Effect of extrusion under controlled temperature and moisture conditions on ileal apparent amino acid and starch digestibility in peas determined with young broilers

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Abstract

The objective of the experimental programme was to examine the influence of extrusion of peas under controlled temperature and moisture conditions on coefficient of ileal apparent digestibility (CIAD) of amino acids and to examine the changes to starch structure (crystallisation) and subsequent effects on CIAD of starch in young broilers. Experiment 1 used one pea cultivar (2.94 mg trypsin inhibitor units (TIU)/g DM) processed under two extrusion temperatures (exit temperatures of 70 and 140 °C) and two moisture levels (0 and 1.96l water added/h) giving a 2 ×ばつ 2 factorial design (with a 5th treatment being the raw pea sample). All pea samples were added at a rate of 500 g/kg into a purified diet where peas were the only source of protein/amino acids. Processing generally led to a reduction in CIAD of amino acids but CIAD of starch for Raw, Treatments 1 and 3 (added water) was lower than that for Treatments 2 and 4 (no water) with data of 0.730, 0.808 and 0.759 vs. 0.852 and 0.834, respectively. Experiment 2 evaluated two near-isogenic lines of peas (HA5 and HB5) processed under the same conditions as for Experiment 1. Extrusion at 70 °C/addition of water had no significant effect on trypsin inhibitor activity but the inhibitor was eliminated at 140 °C with or without addition of water (P<0.001). Pea line HA5 generally had higher CIAD than HB5 (P<0.05 for cystine, lysine, leucine, histidine , alanine; P<0.01 for threonine). There were significant temperature ×ばつ water addition interactions for CIAD (P<0.001 for all amino acids except methionine; P=0.006). For example, data for lysine were 0.760, 0.828, 0.860 and 0.759, respectively for 140 °C/no water, 140 °C/1.96 l/h, 70 °C/no water and 70 °C/1.96 l/h. CIAD for starch improved following processing, attributed to the loss of crsytallinity. The data indicate that peas are sensitive to high processing temperatures which are associated with a reduction in nutritive value , in terms of amino acid digestibility, for young poultry. In contrast, the results reinforce the adverse effect of crystallinity on starch digestibility and show that extrusion in the absence of water improves the digestibility of raw starches due to granular disorganisation (as evidenced by X-ray diffraction patterns) and changes in crystallinity.

Introduction

Peas contain high levels of protein/amino acids and, accordingly, their potential nutritional value is good. However various anti-nutritional factors (ANFs) in peas interfere with digestive processes, thereby reducing their nutritional value for non-ruminants (Savage and Doe, 1989, Gatel and Grosjean, 1990). The major ANFs are trypsin inhibitors and, although levels in peas are 5–20 times lower than in raw soya beans, the amount found in some pea cultivars can be significant and responsible for reduced protein digestibility of pea-based diets (Pisulewski et al., 1983). It has been reported (Valdebouze et al., 1980) that the trypsin inhibitor activity (TIA) of winter types (5700–11,700 U/g) was twice that of those sown in spring (2700–5500 U/g), and that smooth peas had higher TIA than wrinkled-seeded varieties.
Peas also contain appreciable levels of starch although there is considerable variability. Thus the starch content of smooth seed types is higher than wrinkled ones (480 g/kg DM vs. 330 g/kg DM) and for white peas it is slightly different between spring and winter cultivars (500 g/kg DM vs. 475 g/kg DM) (e.g. Gatel and Grosjean, 1990). Potentially, peas are a very valuable feedstuff in terms of their energy-yielding potential. However, as shown by X-ray diffraction studies (French, 1984), there are crystalline regions within the starch granule that are more resistant to acidic and enzymic hydrolysis leading to reduced starch digestibility in raw peas.
TIA levels may be reduced by heat processing and the susceptibility of starch to enzymes can be increased by gelatinisation, or any other process that destroys the granular structure of starch (Holm et al., 1985); heating also may lead to loss of α-amylase inhibitors (Alonso et al., 2000). Extrusion is a process which involves forcing a material to flow under a variety of controlled conditions to pass through a shaped hole or slot at a predetermined rate. The operating conditions of high temperature, pressure and shear, and low or intermediate water content (all of which can be varied during the process) distinguish extrusion cooking from alternative heating processes. However there are conflicting reports on the efficacy of extrusion. Much of the confusion relating to the effects of extrusion is associated with previous reports describing only the name of the process, not the conditions operating.
The objective of the current experimental programme was, accordingly, to extend previous work (Wiseman et al., 2003) and examine the influence of extrusion under controlled temperature and moisture conditions on apparent ileal amino acid digestibility of pea cultivars differing in trypsin inhibitor activity and on ileal starch digestibility in young broilers. The testable hypotheses were that precise control of processing during extrusion of peas would (1) reduce the TIA and hence influence the digestibility of amino acids in peas and (2) result in differences in the physico-chemical properties of pea starch that would be reflected in variations in starch digestibility. The ultimate goal would be to derive processing conditions that would optimise the nutritional value of peas.

Section snippets

Extruder design

A Clextral BC-21 D25 twin-screw extruder (Clextral LTD, Firmney Cedex, France) fitted with a 3 mm circular die was employed. The extruder consisted of two identical co-rotating intermeshing self-wiping screw profiles enclosed within a barrel. Each screw diameter was 25 mm and effective length of the barrel was 400 mm. The throughput range of the extruder was 5–60 kg/h with a motor power of 5.5–8.3 kW. The main components of the extruder were the conveying, compression, mixing and shear zones. Three

Chemical analyses

The results of the chemical analysis of raw and extruded pea samples are given in Table 1a, Table 1b, respectively for Experiments 1 and 2.
Although data for all amino acids were obtained, for brevity those for cystine, methionine, lysine, threonine, leucine, histidine (essential amino acids for poultry) and alanine (non-essential) are presented as described previously (Wiseman et al., 2003). Similar to Experiment 1, the lowest concentrations of amino acids in Experiment 2 were seen in cystine

Amino acids

From the results obtained in Experiment 1 it may be concluded that processing has a limited effect on peas with low levels of TIA. However, in several earlier studies, a negative effect of TIA on ileal protein or amino acid digestibilities has been observed (Huisman et al., 1992, Bengala-Freire et al., 1991, Clarke and Wiseman, 2005). It may also be suggested that the higher the TIA the more resistant the pea protein is to digestion (Huisman et al., 1992).
The level of TIA in legume seeds may be

Conclusions

It may be concluded that processing has a limited effect on the nutritional value peas with low levels of TIA as measured through digestibility of amino acids. However cultivars with high TIA levels may benefit from mild processing although high extrusion temperatures (140 °C) may have an adverse effect on nutritional value that is not overcome by water addition. Thus the testable hypothesis of the current experimental programme has led to the establishment of optimum extrusion conditions for

Acknowledgements

WAM was in receipt of a postgraduate scholarship from Sultan Qaboos University, Sultanate of Oman. The John Innes Centre kindly supplied pea lines. Colleagues in the Division of Food Sciences, School of Biosciences, University of Nottingham are thanked for use of equipment and interpretation of X-ray diffractogram data.

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      The results of the studies which investigated the effect of different hydrothermal processes on ileal digestibility of protein and AA in poultry are shown in Table 4. The only studies reporting reduction in ileal AA digestibility of poultry feed by HTP demonstrated that, extrusion of pea decreased ileal digestibility of AA (Al-Marzooqi and Wiseman, 2009) and the digestibility of Lys, Thr, Met, Val, Ile, and Leu in expeller- extruded soybean meals were lower than the values reported in NRC (Powell et al., 2011). Other studies, conducted to investigate the impact of HTP (particularly extrusion and expansion) on ileal digestibility of protein and AAs, reported identical effects or favorable impact for the hydrothermal treatments (Table 4).

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    Present address: Animal & Veterinary Sciences Department, College of Agricultural and Marine Sciences, Sultan Qaboos University, P.O. Box 34, Al-Khod 123, Muscat, Oman.
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