Pea Carbohydrate and Protein Fractions and the Effect of Processing
Starch (around 450 g kg-1) and crude protein (around 250 g kg-1) are major chemical components in dehulled seeds with a notable variation and a negative relationship between the two components (Daveby et al. 1993 {576}, Reichert and MacKenzie 1982 {690}). The protein in peas is almost completely digested by ruminant animals. Degradation of the total nitrogen of SBM and field pea in the rumen of 2 cows from 0 to 48 hours was estimated at 70% for SBM and 94.7% for field peas (Aufrere et al. 1994 {569}). Intestinal digestibility of rumen undegraded pea protein was measured using two cows and was reported to be 92.5% and DM digestibility was 76.0% (Frydrych 1992 {587}). Intestinal digestion of protein was similar to maize (94.6%) or rye (94.4%) (Frydrych 1992 {587}).
In ruminal fluid untreated pea is characterized by a slow starch degradation rate and a rapid protein solubilization (Focant et al. 1990 {619}). Pea protein is highly soluble with only 22% of CP considered bypass protein or ruminally undegraded protein (NRC 1989). Chemical composition of the different pea protein fractions indicates a high buffer soluble CP level for peas (table 6). Soluble protein estimates range from 40% (Aguileera et al. 1992 as cited by Corbett 1997, Feed Industry Guide) to over 70% CP (Christensen et al. 1998; Mustafa et al. 1998). The non-soluble degradable protein fraction is estimated at 51.2% CP (Walhain et al. 1992 {585}) and 38% (Aguileera et al. 1992 as cited by Corbett 1997, Feed Industry Guide). Walhain et al. (1992 {585}) reported a totally potentially degradable protein fraction of 99.8% CP and a ruminal effective degradability of 88.3%.
The energy content of peas fed to ruminants is high and is comparable to wheat. The starch content of peas ranges from 41-50% DM with 35-50% of the starch being soluble. The non-soluble rumen degradable fraction is characterized by its slow degradation rate (Walhain et al. 1992 {585}; Robinson and McQueen 1989 as cited by Corbett 1997, Feed Industry Guide). Cerneau and Michalet (1991 {598}) investigated pea starch degradability in Holstein cows. Starch degradability of barley, oats, wheat bran, peas and maize was 98, 97, 96, 90 and 58 %, respectively. Maize and pea starches were degraded at the same rate as non-starchy components. Barley, oats and wheat starch were degraded more rapidly than the other DM components. Cerneau and Michalet (1991 {598}) concluded that part of the variation could be contributed to particle size variations.
The search to find a process that will increase bypass protein and ruminal starch degradation has included moist heat treatment, extrusion, steam flaking and micronizing.
Peas would have a higher nutritional
value if ruminal starch utilization was increased and a lower
rate of protein degradation could be obtained (Walhain et al.
1992 {585}). An optimum heat treatment of peas may minimize soluble
CP and maximize NDICP without a substantial increase in ADICP
(Van Soest 1989 as cited by Mustafa et al. 1998). Mustafa et
al. (1998) ground field peas and treated them to moist heat at
127C for 10, 20 and 30 minutes. Heating dramatically reduced
CP solubility, increased NDICP by five times and ADFICP doubled
(table 7). Rumen undegraded protein was increased from 28.6%
to 54.2% by moist heat for ten minutes. The slowly degradable
fraction (fraction B3) was increased from 2.4% to 12.7% at 30
minutes. In conclusion, heating peas for 10 to 30 min will change
the site of CP degradation from the rumen to the small intestine
without affecting the total CP available for digestion postruminally
(Mustafa et al. 1998; Christensen et al. 1998).
Micronizing peas at 115C and held in
a collection bin for ten minutes prior to grinding had little
effect on the carbohydrate fractions. The protein fraction soluble
non-protein nitrogen or soluble NPN (A) increased and soluble
true protein greatly reduced from 68 to 10% CP. The B2 protein
or medium rate available protein increased from 23 to 73% CP (Christensen
et al. 1998).
| Table 6. Effect of moist heat treatment on protein analysis, fractions and degradability of field peas (N=3) | |||||
|---|---|---|---|---|---|
| Heat treatment | |||||
| Control | 10 min | 20 min | 30 min | SEM | |
| Protein Analysis | |||||
| Crude protein (%) | 21.4 | 21.3 | 21.5 | 21.1 | 0.15 |
| Soluble protein (% of CP) | 78.8a | 23.3b | 21.3bc | 19.0c | 0.85 |
| Non-protein nitrogen (% of CP) | 25.0a | 18.4b | 18.1b | 17.1a | 0.74 |
| ND1 insoluble protein (% of CP) | 2.8d | 6.7c | 8.4b | 13.7a | 0.35 |
| AD2 insoluble protein (% of CP) | 0.5 | 0.6 | 0.7 | 0.9 | 0.15 |
| True protein fractions (% of CP) | |||||
| Total | 74.5b | 81.1a | 81.2a | 81.9a | 0.73 |
| Rapidly degradable (B1 fraction) | 53.8a | 4.9b | 3.2b | 1.9b | 1.20 |
| Intermediately degradable (B2 fraction) | 18.4c | 70.1a | 70.3a | 67.3b | 0.72 |
| Slowly degradable (B3 fraction) | 2.4d | 6.1c | 7.8b | 12.7a | 0.33 |
| In situ protein degradability | |||||
| Degradable (% of CP) | 52.1d | 74.5c | 81.0b | 86.6a | 0.78 |
| Degradation rate (% h-1) | 4.8a | 2.9b | 2.9b | 2.6b | 0.15 |
| Effective degradability (%)3 | 73.7a | 51.6b | 47.9c | 43.2d | 0.50 |
| Rumen undegraded protein (% of CP) | 28.6d | 54.2c | 61.3b | 64.4a | 0.71 |
| Intestinally available protein (% of CP) | 16.0d | 47.1c | 51.8b | 58.6a | 1.14 |
| Total available protein (% of CP) | 89.9 | 92.9 | 90.5 | 94.5 | 1.13 |
a-d means in the same row with different letters are different (P < 0.05).
1ND= Neutral detergent.
2AD= Acid detergent.
3Calculated
assuming 5% h-1 rumen outflow rate.
Mustafa et al. (1998).
Extrusion increases the nutritive value of legume
seeds by decreasing anti-nutritional factors, gelatinizing starch
and lowering the rate of protein degradation in the rumen (Walhain
et al. 1992 {585}; Focant et al. 1990 {619}) (table 7).
| Table 7. Chemical composition (%DM), pepsin-insoluble N of the feeds (%N) and characteristics of ruminal degradability of crude peas and extruded peas at 140 Cz | |||
|---|---|---|---|
| Crude peas | Extruded peas | SEM | |
| Chemical composition of peas and extruded peas | |||
| Dry matter | 87.0a | 89.3b | 0.04 |
| Organic matter | 96.7 | 96.6 | 0.02 |
| Crude protein | 24.4 | 24.2 | 0.14 |
| Crude fiber | 6.7a | 5.8b | 0.14 |
| Ether extract | 1.3a | 0.7b | 0.09 |
| Ash | 3.3 | 3.4 | 0.02 |
| Pepsin-insol. N | 97.4a | 95.8b | 0.15 |
| Characteristics of ruminal degradability | |||
| Dry matter | |||
| A (%) | 33.9 | 37.2 | 2.38 |
| B (%) | 65.4a | 53.3b | 2.43 |
| C (%) | 0.19a | 0.25b | 0.014 |
| a+b (%) | 99.2a | 90.6b | 0.77 |
| ED (%) | 83.6 | 79.5 | 0.53 |
| Crude protein | |||
| a (%) | 48.6a | 12.5b | 3.27 |
| b (%) | 51.2a | 71.3b | 2.41 |
| c (%) | 0.20 | 0.18 | .008 |
| a+b (%) | 99.8a | 83.8b | 1.55 |
| ED (%) | 88.3a | 65.5b | 2.02 |
zWalhain et al. 1992 {585}.
Means in the same row with different subscripts differ.
a=rapidly soluble fraction, b=slowly degradable fraction
at time t, c=fractional rate constant of disappearence
of the b fraction, a+b=potentially degradable fraction, ED=ruminal
effective degradability
Extrusion of crude peas at 140 C did not affect rumen dry matter effective degradability (83.6 vs. 79.5%), but dramatically reduced protein effective degradability (88.3 vs. 65.5% at an outflow rate of 0.06/h) in steers and bulls (Walhain et al. 1992 {585}). An in vitro study reported a severe reduction in rumen fluid N digestion from 68.8 to 28.6% after 6 h incubation with rumen fluid. A faster DM and DM-N degradability in the rumen resulted (Focant et al. 1990 {619}). Supplementation of extruded peas resulted in a lower rumen pH, a higher volatile fatty acid production, and a lower ammonia concentration compared to ground peas (Focant et al. 1990 {619}). Friesian heifers (n=6) fed a 40% ground or extruded pea diet reported a reduction in nitrogen solubility by extrusion. This increased non-ammonia-N flow (+36%, P < 0.001) resulting from a higher rate of bacterial N flow (+53%, P < 0.01) and N flow (+19%, numerically higher, but not significantly) (Focant et al. 1990 {619}). Because of the high intestinal protein digestibility (24.9% of crude protein disappearing in the intestine, but the whole tract digestibility=99.8%) it was concluded that extrusion did not over protect proteins rendering them unavailable (Walhain et al. 1992 {585}). In one study, extrusion did not significantly increase the essential amino acid profile in the undegradable ruminal fraction (Walhain et al. 1992 {585}). In other studies, extrusion increased flow of all amino acids into the duodenum (Focant et al. 1990 {619}; Chapoutot and Sauvant 1997 {555}). It was concluded that extrusion temperatures above 140C were unnecessary (Walhain et al. 1992 {585}).
Bulls fed a 34% extruded pea/rapeseed oilmeal mixture reported a reduced rumen degradation of the pea/rapeseed oilmeal mixture by 20.7% compared with the control without interfering in post-rumen protein digestibility (Schmidt et al. 1993 {578}). A study using whole pea and full-fat rapeseed in a 60/40 or 80/20 ratio, raw or extruded was conducted to determine the nutritive value of these blends. Extrusion decreased effective degradation in the rumen by 82, 96 and 93% for crude protein, dry matter and NDF, respectively. A large compensation in nitrogen digestion occurred in the intestine and increased amino acid delivery to the intestine (Chapoutot and Sauvant 1997 {555}).
Extrusion gelatinizes pea starch and increases the degradation of starch in the rumen (Walhain et al. 1992 {585}; Focant et al. 1990 {619}; Chapoutot and Sauvant 1997 {555}). In an in vitro trial starch digestion on a 30 min incubation with pancreatin:maltose yield (mg/g DM) increased from 44.8 to284.0 mg/g DM (Focant et al. 1990 {619}).
Steam flaking at atmospheric pressure is not an effective heat treatment. In one study it did not gelatinize the starch of peas, reduced ruminal digestion of N from 69 to 62% and posed no benefit to any in vivo parameters (Focant et al. 1990 {619}).