Effects of feeding almond hulls to light lambs on carcass characteristics and meat quality

Abstract

BACKGROUND

Almond hulls (AH) are the main by‐product of almond processing for human consumption and contain bioactive compounds that can improve meat quality. Although AH are used as feed for dairy cows in some countries, information on their potential effects on meat quality is limited. This study evaluated the effects of partly replacing conventional feeds with AH in the concentrate of light lambs on carcass traits and meat quality.

RESULTS

Thirty Manchega lambs (15 females and 15 males) were divided into three homogenous groups according to body weight and sex, and each was fed a concentrate containing 0, 60 or 120 g AH kg ⁻¹. Lambs were slaughtered at approximately 23.0 kg of body weight and carcass traits, chemical composition, pH and fatty acid (FA) profile of meat, and changes in color and lipid oxidation of meat over 6 days storage were analyzed. Inclusion of AH in the concentrate did not affect either carcass weight and conformation or meat pH and chemical composition. However, feeding AH significantly improved the meat FA profile by increasing (P < 0.05) its polyunsaturated FA (PUFA) content, which may be related to modifications of ruminal FA biohydrogenation. No significant effects of AH on meat color or lipid oxidation over the storage period were observed. Sex‐related differences were minimal, but males showed higher PUFA content and lower intramuscular fat than females.

CONCLUSION

Replacing conventional feeds with up to 120 g AH kg ⁻¹ in the concentrate of light lambs can enhance the FA profile of their meat without compromising carcass characteristics or meat composition. © 2026 The Author(s). Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

INTRODUCTION

Livestock production is driven by increasing global demand for meat, but has a multifaceted negative impact on the environment, raising the need to seek more sustainable practices in livestock farms. ¹ In this context, the meat market is increasingly demanding not only more environmentally friendly production systems, but also healthier products. One of the strategies to mitigate the negative environmental impact of meat production is reusing agroindustry by‐products in animal feeding. ² Agroindustry by‐products provide nutrients but also functional compounds that can be beneficial to livestock and improve meat quality. ³

The global demand for almonds (Prunus dulcis L.) has markedly increased worldwide in the last years because of the various health benefits of almonds for humans. ⁴ Almond processing generates several products, with almond hulls (AH) being the most abundant. Almond hulls comprise the outer covering of the seed and its hard shell and contain sugars and highly degradable fiber. Almond hulls (AH) are used as feed for dairy cows in the USA and Australia, whereas their use as feed in European countries is not common. ⁵ Previous studies ⁶ , ⁷ with heavy lambs [slaughtered at >37.0 kg body weight (BW)] reported that including up to 300 g AH kg⁻¹ in the diet had no negative effect on lamb growth and meat composition, but improved meat lipid oxidation stability. However, to our knowledge, there is no information on the effects of AH on carcass and meat quality of light lambs (slaughtered at BW < 26 kg), which are in high demand in some Mediterranean countries because of their low‐fat and mild‐flavored meat. ⁸ In a previous study, Recalde et al. ⁵ observed that up to 120 g AH kg⁻¹ can be included in the concentrate of Manchega light lambs without impairing feed intake and animal growth. This is a continuation of that study with the aim of investigating the effects of including two levels of AH (60 and 120 g kg⁻¹) in the concentrate of light lambs on carcass characteristics and meat quality.

MATERIALS AND METHODS

Animals and feeding

The trial lasted for 48 days and involved 30 Manchega lambs, 15 males (12.2 ± 0.26 kg BW) and 15 females (12.5 ± 0.24 kg BW). Within each sex, lambs were distributed into three homogeneous groups according to BW (five lambs per group) and one group of each sex was randomly assigned to one of the three experimental concentrates by generating random numbers in Excel (Microsoft, Redmond, WA, USA). The experimental concentrates included 0 (Control), 60 (AH6) or 120 g (AH12) of AH kg ⁻¹. The AH were obtained from an almond processing plant located in the South of Spain (Écija, Sevilla) and were spread on a smooth concrete surface at room temperature for initial aeration. Then, the AH were maintained under constant ventilation for 2 weeks until reaching approximately 840 g dry matter (DM) kg ⁻¹ and milled. The control concentrate contained 650 g kg⁻¹ of cereal grains (corn, wheat and barley). Barley grains and wheat bran were partially replaced with AH in the AH6 and AH12 concentrates, which also included urea and increased amounts of calcium soap to obtain three isonitrogenous and isoenergetic concentrates (1.95 Mcal net energy for growth kg⁻¹ DM). The chemical composition and fatty acid (FA) profile of concentrates, the AH included in AH6 and AH12 concentrates, and the barley straw fed to lambs are shown in Table 1. The levels of AH were selected from the results of an in vitro study conducted in our laboratory (Recalde A., de Evan, T. Carro M.D.) and from previous studies with fattening lambs. ⁶ , ⁷ Lambs were housed individually in pens (1 × 1 m) with slated floor and were fed concentrate and barley straw ad libitum. Fresh water was always available. More details on animals and experimental protocols are provided in Recalde et al. ⁵

Table 1.

Feed ingredients of experimental concentrates, chemical composition and fatty acid (FA) profile of experimental concentrates, barley straw and the almond hulls (AH) included in the AH6 and AH12 concentrates

Item	Concentrate			Barley straw	AH
	Control	AH6	AH12
Ingredient (g kg−1 fresh matter)
Corn	330	330	330
Wheat	100	100	100
Barley	220	188	156
Soybean meal 47%	110	110	110
Sunflower meal	80.0	80.0	80.0
Wheat bran	120	80.0	40.0
Almond hulls	–	60.0	120
Urea	–	1.70	3.50
Calcium soap	14.0	24.0	34.0
Others a	26.0	26.0	26.0
Chemical composition [g kg−1 dry matter (DM), except as otherwise noted]
DM (g kg−1 fresh matter)	890	886	880	921	847
Ash	49.1	57.8	67.7	78.2	162
Crude protein	145	144	142	57.2	64.5
Ether extract	34.8	46.4	55.3	18.0	54.8
Neutral detergent fiber	199	202	202	686	347
Acid detergent fiber	69.6	84.7	95.1	422	279
Non‐structural carbohydrates b	572	550	533	161	372
FA (g 100 g−1 total FA)
16:0	24.5	27.9	30.0	31.0	13.4
17:0	0.00	0.10	0.12	0.00	0.32
18:0	3.07	3.37	3.86	3.87	4.45
20:0	0.42	0.44	0.46	4.18	1.07
22:0	0.26	0.24	0.22	5.99	1.06
24:0	0.23	0.17	0.20	3.70	0.54
Total saturated FA	28.5	32.2	34.9	48.8	20.9
c9‐18:1	26.9	29.5	31.5	16.8	49.7
C11‐20:1	0.51	0.40	0.35	0.00	0.00
Total monounsaturated FA	27.4	29.8	31.9	16.8	49.7
18:2n‐6	41.3	35.5	31.1	25.3	24.4
18:3n‐3	2.82	2.34	2.10	9.11	5.51
Total polyunsaturated FA	44.1	37.9	33.2	34.4	29.5

^a For all concentrates: 13.0 of calcium carbonate, 8.00 of sodium bicarbonate, 3.00 of salt and 2.00 of mineral‐vitamin premix kg⁻¹ fresh matter.

^b Calculated as 1000 – (ash + crude protein + neutral detergent fiber + ether extract).

Slaughter, carcass measurements and sampling

When reaching approximately 23.0 kg BW, lambs were slaughtered at a commercial slaughterhouse on 2 days after 46 and 48 days of fattening. The slaughterhouse was located 30 km away from the experimental farm, the lairage time was 30 min, and the slaughter procedure followed commercial practices in Spain consisting in electrical stunning followed by cutting both carotid arteries and jugular veins. After evisceration, the carcasses were weighed before and after being stored in a refrigerated chamber (4 °C) for 24 h. Carcass measurements were then performed as previously described by Cañeque et al. ⁹ and Haro et al. ¹⁰ Hind limb length was the length from the perineum to the most distal point of the medial edge of the tarsal‐metatarsal articular surface; thoracic depth was the maximum distance between the sternum and the back of the carcass at the level of the sixth thoracic vertebra; carcass width was the maximum width of the carcass at the level of the ribs; buttock width was the widest buttock measurement in a horizontal plane on the hanging carcass; buttock perimeter was measured at the level of the trochanters of both femurs; and carcass internal length was the distance between the anterior edge of the ischiopubic symphysis and the anterior edge of the first rib at its midpoint. Classification of fatness degree was carried out following Regulations (EEC) No. 2137/92 and 461/93 for light lambs' carcasses of less than 13 kg, on a scale from 1 (low), 2 (slight), 3 (average) to 4 (high). Pelvic‐renal fatness evaluation was performed according to the method proposed by Colomer‐Rocher et al., ¹¹ using a three‐point scale with three subdivisions within each point (−1, 1, 1+, −2, 2, 2+, −3, 3, 3+).

The pH of the longissimus thoracis (13th rib level) and semitendinosus muscles was measured in the carcass immediately after slaughtering using a penetrating electrode adapted to a portable pH‐meter with a temperature probe for automatic pH compensation (pH meter HI‐9025; Hanna Instruments SL, Éibar, Spain) previously calibrated with pH 7.0 and pH 4.0 buffers. These measurements were repeated after 24 h of refrigeration in a cold chamber (4 °C) and two measurements were made for each muscle at each time.

The longissimus lumborum muscles of each lamb, the section between the first and fourth lumbar vertebrae; right‐ and left‐side, were dissected and divided into two portions. A portion from the right‐side muscle was frozen and freeze‐dried for analysis of protein, fat and ash and FA profile and the other portion was frozen for moisture analysis. Moreover, the two portions from the left side were used for determining the changes in color and lipid oxidation over a 6‐day storage period. These meat portions were placed on extruded polystyrene foam trays, overwrapped with an oxygen‐permeable polyvinyl chloride film and stored in darkness at 2 °C. On days 0 (1 h after cutting) and 6, the color was measured on the cut surface of one of the portions as described below. Finally, at each time a subsample of meat was taken and immediately vacuum packaged and frozen (−20 °C) until analysis of lipid oxidation.

Chemical composition and fatty acid profile in feeds and meat

The chemical composition of dietary feeds (both concentrate and barley straw) was performed as described by Recalde et al. ⁵ Meat chemical analyses were performed in longissimus lumborum muscle in duplicate. For DM analysis, 5 g of meat was homogenized in a crucible with 10 g of sea sand, then 5 mL of ethanol was added and samples were dried at 102 °C for 24 h. The ash and ether extract (EE) content of freeze‐dried samples of longissimus lumborum, previously dried at 102 °C for 4 h, were analyzed following the Association of Official Analytical Chemists procedures ¹² (ID 048.13 and 945.16, respectively), whereas the protein content was determined by the Dumas combustion method employing a Leco FP258 N Analyzer (Leco Corporation, St Joseph, MI, USA).

The FA profile in feeds (concentrates, AH and barley straw) and lamb meat was analyzed from 250 mg of concentrate, 400 mg of AH and barley straw or 250 mg of previously freeze‐dried muscle homogenates. The samples were submitted to in situ transesterification. ¹³ Analysis of FA was then carried out with a 7890A gas chromatograph coupled to a 5975C mass spectrometer (Agilent Technologies; Palo Alto, CA, USA) equipped with a HP 88 column (100 m × 0.25 mm × 0.20 mm film thickness) as described by Liu et al. ¹⁴ Helium (3 mL mi^n–1) comprised the carrier gas, and the injector, transfer line and detector were set at 200, 230 and 300 °C, respectively. The sample extract (2 μL) was injected in a 30:1 split ratio mode. The oven program was 170 °C (maintained for 24 min), increased to 220 °C at 7.5 °C min⁻¹ and to 230 °C at 10 °C min⁻¹ (maintained for 5 min). The concentrations of the FA methyl esters were calculated as g FA 100 g⁻¹ total FA and then converted into mg FA 100 g⁻¹ of meat using the lipid conversion factor for lean lamb (0.916) and the fat content of the muscle. ¹⁵ For brevity, only FA with values exceeding 10 mg 100 g⁻¹ of meat are reported, except for 20:5n‐3, 22:5n‐3 and 22:6n‐3, which were included because of their nutritional interest. It should be noted, however, that the total FA sum and the calculated ratios account for all detected FA.

Meat color changes and lipid oxidation

Color measurements at 0 and 6 days of storage were made at 3 different locations of the cut surface of the meat portion perpendicularly to muscle fiber direction using a Konica Minolta Spectophotometer CM‐2600d (Minolta, Osaka, Japan) and the following conditions: illuminant D65, 10° observed angle and 8 mm measurement area. The color coordinates were expressed following the CIELAB Color space system ¹⁶ as L* (brightness), a* (red–green index) and b* (yellow–blue index).

Lipid oxidation was assessed by measuring the concentrations of thiobarbituric acid reactive substances (TBARS) in longissimus lumborum samples at 0 and 6 days of storage. This method is based on the reaction of thiobarbituric acid with malondialdehyde (MDA) and was performed as described by Maraschiello et al. ¹⁷ The extract absorbances were measured using a model Evolution 220 spectrophotometer (Thermo Fisher Scientific, Madrid, Spain) at 532 nm and the results were expressed as mg MDA kg⁻¹ meat.

Statistical analysis

Data on carcass traits and meat composition were analyzed by analysis of variance with the fixed effects of concentrate (Control, AH6 and AH12), sex and their interaction. Analysis of covariance (ANCOVA) was conducted for FA using the intramuscular fat content as covariable and with the above‐mentioned fixed effects. The ANCOVA was used as a result of the high correlation between individual FA and intramuscular fat content to avoid the effect of the amount of intramuscular fat. ANCOVA adjusts for this baseline difference. Values of meat pH, color and lipid oxidation were analyzed as a mixed model with repeated measures, in which the effects of concentrate, sex, time (0 and 6 days) and their interactions were considered fixed, and the effect of lamb was considered random.

The PROC MIXED of the statistical package SAS, version 9.3 ¹⁸ was used for the statistical analyses. P < 0.05 was considered statistically significant. When a significant effect of concentrate was detected, means were compared using Tukey's test. Because of the strong correlation between the amount of intramuscular fat and its constituent FA, ¹⁹ intramuscular fat was included as a covariate to account for the effects of fat content on meat FA concentrations.

RESULTS AND DISCUSSION

The AH used in this trial had 49.7% of 18:1 c‐9, 24.4% of 18:2n‐6 and 13.4% of 16:0 acids (Table 1), confirming that these FA are the most abundant in AH. ²⁰ The high content in 16:0 (50%) and c9‐18:1(35%) in the calcium soap (Magnapac®; Norel SL, Madrid, Spain) explains the increased amounts of both FA observed in the AH6 and AH12 concentrates compared to the control concentrate (Table 1) because the amount of calcium soap was increased in these concentrates to make them isoenergetic to control concentrate.

Carcass measurements

The inclusion of AH in the concentrate did not affect (P > 0.05) hot and cold carcass weights and carcass yield (Table 2) and no concentrate × sex interactions were detected (P > 0.05). These results have already been previously reported ⁵ and are consistent with those reported in other studies, ⁶ , ⁷ , ²¹ such that feeding up to 300 g of AH kg⁻¹ to heavier fattening lambs had no effect on these carcass characteristics.

Table 2.

Slaughter weight, carcass yield and carcass conformation in growing lambs fed barley straw and a concentrate without almond hulls (Control), with 60 (AH6) or with 120 (AH12) g of almond hulls kg⁻¹

Item	Concentrate			SEMc a	Sex		SEMs a	P
	Control	AH6	AH12		Female	Male		Concentrate	Sex	Concentrate × Sex
Hot carcass weight (kg)	10.9	10.7	10.6	0.34	10.6	10.8	0.28	0.735	0.588	0.113
Cold carcass weight (kg)	10.7	10.4	10.4	0.32	10.4	10.6	0.26	0.655	0.518	0.092
Carcass yield (g g−1)	0.458	0.457	0.469	0.045	0.467	0.456	0.036	0.194	0.062	0.152
Chilling losses (%)	1.86	2.37	2.16	0.187	2.28	1.98	0.153	0.175	0.183	0.383
Carcass conformation
Buttock width (cm)	17.5	17.1	17.3	0.23	17.2	17.4	0.19	0.564	0.379	0.131
Buttock perimeter (cm)	50.4	50.6	50.5	0.68	50.7	50.2	0.55	0.978	0.505	0.201
Thoracic width (cm)	17.3	17.4	17.4	0.45	17.3	17.4	0.37	0.986	0.816	0.260
Internal carcass length (cm)	55.7	55.1	54.3	0.35	54.7	55.2	0.28	0.053	0.188	0.086
Thoracic depth (cm)	23.0	22.2	22.5	0.58	22.4	22.7	0.47	0.621	0.620	0.529
Leg length (cm)	23.6	24.0	23.1	0.27	23.6	23.5	0.22	0.095	0.536	0.586
Carcass fatness	2.53	2.11	2.16	0.174	2.41	2.12	0.142	0.218	0.165	0.085
Pelvic‐renal fatness	2.43	2.20	1.97	0.149	2.37	2.03	0.122	0.131	0.067	0.013

^a SEMc, standard error of the mean for concentrate effect (n = 10) and SEMs for sex effect (n = 15).

No differences (P > 0.05) in any carcass conformation measurement were detected among groups (Table 2), although both carcass internal length and leg length tended to decrease (P = 0.053 and 0.090, respectively) in lambs fed the AH concentrates compared to those fed the control concentrate. These results are consistent with the lower, although not significant, final BW of lambs fed the AH concentrates reported by Recalde et al. ⁵ (23.5, 22.8 and 22.1 kg for Control, AH6 and AH12 lambs, respectively). There were no differences (P > 0.05) among groups in carcass fatness or pelvic‐renal fatness.

No effects of sex were observed on any carcass trait, except that females tended to have greater carcass yield (P = 0.062) and more pelvic‐renal fat (P = 0.067) than males (Table 2). Females usually deposit larger amounts of fat at an earlier age than males ²² and have even been reported to have greater expression of lipogenic genes than males. ²³ Among all carcass traits, only pelvic‐renal fatness showed a significant concentrate × sex interaction (P = 0.013) because females had greater (P < 0.05) amounts of pelvic‐renal fat than males when Control (2.93 vs. 1.93) and AH12 (2.20 vs. 1.75) concentrates were fed, although the opposite was observed for lambs fed the AH6 concentrate (2.00 vs. 2.41).

No significant effects of AH were observed in the meat pH at any measurement time (P > 0.05) (Table 3) and after 24 h chilling pH reached normal values for light lamb meat. ¹⁰ , ²⁴ The pH of longissimus thoracis muscle was slightly greater (P = 0.022) in males than in females. Although the results of some previous studies ²⁵ , ²⁶ agree with our findings, others ²⁷ , ²⁸ reported no significant differences between sexes in the pH of meat from light lambs. The higher pH in males than in females has been attributed to greater activity or greater pre‐mortem stress and the consequent lower glycogen content. No significant sex × concentrate interactions were observed on meat pH (P > 0.05) and there were no interactions (P > 0.05) of time with either concentrate or sex.

Table 3.

Meat pH values (at slaughter (0) and after 24 h chilling) in the longissimus thoracis and semimembranosus muscles of growing lambs fed barley straw and a concentrate without almond hulls (Control), with 60 (AH6) or with 120 (AH12) g of almond hulls kg⁻¹

Item	Time	Concentrate			SEMc a	Sex		SEMs a	P b
		Control	AH6	AH12		Female	Male		Concentrate	Sex	Time	Concentrate × Sex	Sex × Time	Concentrate × Time
Longissimus thoracis	0 24	6.73 5.54	6.73 5.73	6.88 5.64	0.034	6.73 5.63	6.83 5.71	0.028	0.369	0.022	<0.001	0.300	0.870	0.287
Semitendinosus	0 24	6.35 5.71	6.37 5.88	6.46 5.76	0.316	6.37 5.70	6.42 5.86	0.258	0.467	0.108	<0.001	0.116	0.402	0.432

^a SEMc, standard error of the mean for concentrate effect (n = 20) and SEMs for sex effect (n = 30).

^b No concentrate × sex × time interactions were detected for any treatment (P > 0.05).

Chemical composition and fatty acid profile of meat

Intramuscular fat is the muscle component with the greatest effect on its eating quality. ²⁹ As shown in Table 4, there were no differences (P < 0.05) between groups in meat chemical composition, which agrees with previous results in heavier lambs fed different levels of AH. ⁶ , ⁷ Males meat had greater (P = 0.016) moisture content and tended to lower protein and fat content (P = 0.082 and 0.057, respectively) than females meat, but no concentrate × sex interactions (P < 0.05) were observed in meat composition.

Table 4.

Chemical composition of the meat (longissimus lumborum muscle) of growing lambs fed barley straw and a concentrate without almond hulls (Control), with 60 (AH6) or with 120 (AH12) g of almond hulls kg⁻¹

Item	Concentrate			SEMc a	Sex		SEMs a	P
	Control	AH6	AH12		Female	Male		Concentrate	Sex	Concentrate × Sex
Moisture (g kg−1)	737	739	748	5.9	733	751	4.9	0.393	0.016	0.910
Ash (g kg−1)	10.4	9.90	10.5	0.31	10.1	10.5	0.26	0.459	0.394	0.788
Protein (g kg−1)	212	222	217	4.2	222	213	3.4	0.235	0.082	0.753
Fat (g kg−1)	39.6	33.1	29.5	3.71	38.4	29.7	3.03	0.187	0.057	0.382

^a SEMc, standard error of the mean for concentrate effect (n = 10) and SEMs for sex effect (n = 15).

The most abundant FA in all lamb groups was c9‐18:1, followed by 16:0 and 18:0 (Table 5), which is consistent with previous observations on lamb meat. ³⁰ , ³¹ No concentrate × sex interactions (P > 0.05) were detected for any FA or the calculated sums and ratios. Feeding the concentrates with AH significantly decreased (P < 0.05) the amounts of odd‐ and branched‐chain FA compared to Control lambs, without differences (P > 0.05) between the two levels of AH. These FA are minor FA specific to ruminant products (meat and milk) and are closely related to rumen fermentation. They originate primarily from microorganisms in the rumen, although odd‐chain FA can also be endogenously synthesized from rumen‐formed propionic acid. ³² , ³³ Therefore, the variation in branched‐ and odd‐chain FA may be related to dietary differences that influence ruminal fermentation. However, identifying a clear mechanism and predicting a well‐defined relationship between the experimental diets and the odd‐ and branched‐chain FA content in lamb meat appears to be a complex task because the underlying mechanism is intricate and currently not fully understood. The main dietary differences potentially influencing the variation in rumen fermentation in the present study were the non‐structural carbohydrate content, which was slightly higher in the Control concentrate (Table 1) and the distinct fiber composition of the concentrates, as AH replaced barley and wheat bran in the Control concentrate. Higher levels of non‐structural carbohydrates in lamb diets ³⁴ or a lower concentration of ruminal cellulolytic bacteria ³⁵ have been associated with increased amounts of odd‐chain FA in lamb meat, which is consistent with the higher odd‐chain FA content observed in the Control lambs in our study.

Table 5.

Fatty acid (FA) content of meat (mg FA per 100 g longissimus lumborum muscle) for an intramuscular fat content of 3.54 g 100 g⁻¹ in growing lambs fed barley straw and a concentrate without almond hulls (Control), with 60 (AH6) or with 120 (AH12) g of almond hulls kg⁻¹

FA	Concentrate				Sex			P
	Control	AH6	AH12	SEMc	Female	Male	SEMs	Concentrate	Sex	Concentrate × Sex
12:0	12.6	17.6	15.4	2.51	14.6	15.8	2.05	0.283	0.636	0.716
14:0	57.4	60.0	59.1	11.00	57.3	60.3	8.98	0.881	0.503	0.900
15:0	20.6 a	13.7 b	11.0 b	4.25	13.3	16.9	3.47	0.037	0.238	0.479
16:0	586 b	623 ab	644 a	84.6	587	599	69.1	0.042	0.467	0.875
17:0	105 a	68.4 b	57.1 b	17.80	74.1	79.6	14.53	0.029	0.700	0.243
18:0	476	499	472	60.3	484	480	49.2	0.766	0.921	0.480
Anteiso‐17:0	30.8 a	22.1 b	18.6 b	5.55	21.5	26.1	4.53	0.033	0.208	0.367
Anteiso‐18:0	12.7	12.5	13.4	1.47	12.3	13.5	1.20	0.763	0.300	0.159
c9‐16:1	34.4	25.6	29.7	5.16	33.6	26.2	4.21	0.413	0.205	0.901
c9‐17:1	63.1 a	36.0 b	33.5 b	6.48	47.9	40.5	5.29	< 0.001	0.166	0.337
t10‐18:1 + t11‐18:1	184	208	210	42.2	189	211	34.5	0.686	0.415	0.255
c9‐18:1	939 a	854 b	825 b	91.1	922	824	74.4	0.016	0.004	0.311
c11‐18:1	109	93.5	90.8	11.77	103	92.5	9.61	0.161	0.207	0.571
c12‐18:1	14.5	10.2	9.9	2.294	12.4	10.6	1.873	0.069	0.311	0.284
18:2n6	254 b	338 a	367 a	34.1	296	344	27.8	0.002	0.046	0.978
18:3n3	11.5	12.7	13.9	1.95	11.9	13.5	1.60	0.248	0.158	0.920
20:3n9	10.7	10.9	10.3	1.331	11.0	10.3	1.087	0.960	0.687	0.363
20:4n‐6	99.6 b	118 a	132 a	10.4	109	117	8.5	0.019	0.491	0.977
20:5n‐3	2.39 b	3.30 a	3.18 a	0.243	2.98	2.93	0.199	0.017	0.845	0.386
22:4n6	11.2 b	14.4 a	15.8 a	1.21	12.7	14.9	0.99	0.014	0.068	0.971
22:5n‐3	6.77 b	10.0 a	10.6 a	0.430	8.72	9.55	0.433	0.005	0.375	0.702
22:6n‐3	0.867 b	1.57 a	1.44 a	0.131	1.36	1.22	0.107	0.004	0.358	0.061
ΣSFA	1342	1345	1320	180.9	1317	1355	147.7	0.830	0.298	0.524
ΣMUFA	1366 a	1244 b	1216 b	149.5	1326	1224	122.1	0.014	0.018	0.612
ΣPUFA	450 b	568 a	621 a	53.0	514	578	43.3	0.001	0.061	0.996
ΣPUFA/ΣSFA	0.371 b	0.456 a	0.505 a	0.0263	0.413	0.475	0.0215	0.003	0.037	0.351
Σn3	30.5 b	38.0 a	40.5 a	3.42	35.3	37.3	2.79	0.001	0.061	0.996
Σn6	370 b	487 a	534 a	44.9	444	493	36.6	0.002	0.082	0.999
Σn6/Σn3	12.4	13.0	13.3	0.61	12.3	13.5	0.50	0.626	0.149	0.821
ΣCLA	15.1	13.1	15.6	2.81	14.4	14.8	2.29	0.558	0.861	0.865
ΣBranched chain FA	65.8 a	48.5 b	46.7 b	10.72	46.7	60.7	8.76	0.034	0.033	0.358
ΣOdd FA	246 a	155 b	135 b	36.84	173	184	30.1	0.004	0.694	0.287

Note: Within values for each concentrate, means in the same row with different lowercase letters differ (P < 0.05; Tukey test). Lowercase letters are only shown when a significant (P < 0.05) effect of concentrate was detected. SEMc, standard error of the mean for concentrate effect (n = 10) and SEMs for sex effect (n = 15).

Abbreviations: CLA, conjugated linoleic acid; MUFA, monounsaturated FA; PUFA, polyunsaturated FA; SFA, saturated FA.

Both the concentration of total PUFA and the PUFA/saturated FA (SFA) ratio were greater (P < 0.05) in the meat of the lambs fed the concentrates with AH than in Control lambs. This indicates a healthier FA profile for humans, especially in AH12‐fed lambs, considering that 0.49 is the PUFA/SFA threshold ratio above which the cardiovascular risk is reduced. ³⁶ These results contrast with the lower PUFA content in the concentrates with AH than in the Control concentrate (Table 1). Differences in the major FA in meat may be related not only to FA intake, but also to other factors related to rumen metabolism, ³³ such as modifications of ruminal FA biohydrogenation. Indeed, AH contains bioactive compounds ³⁷ (e.g. phenolic acids, flavonoids and terpenoids) that can reduce the ruminal biohydrogenation process as suggested by Scerra et al. ⁶ Notwithstanding, the greater amount of PUFA in the meat of AH lambs may also be related to a greater proportion of phospholipids in the intramuscular fat because phospholipids are rich in PUFA and the amount of PUFA tends to be inversely related to intramuscular fat content. ¹⁹ In the present study, the intramuscular fat content of AH‐fed lambs was numerically lower (33.1 and 29.5 g kg⁻¹ for AH6 and AH12, respectively) than that of the control group (39.6 g kg⁻¹), although the differences did not reach the significance level (P > 0.05) (Table 4). Moreover, the Pearson correlation coefficient between the PUFA/SFA ratio and the intramuscular fat content was r = −0.65, indicating a notable negative correlation.

The concentration of both c9‐18:1 and total monounsaturated FA (MUFA) in the meat decreased (P < 0.05) by feeding AH, despite the proportion of MUFA in the concentrates increasing with the AH inclusion (Table 1), which might also be linked to differences in rumen FA biohydrogenation. Finally, including AH in the diet increased the concentrations of 20:5n‐3 (P = 0.017), 22:5n‐3 (P = 0.005) and 22:6n‐3 (P = 0.004), with no differences between the two AH groups.

By contrast to our results, other studies ⁶ , ⁷ reported no changes in the meat FA profile of lambs slaughtered at greater BW (>36 kg) when AH were fed, but, in agreement with our findings, Scerra et al. ³⁸ observed that high levels of AH (400 g AH kg⁻¹ concentrate) tended (P < 0.10) to increase PUFA and odd‐ and branched‐chain FA in the meat of Sarda lambs. Other factors as dietary ingredients, lamb breed and slaughter BW can influence the effects of AH on meat FA profile.

The sum of branched‐chain FA, the amount of 18:2n‐6 and the PUFA/SFA ratio were higher (P ≤ 0.046) in males than in females, and total PUFA content, Σn3 and Σn6 also tended (P ≤ 0.082) to be higher in males (Table 5). By contrast, the amounts of c9‐18:1 and total MUFA were higher (P = 0.004 and 0.018, respectively) in females. Meanwhile, the amount of total SFA, Σ conjugated linoleic acid (CLA) and the Σn6/Σn3 ratio were similar (P > 0.05) for both sexes. The observed differences are broadly consistent with results reported previously, ³¹ , ³⁹ showing that the meat of females had greater content of total MUFA and less PUFA than that of males. The greater PUFA content in the meat of males may be largely a result of sex differences in fattening, namely to the tendency of males to have less intramuscular fat than females (a trend was found in the present study; P = 0.057) (Table 4). This is explained by the general principle that less intramuscular fat corresponds to a higher PUFA content, as previously noted in our study. By contrast to these observations, Horcada et al. ⁴⁰ and Tsiplakou et al. ⁴¹ did not detect differences between sexes in the meat FA profile of lambs slaughtered at 90 and 35 days of age, respectively.

Meat color changes and lipid oxidation

Color is the quality trait that most influences consumers' meat purchasing decisions, with redness (a*) being the color coordinate most closely related to consumer acceptance of fresh lamb meat. ⁴² The lack of effects of AH (P > 0.05) on any color coordinate observed in our study (Table 6) is consistent with previous studies ⁶ , ⁷ in which lambs were fed up to 400 g of AH kg⁻¹.

Table 6.

Color coordinates and lipid oxidation levels (TBARS concentrations) at 24 h after slaughtering and after 6 days of refrigerated storage of longissimus lumborum muscle slices from growing lambs fed barley straw and a concentrate without almond hulls (Control), with 60 (AH6) or with 120 (AH12) g of almond hulls kg⁻¹

Item a	Time	Concentrate			SEMc b	Sex		SEMs b	P
		Control	AH6	AH12		Female	Male		Concentrate	Sex	Time	Concentrate × Sex	Sex × Time	Concentrate × Time
L*	0	47.9	47.2	47.5	1.02	48.3	46.9	0.83	0.649	0.109	< 0.001	0.003	0.343	0.867
	6	51.1	50.5	51.2		51.3	50.6
a*	0	5.94	5.27	6.91	0.210	6.20	5.87	0.171	0.385	0.981	0.417	0.169	0.209	0.015
	6	5.46	6.13	5.91		5.68	5.99
b*	0	10.4	9.63	11.2	0.535	10.6	10.2	0.437	0.375	0.978	0.015	0.080	0.212	0.256
	6	9.40	9.54	9.85		9.39	9.80
TBARS	0	0.199	0.193	0.197	0.0052	0.194	0.198	0.0041	0.056	0.725	< 0.001	0.792	0.726	0.111
	6	0.228	0.214	0.247		0.230	0.230

^a L*: brightness; a*: redness; b*: yellowness; TBARS concentrations are expressed as mg of malondialdehyde (MDA) kg⁻¹ meat.

^b SEMc, standard error of the mean for concentrate effect (n = 10) and SEMs for sex effect (n = 15) effects. No concentrate × sex × time interaction was observed for any parameter (P ≥ 0.143).

No significant effects of sex (P > 0.05) on meat color were observed and no concentrate × sex × time interactions were detected (P > 0.05). This finding is consistent with previous results ⁴⁰ in light lambs from two Spanish breeds. However, a concentrate × sex interaction was detected for L* (P = 0.003). At day 0, sex did not affect L* of the meat of lambs fed Control (49.1 and 46.9 for females and males, respectively) and AH12 concentrates (46.3 versus 48.8), but females had greater (P < 0.05) L* values than males for the AH6 group (49.5 versus 45.0). Moreover, after 6 days of storage the meat of males had greater (P < 0.05) L* values than that of females for Control and AH12 groups (51.4 versus 50.9 and 51.8 versus 50.6, respectively), but the opposite was observed for the lambs fed the AH6 concentrate (51.4 versus 52.2). The underlying reasons for this outcome are unclear.

Storage time led to a significant increase in L* values (P < 0.001) and a decrease in b* values (P = 0.015). The increase in L* indicates increased light scattering, which is influenced by structural changes in the protein matrix, drip on the meat surface or moisture content. ⁴³ The reduction in b* after 6 days of storage is consistent with previous findings in lambs' meat ⁴⁴ and could be attributed to the increased light scattering. However, the change was numerically small, suggesting limited practical relevance. During aerobic storage redness generally tends to decrease as a result of the oxidation of myoglobin, which consequently reduces consumer acceptability. ⁴² However, in our study, no significant effect of storage time on redness was observed, although a concentrate × time interaction was detected (P = 0.015). There was a decrease in the redness of the meat from lambs fed Control (from 5.94 to 5.46) and AH12 (from 6.91 to 5.94) concentrates over the 6‐day storage period, whereas redness increased numerically (from 5.27 to 6.13) in the meat of lambs fed AH6. The reasons for the high redness values after 6 days of storage in the AH6‐meat are unknown.

Concentrations of TBARS tended to be affected (P = 0.056) by the inclusion of AH in the concentrate, with the AH6 group having the lowest concentration. The effects of feeding AH to lambs on meat lipid oxidation reported previously are contradictory and appear to depend on the balance between antioxidant and pro‐oxidant factors. Thus, some studies ⁶ , ³⁸ have reported a significant reduction in TBARS concentration in lamb meat after 7 days of storage when up to 400 g of AH kg⁻¹ was included in the diet, which was attributed to the antioxidant properties of AH. By contrast, Cachucho et al. ⁷ observed no reduction in TBARS concentrations in the meat of Merino Branco lambs by feeding 180 g of AH kg⁻¹. As previously reported, ²⁴ , ⁴⁴ TBARS values increased over a 7‐day meat storage period, which can be attributed to PUFA oxidation. ⁴⁵ , ⁴⁶ Nevertheless, it should be noted that, in all groups, TBARS concentrations in the present study remained below 0.5 mg MDA kg⁻¹ meat, which is the threshold proposed by a trained sensory panel for the detection of rancidity off‐flavors. ⁴⁷ Finally, TBARS were unaffected by lamb sex (P > 0.05) and no concentrate × sex interaction was detected (P > 0.05) (Table 6).

CONCLUSIONS

Almond hulls can be included up to 120 g kg⁻¹ in the concentrate of light lambs to improve the PUFA content of meat without having negative effects on carcass quality and meat pH, color and chemical composition. However, feeding almond hulls did not reduce meat lipid oxidation during storage. The use of almond hulls in practical lamb feeding will depend largely on their cost and availability but could contribute significantly to the sustainability of lamb production systems. The effects of lamb sex on carcass traits and meat quality were subtle.

AUTHOR CONTRIBUTIONS

AR, TdE, AC, MTD‐C, JM and MDC were responsible for investigations. AR, TdE and JM were responsible for formal analysis and writing the original draft. AC, MTD‐C, JM and MDC were responsible for methodology. AC, MTD‐C, JM, RAR, SL‐F and MDC were responsible for resources. AC, MTD‐C, RAR, SL‐F and MDC were responsible for reviewing and editing. RAR, SL‐F and MDC were responsible for conceptualization and funding acquisition. MDC was responsible for supervision, project administration. All authors reviewed the final version of the manuscript submitted for publication.

CONFLICTS OF INTEREST

The authors declare that they have no conflicts of interest.

ETHICAL STATEMENT

Animal care and management followed the European regulations for experimental animal protection and complied with the ARRIVE guidelines (Animal Research: Reporting of In Vivo Experiments). All experimental procedures were approved by the Institutional Animal Care and Use Committee of the Comunidad Autónoma de Madrid (Approval number PROEX 132.6/21).

DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.