Chemical changes in ultra-heat-treated milk during storage: I. Hydrolysis of casein by incubation with pronase and a peptidase mixture

Revision: Indicadores del deterioro de la calidad proteica y del valor nutritivo de la leche / Review: Indicators of damage of protein quality and nutritional value of milk

The nutritional value of milk can be negatively affected by thermal treatments. The quality of the proteins can be modified as a consequence of a set of reactions, the most important of which are the interactions between protein and carbohydrates, also known as Maillard reactions. These reactions can also occur when the storage conditions of moisture and temperature are unfavorable. The differ ent thermal treatments involved in processing milk (pasteurization, sterilization, drying, etc.) lead to different steps of the Maillard reaction and therefore to the formation of different compounds that can be useful as chemical markers of milk alteration, or more exactly of protein alteration. Estimating these markers is useful in monitoring the changes, especially those related to proteins, that happen during processing and storage. A review of the literature on the markers used to evaluate the modi fications that milk undergoes during thermal treatment and subsequent storage was carried out, focusing mainly on proteins. Special attention was paid to the analytical aspects. The chemical mark ers taken into account can be classified into two groups according to their relation with the Maillard reaction:

1. Specific markers of Maillard reaction: undesirable compounds (furosine, lisinoalanine, histidinalanine, furfurals, melanoidins); loss of nutrients (available lysine).

2. Non-markers of Maillard reaction: galactose; lactulose; protein-reducing substances; denaturation of protein; in vitro digestibility of proteins; others—pH, viscosity, free fatty acids.

Chemical changes in ultra-heat-treated milk during storage: II. Lactuloselysine and fructoselysine formation by the maillard reaction

Lactuloselysine (ε-N-deoxylactulosyl-L-lysine) and fructoselysine (ε-N-deoxyfructosyl-L-lysine), formed by Maillard reaction, were identified in enzymic hydrolysates of casein from stored ultra-heat-treated (UHT) milk. Furosine (ε-N-(2-furoylmethyl-L-lysine) and pyridosine (ε-(3-hydroxy-4-oxo-6-methyl-1-pyridinyl)-L-norleucine) were identified in the corresponding acid hydrolysates. Enzymic hydrolysis of the caseins, gel filtration and preparative scale amino acid analysis were used to isolate the compounds which were then identified by reference to synthesized authentic compounds. Lactuloselysine formation was extensive and involved 10–30% of lysine residues in UHT milks stored at 30–37 °C for 6 months to 3 years. Fructoselysine concentration was generally about 10% of the lactuloselysine concentration.

Chemical changes in ultra-heat-treated milk during storage: III. Methods for the estimation of lysine and sugar-lysine derivatives formed by the maillard reaction

Four chemical methods were used in a comparison of the measurement of the extent of modification of lysine residues in casein during storage of ultra-heat-treated (UHT) milk. Determination of homoarginine after reaction with o-methylisourea appeared to be the best measure of total lysine modification. Reduction with borohydride before acid hydrolysis prevented recovery of lysine from lactuloselysine (ε-N-deoxylactulosyl-L-lysine) and fructoselysine (ε-N-deoxyfructosyl-L-lysine). This procedure gave values for these derivatives which were in good agreement with those obtained from the determination of furosine (ε-N-(2-furoylmethyl)-L-lysine) in acid hydrolysates. Determination of lysine after acid hydrolysis showed that lysine loss was accounted for by lactuloselysine and fructoselysine formation in most of the samples.

Browning was most apparent in UHT milk stored at 37 °C for 3 years and in this sample only part of the lysine residues was accounted for as lactuloselysine and fructoselysine, thus indicating that lysine had become involved further in sugar degradation products.

Quantification of glutamine in proteins and peptides using enzymatic hydrolysis and reverse-phase high-performance liquid chromatography

An enzymatic method for hydrolyzing bovine milk proteins was developed. Purified milk proteins (alpha-lactalbumin, beta-lactoglobulin, and beta-casein) were hydrolyzed in 0.1 M Hepes buffer (pH 7.5) containing pronase E, aminopeptidase M, and prolidase at 37 degrees C for 20 h. Free glutamine and other amino acids were derivatized with phenylisothiocyanate and separated using a C18 Pico-Tag column. Amino acids were eluted from the column with an aqueous sodium acetate-acetonitrile gradient with detection at 254 nm. Glutamine recoveries from hydrolyzed alpha-lactalbumin, beta-lactoglobulin, and beta-casein were 78 +/- 4, 98 +/- 3, and 101 +/- 3% of the theoretical values, respectively. The recoveries of most amino acids were comparable with those obtained using acid hydrolysis, except for the recoveries of proline and acidic amino acids. These peptide bonds appeared to be resistant to enzymatic hydrolysis and also to inhibit the hydrolysis of adjacent amino acids. Free glutamine was found to be very stable (97% recovery) under the enzymatic hydrolysis conditions.

Evaluation of the extent of the early Maillard-reaction in milk products by direct measurement of the Amadori-product lactuloselysine

A new method for the evaluation of the extent of lysine modification caused by the Maillard reaction is presented, based on the direct determination of the Amadori product lactuloselysine plus unmodified lysine after complete enzymatic hydrolysis by ion-exchange chromatography. Using a simple mathematical relation, the amount of modified lysine in milk products can be estimated without knowledge of the initial lysine value before heating or storage.

Nutritional and toxicological aspects of the Maillard browning reaction in foods

The Maillard, or nonenzymatic, browning reaction between carbonyl and amino groups is a common reaction in foods which undergo thermal processing. The Maillard reaction is a desirable consequence of many industrial and domestic processes and is responsible for the attractive flavor and brown color of some cooked foods. An early recognized consequence of the Maillard reaction was the destruction of some essential amino acids, such as lysine. More recently, research interest has focused on the production of toxic and antinutritive compounds. This review examines the nutritional and toxicological consequences of the Maillard reaction in light of the findings of such research. In particular, the effect of Maillard reaction products on the digestion, absorption, and excretion of nutrients is considered. The cytotoxicity, mutagenicity, and immunochemical aspects of selected Maillard reaction products are also examined and suggestions are made for future areas of investigation.

Effects of small concentrations of Mucor miehei protease on stability of sterile milk-based dietetic foods

Four commercial sterile milk-based dietetic products were inoculated aseptically with small concentrations of a commericial milk clotting enzyme derived from Mucor miehei and incubated at 30 C for 32 wk. One of the products, Sustacal, exhibited no observable changes in body and texture during storage. Enzyme concentrations of 1 x 10(-4) chymosin units per milliliter or higher induced undesirable changes in the body of the other three products, Enfamil Ready-to-Use, Enfamil Concentrate, and Metrecal Shape. None of the products was affected visibly by enzyme concentrations of 1 x 10(-5) chymosin units per milliliter or less. In-can sterilization of Enfamil Ready-to-Use at 126.1 C for 4.5 min and Enfamil Concentrate at 126.7 C for 4.23 min completely destroyed all measurable enzyme activity at concentrations (up to 5.5 x 10(-2) chymosin units per milliliter) in these experiments. However, 1 to 3 h between formulation and sterilization allowed time for the enzyme at concentrations of 1 x 10(-2) chymosin units per milliliter or more to cause coagulation during heat sterilization.