Rapeseed is the second most produced oilseed in the world after soybean, and after the oil is extracted, a protein-rich meal is the resulting byproduct containing greater concentrations of sulfur amino acids (AA; i.e., Met and Cys) and Lys compared with legumes and cereal grains. Rapeseed proteins have great potential as a high-quality plant-based protein for humans due to their well-balanced AA profile, high metabolic utilization of protein, a protein digestibility corrected amino acid score (PDCAAS) similar to soy and milk proteins, and easily separable antinutritional factors. However, to our knowledge, digestible indispensable amino acid score (DIAAS) has not been determined for rapeseed protein isolate and the level of processing required to concentrate rapeseed into a protein isolate warrants further evaluation of its protein quality. Therefore, this experiment was conducted to test the hypothesis that heat treatment of rapeseed protein isolate will increase the digestibility of AA by growing pigs and result in a DIAAS that is comparable to soy and animal protein isolates.

Experimental design

Six ingredients were tested: whey protein isolate, soy protein isolate, pea protein concentrate, brown rice protein concentrate, rapeseed protein isolate, and heat-treated rapeseed protein isolate. The rapeseed protein isolates were sourced from DSM, Kaiseraugst, Switzerland and the heat treatment for the rapeseed protein isolate was as followed: rapeseed protein isolate was dissolved in osmosed water at 55^oC with a high-speed turbine mixer. The temperature was increased to 90^oC and maintained for 10 min, then decreased to 70^oC and the mixture was ground with a high-speed turbine mixer for 15 min. The resulting suspension was then spray dried producing the heat treated rapeseed protein isolate.

A total of seven ileal cannulated barrows (initial BW: 36.51 ± 1.61 kg) were randomly allotted to a 7 × 7 Latin square design with 7 diets and seven 7-d periods with ileal digesta collection for 9 h on d 6 and 7. Pigs were housed in individual pens (1.2 × 1.5 m) equipped with smooth plastic siding, fully slatted tribar floors, a feeder, and a nipple drinker. The 6 test ingredients were included in a diet as the only AA containing source, and a nitrogen-free diet was also formulated to measure basal endogenous losses of AA and crude protein (CP). All diets also contained 0.4% titanium dioxide as an indigestible marker. Pigs were fed their assigned diets in a daily amount of 3.3 times the estimated energy requirement for maintenance (i.e., 197 kcal metabolizable energy per kg^0.60; NRC, 2012). At the conclusion of the experiment, all diets, ingredients, and ileal digesta samples were analyzed for CP, AA, and titanium. Values for standardized ileal digestibility (SID) were calculated, and DIAAS was calculated for 2 age groups established by the FAO; 1) children 6 mo to 3 yr and 2) children > 3 yr, adolescents, and adults.

Results

Whey protein isolate had a greater (P < 0.05) SID of CP and most indispensable AA (IAA) compared with the other ingredients (Table 1), but the SID of Thr and Trp did not differ among whey protein isolate, soy protein isolate, and heat-treated rapeseed protein isolate. The heat-treated rapeseed protein isolate had a greater (P < 0.05) SID of all IAA than rapeseed protein isolate and brown rice protein concentrate, and a greater (P < 0.05) SID of all IAA, except Arg and Lys, than pea protein concentrate, but the SID of most IAA did not differ between heat-treated rapeseed protein isolate and soy protein isolate. Pea protein concentrate had a greater (P < 0.05) SID of all IAA compared with brown rice protein concentrate, except Trp, and rapeseed protein isolate.

For DIAAS calculated for children from 6 mo to 3 y (Table 2), heat-treated rapeseed protein isolate was the only ingredient with a DIAAS of 100 or greater. Whey protein isolate had a DIAAS less (P < 0.05) than heat-treated rapeseed protein isolate but greater (P < 0.05) than soy protein isolate. The DIAAS for rapeseed protein isolate was less (P < 0.05) than soy protein isolate and greater (P < 0.05) than pea protein concentrate, which had a DIAAS greater (P < 0.05) than brown rice protein concentrate. The first limiting AA for these ingredients compared with the reference protein pattern are as follows: Leu (rapeseed protein isolate), Lys (heat-treated rapeseed protein isolate and brown rice protein concentrate), His (whey protein isolate), and sulfur AA (soy protein isolate and pea protein concentrate).

For DIAAS calculated for older children, adolescents, and adults, whey protein isolate had the greatest (P < 0.05) DIAAS followed by heat-treated rapeseed protein isolate, but both ingredients has a DIAAS greater than 100. The other ingredients had a DIAAS less than 100 and followed the same order as the DIAAS calculated for children from 6 mo to 3 y. The first limiting AA for these ingredients compared with the reference protein pattern for older children, adolescents, and adults were the same as for children from 6 mo to 3 y.

Key points

• Moderately heating rapeseed protein isolate increased the SID of CP and AA and the DIAAS for the ingredient.
• Heat-treated rapeseed protein isolate and whey protein isolate had a DIAAS greater than 100 for older children, adolescents, and adults indicating the potential of heated rapeseed protein isolate to complement low quality proteins to produce a diet adequate in all IAA.
• Heating did not negatively affect DIAAS, and moderate heating may be an opportunity to increase the protein quality of oilseeds.

Table 1. Standardized ileal digestibility (SID) of amino acid (AA) in ingredients^1,2

^a-eMeans within a row lacking a common superscript letter differ (P < 0.05).

¹Data are least squares means of 6 to 8 observations.

²Standardized ileal digestibility values were calculated by correcting values for apparent ileal digestibility for the basal ileal endogenous losses. Endogenous losses (g/kg of dry matter intake) AA were as follows: crude protein, 20.84 Arg, 0.82; His, 0.21; Ile, 0.34; Leu, 0.58; Lys, 0.46; Met, 0.08; Phe, 0.36; Thr, 0.61; Trp, 0.11; Val, 0.58; Ala, 0.70; Asp, 0.83; Cys, 0.20; Glu, 1.03; Gly, 2.06; Pro, 8.90; Ser, 0.55; Tyr, 0.28.

Table 2. Digestible indispensable amino acid scores (DIAAS) of ingredients¹

^a-fMeans within a row lacking a common superscript letter differ (P < 0.05).

¹First-limiting AA is in parentheses.

²DIAAS were calculated using the recommended AA scoring pattern for a child (6 mo to 3 yr). The indispensable AA reference patterns are expressed as mg AA/g protein: His, 20; Ile, 32; Leu, 66; Lys, 57; sulfur AA, 27; aromatic AA, 52; Thr, 31; Trp, 8.5; Val, 43 (FAO, 2013).

³DIAAS were calculated using the recommended AA scoring pattern for older child, adolescent, and adult. The indispensable AA reference patterns are expressed as mg AA/g protein: His, 16; Ile, 30; Leu, 61; Lys, 48; sulfur AA, 23; aromatic AA, 41; Thr, 25; Trp, 6.6; Val, 40 (FAO, 2013).