The availability of lysine, methionine and tryptophan in condensed milk and milk powder; in vitro digestion studies

"Milk on Ice": A detailed analysis of Ernest Shackleton's century-old whole milk powder in comparison with modern counterparts

Whole milk powder (WMP) manufactured in New Zealand in 1907 was sent to the Antarctic continent with the Shackleton-led British Antarctic Expedition from 1907 to 1909. This powder was stored at ambient conditions at Shackleton's Hut at Cape Royds, Antarctica, for over 100 yr before a sample was collected on behalf of Fonterra by the Antarctic Heritage Trust. Having spent most of its existence both dried and in frozen storage, any deleterious reactions within the WMP would have been markedly retarded. The composition and some properties of the roller-dried Shackleton's WMP are reported along with those of 2 modern spray-dried New Zealand WMP. The Shackleton powder was less white and more yellow than the modern WMP and was composed of flakes rather than agglomerated particles, consistent with that expected of a roller-dried powder. Headspace analysis showed lipolytic and oxidative volatile compounds were present in the Shackleton WMP, indicting some deterioration of the milk either before powder manufacture or on storage of the finished product. On a moisture-free basis, the Shackleton WMP had higher protein, higher fat (with a markedly higher free fat level), higher ash, and a lower lactose level than the modern WMP. The lysine level was lower in the Shackleton WMP compared with the spray-dried powders, whereas the fatty acid composition was relatively similar. The sodium level was markedly higher in the Shackleton WMP compared with the spray-dried powder, which is probably due to the addition of an alkaline sodium salt to adjust the pH of the milk before roller drying. Lead, iron, and tin levels were markedly higher in the Shackleton WMP compared with the spray-dried powders, possibly due to the equipment used in powder manufacture and the tin-plated cases used for storage. The proteins in the Shackleton WMP were more lactosylated than in the spray-dried powders. The Shackleton WMP had a higher ratio of κ-casein A to B variants and a higher ratio of β-lactoglobulin B to A variants than the spray-dried powders, whereas the αS1-casein, β-casein, αS2-casein, and α-lactalbumin protein variants were similar in all powders. The total phospholipid content was markedly lower in the Shackleton WMP than the spray-dried powders, primarily due to a lower phosphatidylethanolamine concentration. The molecular species distributions within the phospholipid classes were generally similar in the 3 powders. Claims are sometimes encountered that the milk of today is different from that consumed by previous generations. However, this comparative study has shown that the Shackleton WMP was generally similar to modern WMP. Although differences in some components and properties were observed, these were attributable to the manufacturing equipment and processes used in the pioneering years of WMP manufacture.

In Vitro Methods of Assessing Protein Quality for Poultry

Protein quality assessment of feed ingredients for poultry is often achieved using in vitro or in vivo testing. In vivo methods can be expensive and time consuming. Protein quality can also be evaluated using less expensive and time consuming chemical methods, termed in vitro. These techniques are used to improve the user's efficiency when dealing with large sample numbers, and some mimic the physiological and chemical characteristics of the animal digestive system to which the ingredient will be fed. The pepsin digestibility test is the in vitro method of choice for quick evaluation of protein sample during quality control and in most research settings. Even though the pepsin digestibility test uses enzymes to liberate the amino acids from the protein, it does not mimic normal in vivo digestive conditions. The results obtained with this method may be misleading if the samples tested contain fats or carbohydrates which they often do. Multi-enzyme tests have been proposed to overcome the problem encountered when using the pepsin digestibility test. These tests use a combination of enzymes in one or multiple steps customized to simulate the digestive process of the animal. Multi enzyme assays can predict animal digestibility, but any inherent biological properties of the ingredients on the animal digestive tract will be lost.

Baking, ageing, diabetes: a short history of the Maillard reaction

The reaction of reducing carbohydrates with amino compounds described in 1912 by Louis-Camille Maillard is responsible for the aroma, taste, and appearance of thermally processed food. The discovery that non-enzymatic conversions also occur in organisms led to intensive investigation of the pathophysiological significance of the Maillard reaction in diabetes and ageing processes. Dietary Maillard products are discussed as "glycotoxins" and thus as a nutritional risk, but also increasingly with regard to positive effects in the human body. In this Review we give an overview of the most important discoveries in Maillard research since it was first described and show that the complex reaction, even after over one hundred years, has lost none of its interdisciplinary actuality.

Effects of different industrial heating processes of milk on site-specific protein modifications and their relationship to in vitro and in vivo digestibility

Heating processes are applied to milk and dairy products to ensure their microbiological safety and shelf lives. However, how differences in "industrial" thermal treatments affect protein digestibility is still equivocal. In this study, raw milk was subjected to pasteurization, three kinds of ultra-high-temperature (UHT) treatment, and in-can sterilization and was investigated by in vitro and in vivo digestion and proteomic methods. In-can sterilized milk, followed by UHT milk samples, showed a rapid decrease in protein bands during the course of digestion. However, protein digestibility determined by a Kjeldahl procedure showed insignificant differences. Proteomic analysis revealed that lactulosyllysine, which reflects a decrease in protein digestibility, in α-lactalbumin, β-lactoglobulin, and caseins was higher in in-can sterilized milk, followed by UHT milk samples. Thus, industrial heating may improve the digestibility of milk proteins by denaturation, but the improvement is likely to be offset by heat-derived modifications involved in decreased protein digestibility.

Amino acid analysis

Amino acid analysis is used to determine the amino acid content of amino acid-, peptide- and protein-containing samples. With minor exceptions, proteins are long linear polymers of amino acids connected to each other via peptide bonds. The first step of amino acid analysis involves hydrolyzing these peptide bonds. The liberated amino acids are then separated, detected, and quantified. The method was first developed by Moore, Stein and coworkers in the 1950s using HCl acid hydrolysis, and, despite considerable effort by many workers, the basic methodology remains relatively unchanged. This unit provides an overview and strategic planning for amino acid analysis, discussing a range of methodologies and issues. In addition, several common methods used for analysis of L-amino acids are described in detail, including: HCl acid hydrolysis, performic acid oxidation for methionine and cysteine analysis, base hydrolysis for tryptophan analysis, analysis of free amino acids, and analysis of reactive lysine.

Analysis of advanced glycation endproducts in dairy products by isotope dilution liquid chromatography-electrospray tandem mass spectrometry. The particular case of carboxymethyllysine

A fully validated multiple-transition recording isotope dilution liquid chromatography-electrospray tandem mass spectrometry (LC-MS/MS) method for the simultaneous quantitative determination of N(epsilon)-carboxymethyllysine (CML) and lysine in dairy products is described. Internal standards were [N-1',2'-(13)C(2)]CML and [1,2,3,4,5,6-(13)C(6)-2,6-(15)N(2)]lysine, and the method was validated by evaluating the selectivity, linearity, precision (repeatability and reproducibility) and trueness, using both powder and liquid products. For liquid dairy products, the repeatability and reproducibility was 2.79% and 11.0%, while 4.85% and 4.92% were determined for powder dairy products, respectively. The trueness of the method ranged from -9.6% to -3.6% for powder and from -0.99% to 6.8% for liquid dairy products. The limit of detection for CML was estimated to be 8 ng CML per mg protein while the limit of quantification was 27 ng CML per mg protein. The method encompasses a proteolytic cleavage mediated by enzymatic digestion to reach a complete release of the amino acids prior to a sample cleanup based on solid phase extraction, and followed by LC-MS/MS analysis of CML and lysine residues. To ensure a suitable performance of the enzymatic digestion, CML measurements were compared to values obtained with an acid hydrolysis-mediated proteolysis. Finally, the method was employed for the analysis of CML in various dairy products. The values compare well to the data available in the literature when similar methods were used, even if some discrepancies were observed upon comparison with the results obtained by other techniques such as enzyme-linked immunosorbent assay and GC-MS.

Development of a novel bioassay for determining the available lysine contents of foods and feedstuffs

Lysine is an important indispensable amino acid, and describing the lysine content of a food or feedstuff provides useful information about nutritional value. However, when a food or feedstuff is subjected to heating the lysine present can be altered to nutritionally unavailable derivatives. These derivatives can revert back to lysine during the acid hydrolysis step used in amino acid analysis causing an overestimate of the lysine content. There have been many chemical methods developed to determine the reactive (unmodified) lysine content of foods and feedstuffs, but these do not take into account the incomplete absorption of lysine from the small intestine. There are also a number of animal-based assays for determining available lysine (the lysine that can be absorbed in a form that can be used for protein synthesis). The true ileal amino acid digestibility assay is commonly used to determine amino acid availability and is accurate for application to unprocessed foods and feedstuffs but is not accurate for lysine and possibly other amino acids when applied to heat-processed foods or feedstuffs. For such protein sources, assays such as the slope-ratio assay, indicator amino acid oxidation assay and the BIOLYSINE™ assay (true ileal digestible reactive lysine assay) have been developed to determine available lysine. The present paper discusses the efficacy of the BIOLYSINE™ assay as well as other assays for determining available lysine in processed foods and feedstuffs.

Historical perspective of the Maillard reaction in food science

Maillard's paper of 1912 describing the reaction between amino acids and sugars is both innovative and visionary. It provides original and still-valuable data on the chemistry of a new reaction and foresees its involvement in many scientific and biological domains, even in human pathology. This paper was ignored by the scientific community until 1941. In 1948 the Maillard reaction was definitely recognized as being responsible for the browning and loss of nutritive value of heated milk powders. There was then a continuous increase in papers on the chemistry of this complex reaction to identify its various pathways: in food science, to evaluate the influence of reaction parameters (pH, T degrees , time, sugar reactivity, concentration of the reagents, water activity, glass transition temperature) on the evolution of the reaction and on changes in food quality; in nutrition, to quantify the loss of bioavailability of essential amino acids; on the metabolism of the reaction products and on the physiological effects of the ingested Maillard reaction products. The significant scientific advances and the key persons and pioneers who contributed much to the understanding of the Maillard reaction are presented. The food industry is directly concerned with the occurrence of this reaction in processed foods and contributed significantly by its own research to understanding the phenomena and to optimizing the processes and conditions of food preparation in order to preserve the nutritional, safety, and organoleptic qualities of foods.

Extent of damage to amino acid availability of whey protein heated with sugar

The effect of heat treatments, at various water activities (aW), on digestibility and on the availabilities of amino acids of whey protein samples in the presence of lactose was estimated by an in vitro digestion method with continuous dialysis. Four aW (0.3, 0.5, 0.7 and 0.97), three temperatures (75, 100 and 121 degrees C) and three heating periods (50, 500 and 5000 s) were selected. The initial lysine:lactose molar ratio was 1:1. Amino acid profiles showed that excessive heating of whey (121 degrees C, 5000 s) destroyed a significant proportion of cystine at all aW, lysine at aW 0.3, 0.5 and 0.7, and arginine at aW 0.5 and 0.7. At aW 0.3, 0.5 and 0.7, protein digestibility decreased (P less than 0.05) as the temperature increased from 75 to 121 degrees C for a heating period of 5000 s, and as the heating time was prolonged, from 500 to 5000 s at 121 degrees C. Excessive heating also decreased (P less than 0.05) the availabilities of all amino acids at aW 0.3, 0.5 and 0.7. The availabilities of lysine, proline, aspartic acid, glutamic acid, threonine, alanine, glycine and serine were particularly affected. Severe heating at aW 0.97 did not seem to favour the Maillard reaction, but the availabilities of cystine, tyrosine and arginine were decreased, probably as a result of structural modifications of the protein upon heating. Heating whey protein concentrates in the presence of lactose not only affected lysine, but also impaired enzymic liberation of other amino acids, according to the severity of heat treatments and aW.

Effect of the drying process on the nutritive value of milk. Part 2. Biological evaluation

Five different brands of dry milk and freeze dried milk were used in rat feeding trials to study their nutritive value and to assess the effect of the drying process on their nutritive value by using growth response, weight gain, food efficiency and relative nutritive value (RNV). Brand I milk gave better growth response, weight gain and food efficiency than any other dry milk studied, while brand V gave the least values. Freeze dried milk gave better growth response, weight gain and food efficiency than brand I milk. Using Slope-ratio assay, freeze dried milk gave the best RNV compared to all dry milks tested on the basis of the two response parameters used, weight gain and moisture gain.

Stability of tryptophan during food processing and storage. 1. Comparative losses of tryptophan, lysine and methionine in different model systems

The stability of tryptophan was evaluated in several different food model systems using a chemical method (high pressure liquid chromatography after alkaline-hydrolysis) and rat assays. Losses of tryptophan were compared with the losses of lysine and methionine. Whey proteins stored in the presence of oxidizing lipids showed large losses of lysine and extensive methionine oxidation but only minor losses of tryptophan as measured chemically. The observed decrease in bioavailable tryptophan was explained by a lower protein digestibility. Casein treated with hydrogen peroxide to oxidize all methionine to methionine sulphoxide showed a 9% loss in bioavailable tryptophan. When casein was reacted with caffeic acid at pH 7 in the presence of monophenol monooxygenase (tyrosinase; EC 1.14.18.1), no chemical loss of tryptophan occurred, although fluorodinitrobenzene-reactive lysine fell by 23%. Tryptophan bioavailability fell 15%, partly due to an 8% reduction in protein digestibility. Alkali-treated casein (0.15 M-sodium hydroxide, 80 degrees, 4 h) did not support rat growth. Chemically-determined tryptophan, available tryptophan and true nitrogen digestibility fell 10, 46 and 23% respectively. Racemization of tryptophan was found to be 10% (D/(D+L)). In whole-milk powder, which had undergone "early' or "advanced' Maillard reactions, tryptophan, determined chemically or in rat assays, was virtually unchanged. Extensive lysine losses occurred. It was concluded that losses of tryptophan during food processing and storage are small and of only minor nutritional importance, especially when compared with much larger losses of lysine and the more extensive oxidation of methionine.

[Advances in possibilities for protein evaluation]

With protein evaluation the changing abilities of food proteins are characterized with regard to their covering the requirements of the organism for protein or amino acids, respectively. The biological value of proteins is influenced by several factors. Measuring the reaction of the whole organism (of man or animal) to the food protein the sum of all influencing factors is covered. Hence, these direct evaluation methods have priority. The general dose-reaction curve demonstrates the direct evaluation methods: The evaluation criteria protein efficiency or minimum protein requirement are presented and the different possibilities of interpretion are compared. Most direct evaluation methods are performed with suboptimal protein supply, therefore values for an optimum protein supply can be derived only with reservation. Improved values for protein evaluation can be obtained from investigations of the protein metabolism. With that the substrat-related evaluation can be more orientated to a organism-related one. Indirect biochemical evaluation methods are less important. That does not concern in-vitro evaluation by amino-acid analysis and/or in-vitro digestibility tests. Possibilities and limits of these methods are discussed.

Effect of protein treatment on the enzymatic hydrolysis of lysinoalanine and other amino acids

Lysinoalanine (LAL) release from alkali-treated proteins by proteolytic enzymes could be measured on a large scale using an in vitro digestion method based on a two-step hydrolysis with pepsin and pancreatin. The hydrolysis was carried out in a dialysis bag, and the digestion products collected and analyzed. Applying this procedure to alkali-treated soybean and rapeseed protein showed that the release of LAL was not related to its concentration in the protein. Instead, LAL release depended on the nature of the protein, the length of the treatment, and the presence of untreated protein. The enzymatic procedure was also used to measure the release of other amino acids in treated and untreated proteins. This in vitro method could directly measure beneficial and adverse effects of processing on amino acid digestibility.

Assessment of protein digestibility by in vitro enzymatic hydrolysis with simultaneous dialysis

An in vitro digestion method to assess the quality of proteins based on enzymatic hydrolysis is presented. The method consists of a two-step proteolysis at 37 degrees, a 30-minute incubation of the protein with pepsin at pH 1.9, followed by pancreatin digestion at pH 8 for 24 hours in a dialysis bag with a 1000 molecular weight cutoff for the continuous elimination of the digested products into a replaceable buffer. Three types of buffer replacement were tried. These were, in increasing order of efficiency: intermittent with either six changes (infrequent buffer replacement) or eleven changes (frequent buffer replacement) or continuously circulated at the rate of 212 ml/hour (continuous buffer replacement). By using three protein sources, i.e., casein, soybean and rapeseed proteins, it was found that the degree of digestion (dialyzed N) as well as the regularity of the process were markedly improved by increasing the frequency of buffer replacement. In spite of different pepsin digestibilities, as determined from the production of trichloroacetic acid-soluble N, the digestion of the three proteins gave equivalent results when the best procedure of buffer replacement was used (continuous buffer replacement). The deleterious effect of heat and alkali treatment on protein digestion was readily shown by this procedure and indicated casein to be a very heat-sensitive protein as compared to the others.

[Artificial macaroni. 2. Biological value of synthetic macaroni]

The authors determined (in vitro) the amino-acid composition and the enzymatic cleavage of protein and starch as well as (in vivo) the biological value of synthetic macaroni produced on the basis of casein, soybean protein and cotton-seed protein. The products based on casein and soybean protein have a great biological value. Cotton-seed protein has a toxic effect. The biological value of synthetic macaroni is mainly determined by the composition of the initial mixture. The processes of production and culinary processing exert no essential effects on protein content, amino-acid composition of the protein and enzymatic cleavage.