Protein malnutrition is a serious problem in both underdeveloped countries and the U.S., where many children and elderly people don't get enough protein. To determine the quality of food proteins, experts use a method called digestible indispensable amino acid score (DIAAS), which measures how well the body can digest the amino acids in a food item. The FAO recommended this method in 2011. According to the DIAAS method, protein claims can only be made for foods with a score above 75. Eggs are rich in protein and essential amino acids needed for muscle development and bodily functions. However, it's important to consider how eggs are cooked, as different methods can affect the availability of amino acids. Eggs are often eaten with other foods, and it's believed that the high-quality protein in eggs can make up for the lower quality of protein in plant ingredients when eaten together, but more research is needed to confirm this. Therefore, the objectives of this experiment were to determine DIAAS for eggs cooked in different forms and in traditional egg-style combinations with breads or hash brown, and test the hypothesis that protein quality in eggs can compensate for the low protein quality of plant-based ingredients and that DIAAS in different combined meals is additive.

Experimental Procedure

Eggs cooked in different styles including frying, boiling, and scrambling were used. All eggs were cooked until the yolks were firm according to common recipes as usually consumed by humans. Plant-based proteins such as English muffins, Texas toast, and hash browns were also included in this study. No preparation was made for the breads, but hash browns were baked. Ten diets were used. Six diets contained one source of protein (egg or plant-based food) and another three diets were made up of a combination of one type of egg with one type of plant-based protein (fried egg + English muffin, boiled egg + Texas toast, scrambled egg + hash brown). The last diet was a protein-free diet as a control diet. Nine growing pigs with a T-cannula in the distal ileum were fed the experimental diets in a randomized trial and no pig received the same diet more than once. There was a total of 6 replicate pigs per treatment. Each experimental period lasted 7 days with the initial 5 days being considered an adaptation period to the diet and the last two days being used for digesta collection. Chemical analysis was performed in all ingredients, diets, and digesta samples. Values for digestibility of protein and each amino acid were calculated. Protein quality was measured using the DIAAS method calculated for different ages including infants, toddlers, older children, adolescents, and adults. Cut-off values for DIAAS were considered “good” protein quality if numbers between 75 and 99. “Excellent” protein quality was considered at a value of 100 or more. Predicted digestibility and DIAAS values were also calculated and compared with the measured values to check for additivity.

Results

The SID of alanine was greater (P < 0.05) in hash browns than in all other ingredients and the SID of glutamic acid was greater (P < 0.05) in scrambled eggs, English muffin, and Texas toast than in fried eggs and boiled eggs. The SID of serine was also greater (P < 0.05) in hash browns, English muffin, and Texas toast than in boiled or fried eggs. In contrast, the SID of tyrosine was lower (P < 0.05) in hash brown compared with fried egg, scrambled egg, and the two breads. The SID for the predicted meal was in generally not different from the calculated meal, being additive in the mixed meals. For all ages (infants, children, and older individuals), all three cooked eggs had greater (P < 0.05) DIAAS compared with the other food ingredients (Table 1) and hash brown had greater (P < 0.05) DIAAS than both breads (English muffin and Texas toast). For infants, the first limiting AA for the cooked eggs was leucine. Lysine was the first limiting AA in both breads due to possible heat damage in the processing using to produce the breads, and tryptophan was the first limiting AA for hash brown because infants have a high requirement for tryptophan. For individuals older than 6 months, there was no limiting AA (DIAAS ≥ 100) for the cooked eggs, but lysine was the first limiting AA in both breads and in hash brown. The first limiting AA for the combined meals of egg and bread was lysine (Table 2). There were no differences between the predicted DIAAS and the calculated DIAAS for all mixed meals.

Key points

• All tested proteins have high digestibility of protein and amino acids.
• The breads have lower digestibility of lysine due to processing.
• Digestibility values were additive in all combined meals.
• All cooked eggs had greater DIAAS than the plant-based ingredients.
• For individuals older than 6 months, no limiting amino acid was observed in eggs, but lysine was the first limiting amino acid in all plant-based foods.
• Hash browns are considered "good" source of protein for individuals older than 3 years.
• No protein claims can be made for the breads for all ages.
• Cooking eggs using specific methods increased DIAAS.
• Boiling, frying, or scrambling did not influence protein quality of the eggs.
• All cooked eggs were considered “excellent” protein quality for individuals older than 6 months.
• For individuals older than 6 months, eggs can compensate for the low DIAAS in breads providing a meal with “good” protein quality, but lysine was still limiting. However, the combination of scrambled eggs and hash brown provided an "excellent" protein quality.
• DIAAS was additive in all combined meals.

Table 1. Digestible indispensable amino acids (DIAA) reference ratio and digestible indispensable amino acid score (DIAAS) in food ingredients^1,2

^a-cMean values within a row with unlike superscript letters are significantly different (P < 0.05).

¹AAA, aromatic amino acids (phenylalanine + tyrosine); SAA, sulfur amino acids (methionine + cystein); SEM, standard error of the mean.

²First limiting AA in parenthesis.

³The DIAA reference ratios and DIAAS values were calculated using the recommended AA scoring pattern for an infant (birth to 6 months). The DIAA reference patterns are expressed as mg AA/g protein: His, 21; Ile, 55; Leu, 96; Lys, 69; SAA, 33; AAA, 94; Thr, 44; Trp, 17; Val, 55 (FAO, 2013).

⁴The DIAA reference ratios and DIAAS values were calculated using the recommended AA scoring pattern for a child (6 months to 3 years). The DIAA reference patterns are expressed as mg AA/g protein: His, 20; Ile, 32; Leu, 66; Lys, 57; SAA, 27; AAA, 52; Thr, 31; Trp, 8.5; Val, 43 (FAO, 2013).

⁵The DIAA reference ratios and DIAAS values were calculated using the recommended AA scoring pattern for an older child, adolescent, and adult (older than 3 years). The DIAA reference patterns are expressed as mg AA/g protein: His, 16; Ile, 30; Leu, 61; Lys, 48; SAA, 23; AAA, 41; Thr, 25; Trp, 6.6; Val, 40 (FAO, 2013).

Table 2. Measured and predicted values for digestible indispensable amino acids (DIAA) reference ratio and digestible indispensable amino acid score (DIAAS) in combined meals of eggs and plant-based proteins^1,2,3

¹Means in a row differ if ^*Measured vs. predicted P < 0.05, ^**Measured vs. predicted P < 0.01, or tend to differ if ⁺Measured vs. predicted 0.05 ≤ P < 0.10.

²AAA, aromatic amino acids (phenylalanine + tyrosine); SAA, sulfur amino acids (methionine + cystein); SE, standard error.

³First limiting AA in parenthesis.

⁴The DIAA reference ratios and DIAAS values were calculated using the recommended AA scoring pattern for an infant (birth to 6 months). The DIAA reference patterns are expressed as mg AA/g protein: His, 21; Ile, 55; Leu, 96; Lys, 69; SAA, 33; AAA, 94; Thr, 44; Trp, 17; Val, 55 (FAO, 2013).

⁵The DIAA reference ratios and DIAAS values were calculated using the recommended AA scoring pattern for a child (6 months to 3 years). The DIAA reference patterns are expressed as mg AA/g protein: His, 20; Ile, 32; Leu, 66; Lys, 57; SAA, 27; AAA, 52; Thr, 31; Trp, 8.5; Val, 43 (FAO, 2013).

⁶The DIAA reference ratios and DIAAS values were calculated using the recommended AA scoring pattern for an older child, adolescent, and adult (older than 3 years). The DIAA reference patterns are expressed as mg AA/g protein: His, 16; Ile, 30; Leu, 61; Lys, 48; SAA, 23; AAA, 41; Thr, 25; Trp, 6.6; Val, 40 (FAO, 2013).