ W, 162 m above sea level).
2.2. Animals, Experimental Design and Management
Twelve multiparous lactating goats were used in a triple Latin square experimental design, consisting of two squares with four Saanen goats and one square with four Anglo-Nubian goats (multiparous, average weight of 46.9 ± 9.4, average of 100 days in milk and average production of 0.7 kg day−1).
The experiment lasted 71 days, which included 15 days for the animals to acclimate themselves to the facilities, milking management and diets. The remaining 56 days were divided into four experimental periods of 14 days each, of which ten days were used for the animals to adapt to the treatments and four for data collection.
The diet was formulated according to the NRC [
12], to meet the requirements for maintenance and milk production. The experimental treatments (
Table 1) consisted of the inclusion of PKC at the levels of 0, 80, 160 and 240 g kg
−1. The diets were supplied as a total mixture, twice daily (8 a.m. and 3 p.m.). A forage:concentrate ratio of 40:60 was adopted, with maize silage used as forage.
The goats were housed in individual pens with an area of 1.5 m2, which were equipped with a drinker and a feeding trough. Water was provided in adequate quantity and quality, and feed was supplied with daily adjustments to allow around 10% orts.
Milking was performed at 7 a.m., after pre-dipping the teats with a 0.5% glycerin iodine solution. After milking, post-dipping was performed by immersing the teats in a 0.5% glycerin iodine solution. Hygiene measures for milkers were followed and the place and utensils used for milking were cleaned.
2.3. Intake
Intake was calculated as the difference between the amount of the component present in the feed supplied and in the orts.
2.4. Chemical Analysis
During the experimental period, samples of ingredients and orts were collected and dried in a forced-air oven at 55 °C for 72 h. Once dried, the samples were divided into two portions that were either ground in a Wiley knife mill into 1 mm particles for chemical composition analysis, or into 2 mm particles to determine the neutral detergent fiber (NDF) content. These samples were then used to measure the DM (934.01), ash (930.05), CP (981.10) and EE (920.39) contents, following the methodology proposed by the Association of Official Agricultural Chemists (AOAC) [
13].
Neutral detergent fiber and acid detergent fiber (ADF) were determined as proposed by Van Soest et al. [
14], with the adaptations described by Mertens [
15]. Neutral detergent fiber corrections for ash and protein (NDFap) were performed according to Sniffen et al. and Licitra et al. [
16,
17], respectively. Lignin was determined, according to the AOAC method 973.18 [
18], by immersing the ADF residue in a 72% sulfuric acid solution.
Indigestible neutral detergent fiber (iNDF) was determined by the in situ incubation of samples inside non-woven fabric (“TNT”) bags weighing 100 g m
2, following the methodology described by Valente et al. [
19]. Potentially digestible neutral detergent fiber (pdNDF) was determined as the difference between neutral detergent fiber corrected for ash and protein (NDFap) and iNDF.
2.5. Milk Composition
Milk production was determined per animal and per day during the last four days of each experimental period. After collection, milk was measured using a graduated measuring cylinder of one liter in volume.
Milk samples were collected and a portion of each was placed in a plastic bottle containing the preservative 2-bromo-2-nitropropane-1,3-diol (bronopol) for the analysis of protein, fat, lactose, urea nitrogen and total solids, using the Bentley-2000 infrared analyzer, as well as somatic cell count, using the Somacount-500 instrument. These analyses were performed at the laboratory of the Clínica do Leite at ESALQ/USP, in Piracicaba-SP, Brazil.
To determine the values of the milk components in g day−1, the percentage of each component (fat, protein, lactose and total solids) was multiplied by the volume of milk produced (g day−1).
2.6. Milk Fatty Acid Profile and Fat Quality Analysis
Milk samples were stored in airtight containers and kept frozen at −20 °C until the moment of fatty acid profile analysis. The milk was slowly thawed in a refrigerator and homogenized, and an aliquot (10 mL) was collected and centrifuged. After centrifugation, the supernatant was subjected to a fat extraction procedure with the organic solvent hexane. For the methylation of fatty acids, a basic catalyst (sodium methoxide) and an acid catalyst (acetyl chloride) were used in a two-step methylation process [
20].
Fatty acid methyl esters were quantified by gas chromatography (Focus GC-Thermo Scientific, Thermofisher, São Paulo, Brazil) with a flame ionization detector (CG-DIG) and an SP-2560 capillary column (Supelco, 100 m × 0.25 mm × 0.2 µm). Hydrogen was used as carrier gas at the rate of 1.5 mL min
−1. Detector and injector temperatures were fixed at 250 °C. The initial temperature of the column was set at 70 °C, held for 4 min, raised to 175 °C at a rate of 13 °C per minute, held for 27 min, and finally raised again up to 215 °C at a rate of 4 °C per minute and held for 31 min [
20]. The fatty acid methyl esters were identified based on the retention times of the FA 275 standard (GLC-674, Nu-Chek Prep Inc., Elysian, USA).
The obtained results were used to calculate the total saturated (SFA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids and the ratio of omega-6 (n-6) to omega-3 (n-3) fatty acids.
The indices that indicate the quality of milk fat were calculated from the equations proposed by Ulbrich and Southage [
21].
2.7. Cheese-Making Process
Milk was collected in the last four days of each experimental period for the production of Minas Frescal cheese. The milk was weighed, sieved, and stored individually in airtight containers at −20 °C.
Later on, the milk was thawed in a refrigerator and sieved again. For the manufacture of Minas Frescal cheese, the sanitary norms set forth by Ordinance no. 326 of the Brazilian Ministry of Health were followed to ensure its safety and quality for human consumption [
22].
The milk was pasteurized at a temperature of 60 °C for 30 min and then cooled in an ice bath until it reached 38 °C. The cheese-making procedures followed the steps described by Malheiros et al. [
23], with some adaptations, using potassium chloride (0.02%; Rica Nata, Piracema, Brazil), cultures provided by natural low-fat yogurt (1.8%; Nestlé, São Paulo, Brazil), sodium chloride (0.8%; Sal Lebre, São Paulo, Brazil) and liquid coagulant (CHY-MAX
®) as ingredients. The curds were placed in perforated and sterilized cylindrical molds that were kept at room temperature and turned every 1 h until the final dripping.
After production, the cheeses were kept refrigerated at 4 ± 1 °C for approximately 24 h until the physicochemical and sensory analyses were conducted. Samples were collected for physicochemical characterization, including moisture, by the gravimetric method. The cheese samples were pre-dried in a LV2000
® lyophilizer (Equipamentos Terroni Científicos, São Carlos, SP, Brazil). Ash (930.05), CP (981.10) and EE (920.39) were determined by the AOAC [
13] methods, and yield was calculated as proposed by El-Gawad and Ahmed [
24].
2.8. Sensory Analysis
Sensory evaluation was carried out through the application of a questionnaire in which five sensory attributes were assessed, namely, color, aroma, taste, texture and overall acceptability. Each attribute was assigned a score on a nine-point scale, as follows: dislike extremely (1), dislike very much (2), dislike moderately (3), dislike slightly (4), neither like nor dislike (5), like slightly (6), like moderately (7), like very much (8), and like extremely (9) (
Appendix A).
The test was conducted with 103 untrained tasters who had been previously selected to include only dairy product consumers without allergies and who were interested in participating in sensory analysis. Of the 103 participants, 65% were female and 35% male; 86.3% were aged between 18 and 30 years, 9.7% between 31 and 40 years, 2.0% between 41 and 50 years and 2.0% between 51 and 60 years. Regarding the frequency of goat cheese consumption, 81% declared that they rarely consumed it, 15% consumed it occasionally and only 4% consumed it often.
The samples corresponding to the four treatments (0, 80, 160 and 240 g kg
−1 of PKC inclusion) were offered in 50-mL cube-shaped cups with an average size of 3.4 cm
3 to each of the tasters, together with crackers and mineral water. To avoid the mixing of tastes between samples, which might interfere with the sensory analysis, the tasters were advised to ingest a cracker and some water between tastings [
25]. The samples were identified by codes with three random digits and were provided to the tasters in closed jars to maintain the sensory characteristics. The evaluation took place in the morning (between 9 a.m. and 12 p.m.).
2.9. Calculations
The atherogenicity index (AI) = [12:0 + (4 × 14:0) + 16:0]/(Σn-6 + Σn-3 + ΣMUFA n-9), where 12:0 = lauric acid; 14:0 = myristic acid; 16:0 = palmitic acid; Σn-6 = sum of omega-6 polyunsaturated fatty acids; Σn-3 = sum of omega-3 polyunsaturated fatty acids; and ΣMUFA n-9 = sum of omega-9 monounsaturated fatty acids.
The thrombogenicity index (TI) = (14:0 + 16:0 + 18:0)/(0.5 × ΣMUFA) + (0.5 × Σn-6) + (3 × Σn-3) + (Σn-3/Σn-6), where 14:0 = myristic acid; 16:0 = palmitic acid; 18:0 = stearic acid; Σn-6 = sum of omega 6 fatty acids; Σn-3 = sum of omega-3 fatty acids; and ΣMUFA = sum of monounsaturated fatty acids.
The hypocholesterolemic-to-hypercholesterolemic fatty acid ratio (h:H) was evaluated and adapted in accordance with the method used by Bessa and Santos-Silva et al. [
26,
27]: h:H = (C18:1 cis9 + C18:2 n-6 + 20:4n-6 + C22:5n-3)/(C14:0 + C16:0).
Milk yield = [(0.93 F + C − 0.1) × 1.09 × 100]/(100 − M), where F is milk fat (%), C is casein (%) and M is moisture (%).
Non-fibrous carbohydrates (NFC) were calculated by the following equation [
28]: NFC = 100 − (%NDFap + %CP + %EE + %ash).
The apparent digestibility of the nutritional components was estimated using the following formulae proposed by da Cruz et al. [
29] for small ruminants:
- (1)
adCP = 0.7934 × CP% − 0.44
- (2)
adEE = 0.9107 × EE% − 0.33
- (3)
adNDFap = {0.7877 × (NDF − LIGNIN) + [1 − LIGNIN ÷ NDF)0.85]}
- (4)
adNFC = 0.9041 × NFC% − 3.22
where: adCP = apparent digestibility of crude protein; adEE = apparent digestibility of ether extract; adNDFap = apparent digestibility of neutral detergent fiber; and adNFC = apparent digestibility of non-fibrous carbohydrates.
After calculating the apparent digestibility of the nutritional components, the following formula was used to determine the total digestible nutrients (TDN): TDN = adCP + (adEE × 2.25) + adNDFap + adNFC.
2.10. Statistical Analysis
Analyses of normality, variance, and regression were performed for the variables of cheese yield, proximate composition, fatty acid profile, AI, throTI and h:H ratio, with decomposition into linear and quadratic effects, considering a 5% probability level. For the analyses, the SAS statistical software version 9.2 (Statistical Analysis System, 2009) [
30] was used.
The mathematical model below was applied:
where Ŷij = value observed in the plot that received treatment i in replicate j; μ = overall mean; ILi = fixed effect of PKC inclusion level i (i = 0, 80, 160 and 240 g kg
−1); and Ɛij = random experimental error associated with each observation, with NID~(0, σ2) assumption.
The scores obtained in sensory analysis constituted a set of multivariate data that were arranged in a matrix (412 × 6) and interpreted using principal component analysis (PCA). For this analysis, SAS software version 9.2 (Statistical Analysis System) [
30] was used with data centered on the mean.
5. Conclusions
According to the results, the estimated inclusion of 80 g kg−1 palm kernel cake in the total diet of lactating goats is recommended, as it maintains the quality of the cheese made from the milk of these animals. Furthermore, the inclusion of palm kernel cake also improved the quality of the milk by reducing its thrombogenicity index.
Above the estimated level of 80 g kg−1 palm kernel cake in the total diet of lactating goats, the decreasing value of the parameters affects the productivity of the goats.