Influence of duration of storage on protein quality traits of soybean meals

Transcription

1 2013 Poultry Science Association, Inc. Influence of duration of storage on protein quality traits of soybean meals M. P. Serrano,* P. G. Rebollar,* S. Sueiro, M. Hermida, and G. G. Mateos * 1 * Departamento de Producción Animal, Universidad Politécnica de Madrid, Ciudad Universitaria, Madrid, Spain; and Laboratorio de Mouriscade, Vilanova s/n, Lalín, Pontevedra, Spain Primary Audience: Nutritionists, Plant Managers, Quality Assurance Personnel, Researchers SUMMARY Two experiments were conducted to determine the influence of duration of storage of soybean meal (SBM) on variables that define the quality of the protein fraction. Urease activity, protein dispersibility index (PDI), KOH protein solubility (KOHsol), and trypsin inhibitor activity were determined. In experiment 1, 8 samples of SBM, ranging in CP content from 55.4 to 56.5% DM, were collected from a US crushing plant at weekly intervals and analyzed at arrival to the laboratory and after 30, 60, 90, and 120 d of storage. In experiment 2, 7 samples of SBM, ranging in CP content from 49.0 to 55.0% DM, were collected from different Argentinean crushers and analyzed at arrival and after 24, 48, 80, and 136 wk of storage. In both experiments, samples were stored in hermetic glass containers in a laboratory room at 12 ± 2 C and a relative humidity of 70 ± 3%. Duration of storage did not affect urease activity or trypsin inhibitor activity values in either of the 2 experiments. However, PDI values decreased linearly with time of storage in both experiments (P < 0.001). Also, KOHsol decreased linearly (P < 0.05) with duration of storage in experiment 2 (long-term storage) but not in experiment 1 (shorter term storage). Therefore, PDI values might not be adequate to compare protein quality of commercial SBM samples that have been stored for different periods of time. The KOHsol values are less affected by length of storage of the meals under current commercial practices. Key words: KOH solubility, protein dispersibility index, soybean meal, storage, trypsin inhibitor activity, urease activity 2013 J. Appl. Poult. Res. 22 : DESCRIPTION OF PROBLEM Soybean meal (SBM) is a valuable protein source in poultry diets because of its amino acid profile and high CP content. However, the nutritional value of SBM varies among batches depending on the presence of antinutritional factors (ANF), such as trypsin inhibitors (TI) and saponins [1, 2]. Processing conditions of the beans, including time, temperature, moisture, and friction, inactivates to a certain extent the heat-labile ANF of SBM by denaturing the proteins [3, 4] and exposing new sites for enzymatic hydrolysis [5, 6]. When the thermal process is insufficient, part of the TI present in the SBM will remain intact and the integrity of the diges- 1 Corresponding author: gonzalo.gmateos@upm.es

2 424 JAPR: Research Report tive mucosa, incidence of enteric problems, and bird performance will be affected [7]. Excessive heating will reduce to a minimum the concentration of TI but will result in increased Maillard reactions, which reduces Lys availability [8 10] and, to a lesser extent, that of other amino acids [10, 11]. Therefore, the challenge in SBM processing consists in applying the optimal amount of heat to reduce to a minimum the presence of heat-labile ANF while minimizing the incidence of Maillard reactions. Direct analyses of the ANF and Maillard compounds present in SBM are expensive and time consuming in routine operations and, consequently, indirect tests, such as urease activity (UA) [12, 13], protein dispersibility index (PDI) [14, 15], and KOH solubility (KOHsol) [16, 17], as well as direct measurement of TI activity (TIA) [18 20], are routinely used to estimate the quality of the protein fraction. Urease activity might have some merit to evaluate under-processed SBM but not for over-processed SBM, because the UA scale only has positive values [21]. Similarly, high TIA values are indicative of under-processed SBM, but low values indicate that the sample was either well-processed or over-processed [2, 22]. The PDI and KOHsol methods measure the decrease in solubility of the protein fraction of the ingredient in response to heat processing and, consequently, values will vary with the conditions (i.e., grinding, stirring, temperature, ph of the solution, and time) applied during analytical determination [23]. In general, values below 0.05 mg of N/g min for UA, between 15 and 30% for PDI, and between 78 and 85% for KOHsol are considered acceptable for wellprocessed SBM [21, 24]. Similarly, TIA values ranging from 2.5 to 4.0 mg/g are considered best [25]. However, disparity of criteria exists on the preferred chemical indicator to evaluate protein quality in commercial SBM. In general, PDI and KOHsol evaluate better than UA the quality of processed of SBM in pigs [26] and poultry [1]. In fact, Batal et al. [15] concluded that the PDI value of SBM was the best predictor of ADG in chickens. However, Frikha et al. [2] evaluated the coefficient of standardized ileal digestibility in broilers of CP and Lys of 22 SBM samples and reported that KOHsol was a better predictor of protein quality than PDI. We have not found any research reporting the influence of duration of storage on variables used to measure the quality of the protein fraction of commercial samples of SBM. Therefore, the objective of this research was to study, in 2 different experiments, the influence of duration of storage on UA, PDI, KOHsol, and TIA values of commercial SBM samples. MATERIALS AND METHODS Sample Procurement and Laboratory Analyses In experiment 1, 8 representative samples (3 kg each) of SBM were collected from the same US crushing plant [27] at weekly intervals and air mailed to the laboratory [28] where they were analyzed after 0, 30, 60, 90, and 120 d of storage. In experiment 2, 7 representative samples (3 kg each) were collected within a week by specialized quality control personnel from 7 different Argentinean crushing plants close to Rosario port. Samples were analyzed at arrival and at the same laboratory used for the US meals, and at 24, 48, 80, and 136 wk of storage. All samples were stored until analyses in hermetic glass containers in a laboratory room at 12 ± 2 C and a relative humidity of 70 ± 3%. The SBM samples were analyzed for moisture by oven-drying (method ), total ash with a muffle furnace (method ), and nitrogen by Kjeldahl (method ) as described by AOAC International [29]. Crude protein content was calculated as N The CF content was determined by sequential extraction with diluted acid and alkali (method ) as indicated by AOAC International [29], and the NDF as described by Van Soest et al. [30] and expressed on an ash-free basis. Ether extract was analyzed by Soxhlet fat analysis after 3 N HCl acid hydrolysis (method 4.b) as described by Boletín Oficial del Estado [13]. Urease activity was determined according to the International Organization for Standardization [12]. Briefly, the samples were ground using a hammer mill [31] fitted with a 0.2-mm screen, and a 0.2-g subsample was prepared. The enzymatic reaction between the urease content of the SBM sample and the buffered urea solution added (ph = 7.0) was carried out. Also, 10 ml of PBS was added to the blank sample. Solutions were incubated in a water bath at 37 C for

3 Serrano et al.: SOYBEAN MEAL STORAGE min and the N generated during the reaction was determined. The UA was expressed in milligrams of N per grams of sample and minute. The PDI of the SBM was determined according to method Ba of AOCS [14] using a Hamilton blender [32]. Briefly, a 20-g sample was weighed and processed for 10 min at 8,500 rpm in distilled water (ph 7.0) at 25 C. The mixture was centrifuged for 10 min at 1,400 g at room temperature (24 C), and the N of supernatant was determined. Results were expressed as the percentage of dispersible protein respect to the protein content of the original sample. The KOHsol of the SBM was measured as indicated by Araba and Dale [16]. Briefly, 1.5 g of the SBM were ground (0.5-mm screen) and incubated with 75 ml of 0.2% KOH (wt/vol; N, ph 12.5) solution for 20 min at 22 C using a magnetic stirrer. Following incubation, samples were centrifuged for 10 min at 1,400 g at room temperature (24 C), and the supernatant was analyzed for N content. Results were expressed as the percentage of soluble protein in the KOH solution respect to the protein content of the original sample. The TIA was assayed according to the method of Hamerstrand et al. [18] and expressed in milligrams per grams of DM sample. Briefly, samples (1 g) of SBM were ground with a grinder [31] provided with a 0.2-mm screen and extracted with 50 ml of 0.01 N NaOH using a magnetic stirrer for 3 h at ambient temperature and a ph between 8.4 and The suspension was then diluted to ensure that 2 ml of the sample extract inhibited 40 to 60% of the trypsin used as a standard in the analysis. All the analyses were conducted in duplicate. Statistical Analysis Data were analyzed as a completely randomized design using the GLM procedure of SAS [33] with duration of storage of SBM as main effect of the model. Orthogonal polynomial contrasts were performed to study the effect of duration of storage on the traits studied. In addition, Pearson correlation (r) analyses were also determined to study the relation between the different protein quality traits of the samples in each of the 2 experiments. An α value of <0.05 was considered as a significant difference. RESULTS AND DISCUSSION In experiment 1, the average CP content of the SBM, on a DM basis, was 55.9% ± 4.5 (mean ± SD; Table 1). The mean values for the protein quality variables studied at d 0 were 0.00 ± 0.00 mg of N/g min for UA, 21.9 ± 0.88% for PDI, 83.8 ± 0.86% for KOHsol, and 3.0 ± 0.23 mg/g of DM for TIA (Table 2). In experiment 2, the average CP content (DM basis) of the SBM used was 50.5 ± 2.1%. At d 0, the mean values of the variables studied were 0.03 ± 0.03 mg of N/g min for UA, 19.5 ± 3.8% for PDI, 84.8 ± 2.9% for KOHsol, and 2.4 ± 0.50 mg/g of DM for TIA (Table 3). Urease activity is an indirect indicator of the TIA content of commercial SBM [15, 24]. In the current study, the UA value of all SBM samples were below 0.05 mg of N/g min except for 1 of the samples in experiment 2 (value of 0.09). The PDI values were within the 15 to 30% range reported by Van Eys et al. [21] and Balloun [24] for correctly processed SBM. The KOHsol values for all samples in both experiments were in all instances within the range of 78 to 85% considered as adequate for well processed SBM [21]. Also, all TIA values were within the optimal range recommended by the United States Soybean Export Council [25]. A positive correlation was found at d 0 between KOH and TIA of the SBM in both experiments (r = 0.43, P < 0.01 and r = 0.77, P < 0.001, respectively), but no significant correlation was observed between PDI and TIA (data not shown). Duration of storage did not affect UA values of the SBM in either of the 2 experiments. However, PDI values decreased linearly with length of storage from 21.9% at d 0 to 17.7% at d 120 of storage in experiment 1 ( 0.26 ± 0.02 percentage units per week; P < 0.001) and from 19.5% at d 0 to 9.8% at wk 136 of storage in experiment 2 ( ± 0.01 percentage units per week; P < 0.001). Also, KOHsol decreased linearly with duration of storage ( ± 0.01 percentage units per week; P < 0.05) in experiment 2 (longterm storage), but no differences were detected in experiment 1 (shorter duration of storage). Most likely, the formation of insoluble disulfide polymers and the loss of moisture that occurred during storage favored the aggregation of some of the protein fractions of the SBM, decreasing

4 426 JAPR: Research Report Table 1. Chemical composition and protein quality variables of the soybean meals at d 0 of storage 1 Sample (% on DM basis) DM Ash CP CF NDF EE UA 2 (mg of N/g min) PDI 3 (%) KOHsol 4 (%) TIA 5 (mg/g of DM sample) Experiment Average Experiment NA NA NA NA NA NA NA Average NA Analyses per sample were conducted in duplicate for each chemical component and protein quality variables. 2 Urease activity. 3 Protein dispersibility index. 4 Potassium hydroxide solubility. 5 Trypsin inhibitor activity. 6 Samples collected at weekly intervals from the crushing plant [27]. 7 Samples collected at random in the same week from different Argentinean crushing plants close to Rosario port. 8 Not analyzed. their solubility. Narayan et al. [34] studied the influence of storage under ambient conditions for 1, 2, 3, and 9 yr on the physico-chemical characteristics of soybeans, and observed that the density and hardness of the kernel, and the solubility of N in water decreased with storage time, in agreement with the results of the current trial. Saio et al. [35] reported that protein solubility in water and diluted alkaline solutions of 7 samples of SBM collected in different locations within the United States (Indiana, Ohio, and Michigan) was reduced after 6 mo of storage, in agreement with the results of the current experiments. These authors reported that the reduction Table 2. Influence of duration of storage of soybean meal on protein quality traits 1,2 in experiment 1 Duration of storage (d) Probability 4 Item SEM 3 L Q Urease activity (mg of N/g min) NS NS Protein dispersibility index (%) *** NS KOH solubility (%) NS NS Trypsin inhibitor activity (mg/g of DM sample) NS NS 1 Samples collected at weekly intervals from the crushing plant [27]. 2 Analyses per sample and day were conducted in duplicate for each protein quality variable. 3 Eight replicates (individual soybean meal samples) per treatment. 4 L = linear; Q = quadratic. ***P

5 Serrano et al.: SOYBEAN MEAL STORAGE 427 Table 3. Influence of duration of storage of soybean meal on protein quality traits 1,2 in experiment 2 Duration of storage (wk) Probability 4 Item SEM 3 L Q Urease activity (mg of N/g min) NS NS Protein dispersibility index (%) *** NS KOH solubility (%) * NS Trypsin inhibitor activity (mg/g of DM sample) NS NS 1 Samples collected at random from different Argentinean crushing plants close to Rosario port during the same week. 2 Analyses per sample and day were conducted in duplicate for each protein quality variable. 3 Seven replicates (individual soybean meal samples) per treatment. 4 L = linear; Q = quadratic. *P < 0.05; ***P < in solubility observed was higher under more severe conditions during storage (35 C and 85% relative humidity vs. 25 C and 65% relative humidity). Moreover, they observed that the proportion of the different proteins of the SBM, as determined by electrophoresis, varied with changes in moisture, temperature, and duration of storage. In fact, the differences observed between the 2 current experiments in respect to changes in KOHsol with time, might be related with the duration of storage of the samples (17 wk vs. 136 wk in experiments 1 and 2, respectively). Protein solubility of SBM varies with the ph of the solution and the duration of storage [35] as well as with the environmental conditions during storage [34]. In the current study, when storage length was short (experiment 1), the reduction in protein solubility with time was more noticeable when the protein fraction of the SBM was solubilized under neutral (ph 7.0 for PDI) rather than under alkaline conditions (ph 12.5 for KOHsol). Protein solubility is affected by the ph of the solution and decreases with ph close to the isoelectric point. The isoelectric region of SBM proteins occurs at ph within the 4.2 to 4.6 range [36, 37]. In consequence, small changes in protein solubility might be more easily detected at a ph of 7 (water solubility for PDI values) than at a more alkaline ph (potassium hydroxide for KOHsol values) because the ph of the solution is closer to the isolectric point of SBM proteins. In this respect, Saio et al. [35] reported that the changes in protein solubility that occurred when the SBM were stored under poor conditions (35 C and 85% relative humidity) were more evident when a water solution was used than when PBS containing either 0.4 M of NaCl or diluted NaOH (ph 8.0) was used, in agreement with the results of the current study in either experiment. Duration of storage did not affect TIA of the SBM in either of the 2 experiments. In contrast, Narayan et al. [34] reported that TIA of soybeans decreased with time. These authors indicated that, because of its proteinaceus nature, TI might be denatured during storage, especially under high temperature and moisture conditions. However, in the current study, the moisture content of the samples at arrival to the laboratory was below 13.3% and samples were maintained under adequate environmental conditions for limited time (17 wk and 136 wk in experiments 1 and 2, respectively) as compared with the samples in the study of Narayan et al. [34] (432 wk). Therefore, less effect of duration of storage on TI values should be expected in the current research than in that of Narayan et al. [34]. CONCLUSIONS AND APPLICATIONS 1. Duration of storage did not affect UA or TIA of SBM. However, PDI decreased with the duration of the storage in both experiments, whereas KOHsol values decreased only in experiment 2 where the duration of the storage was longer. 2. The PDI methodology might not be recommended to compare the quality of the protein of SBM that have been stored for different periods of time under different management conditions.

6 428 JAPR: Research Report 3. The KOHsol values are less influenced than PDI values by the length of storage of the SBM. Therefore, the KOH method might be preferred to the PDI method to compare the quality of processing of SBM stored for less than 17 wk. REFERENCES AND NOTES 1. de Coca-Sinova, A., D. G. Valencia, E. Jiménez- Moreno, R. Lázaro, and G. G. Mateos Apparent ileal digestibility of energy, nitrogen, and amino acids of soybean meals of different origin in broilers. Poult. Sci. 87: Frikha, M., M. P. Serrano, D. G. Valencia, P. G. Rebollar, J. Fickler, and G. G. Mateos Correlation between ileal digestibility of amino acids and chemical composition of soybean meals in broilers at 21 days of age. Anim. Feed Sci. Technol. 178: Björck, J., and N. G. Asp The effects of extrusion-cooking on nutritional value A literature review. J. Food Eng. 2: Petres, J., and B. 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