The minerals of milk Frédéric Gaucheron To cite this version: Frédéric Gaucheron. The minerals of milk. Reproduction Nutrition Development, EDP Sciences, 2005, 45 (4), pp.473-483. 10.1051/rnd:2005030. hal-00900570 HAL Id: hal-00900570 https://hal.archives-ouvertes.fr/hal-00900570 Submitted on 1 Jan 2005 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Reprod. Nutr. Dev. 45 (2005) 473–483 473 © INRA, EDP Sciences, 2005 DOI: 10.1051/rnd:2005030 Review The minerals of milk Frédéric GAUCHERON* INRA, UMR Science et Technologie du Lait et de l’Œuf, 65 rue de Saint-Brieuc, 35042 Rennes, France Abstract – The salt of milk constitutes a small part of milk (8–9 g·L–1); this fraction contains calcium, magnesium, sodium and potassium for the main cations and inorganic phosphate, citrate and chloride for the main anions. In milk, these ions are more or less associated between themselves and with proteins. Depending on the type of ion, they are diffusible (cases of sodium, potassium and chloride) or partially associated with casein molecules (cases of calcium, magnesium, phosphate and citrate), to form large colloidal particles called casein micelles. Today, our knowledge and understanding concerning this fraction is relatively complete. In this review, the different models explaining (i) the nature and distribution of these minerals (especially calcium phosphate) in both fractions of milk and (ii) their behaviour in different physico-chemical conditions, are discussed. milk / minerals / salts / calcium / phosphate / casein micelle 1. INTRODUCTION important characteristics of this salt frac- tion in different conditions are covered with The milk components (proteins, fat, lac- an emphasis on recent findings. Thus, after tose, vitamins and minerals) are necessary indicating the milk mineral composition, to nurture the new-born and consequently we describe the distribution of ions between they contribute to its growth. The mineral fraction, which is a small fraction of milk the different fractions of milk and the fac- (about 8–9 g·L–1), contains cations (cal- tors influencing this distribution. Then, par- cium, magnesium, sodium and potassium) ticular attention will be paid to the structure and anions (inorganic phosphate, citrate and stability of casein micelles in relation and chloride). In milk, these ions play an with this mineral fraction. important role in the structure and stability of casein micelles [1–3]. A good under- standing of the properties of milk minerals 2. MINERAL COMPOSITION is important for fundamental research but OF COW MILKS also for the development of dairy products in which this fraction appears to be com- Concentration ranges of the different plex, dynamic, and in strong interaction minerals in cow milk (expressed in massic with the protein fraction. In this review, the and molar concentrations) are indicated in * Corresponding author: [email protected] Article published by EDP Sciences and available at http://www.edpsciences.org/rnd or http://dx.doi.org/10.1051/rnd:2005030 474 F. Gaucheron Table I. Mineral composition of cow milk. Mineral Concentration (mg·kg–1) Concentration (mmol·kg–1) Calcium 1043–1283 26–32 Magnesium 97–146 4–6 Inorganic phosphate 1805–2185 19–23 Total phosphore 930–992 30–32 Citrate 1323–2079 7–11 Sodium 391–644 17–28 Potassium 1212–1681 31–43 Chloride 772–1207 22–34 Table II. Total and soluble salt concentration (mM) in the skim milk of Ayrshire in early, middle and late lactation and from cows with subclinical mastitis [4]. Bulk Early Middle Late Mastitic Total calcium 30.1 33.2 29.4 32.1 29.4 Soluble calcium 9.5 12.8 9.4 8.0 9.1 Total magnesium 5.1 5.7 5.0 5.4 4.9 Soluble magnesium 3.3 3.9 3.2 3.3 3.2 Total inorganic phosphate 20.9 19.4 20.9 18.4 19.0 Soluble inorganic phosphate 11.2 10.5 11.7 7.0 9.2 Total citrate 9.8 9.8 9.1 8.5 8.8 Soluble citrate 9.2 9.1 8.9 7.8 8.3 Total sodium 25.5 29.7 24.8 48.8 34.5 Total potassium 36.8 41.8 40.3 26.9 36.1 Total chloride 30.3 36.7 29.7 46.5 40.5 pH 6.72 6.53 6.73 6.98 6.87 Table I. This composition is considered as also modified during mastitis and this spe- relatively constant but slight variations can cially concerns concentrations of sodium and be observed in some cases. Thus, the cal- chloride ions which are strongly increased cium and phosphate contents are higher in (Tab. II, last column). Depending on the milks rich in proteins. For example, the cause and severity of mastitis, these changes milk from Normandy cows has a higher min- in composition can be more or less impor- eral content than the milk from Friesian, tant. Concerning the effect of diet on the Red Pied and Holstein cows. The concen- mineral composition of milk, there are few tration of minerals also varies with the time scientific studies. However, some differ- of the lactation period (Tab. II) and the ences in the milk salt concentration and most important changes in composition especially in the citrate concentration are occur at around parturition; thus the cal- related during the change in season. In fact, cium concentration in colostrum is much there is a causal effect of diet on citrate con- higher than that of normal milk and near the centration related to the de novo synthesis end of lactation. Mineral composition is of fatty acids by the mammary gland [4]. The minerals of milk 475 Table III. Salts partition in cow milk [35]. [5]. No macro-element is bound in impor- tant amount to the fat globules and lactose. Constituents Concentration –1 Casein 26.1 g·L 3.1. Minerals in the diffusible fraction Total Ca 29.4 mM Soluble Ca 9.2 mM 3.1.1. Preparation of diffusible fraction % of soluble Ca 31% Experimentally, the determination of Micellar Ca 20.2 mM salt content in the aqueous phase is not Micellar calcium·g–1 of casein 0.77 mM problematic but implicates, before analysis, Total Pi 20.9 mM the obtention of this fraction. Four different Soluble Pi 11.2 mM methods to obtain this phase are used: dial- ysis, ultrafiltration, ultracentrifugation and % of soluble Pi 54% rennet coagulation followed by recovery of Micellar Pi 9.7 mM whey. During the preparation of the sample Micellar Ca/micellar Pi 2.08 by one of these techniques, it is imperative to work at a controlled pH and temperature Total Mg 5.1 mM since mineral equilibrium is very depend- Soluble Mg 3.3 mM ent on these physico-chemical parameters. % of soluble Mg 65% For dialysis and ultrafiltration, it is recom- mended to use membranes with a molecular Micellar Mg 1.8 mM weight cut-off inferior to 10 000–15 000 Da Ester phosphate 3.5 mM in order to avoid a transfer of small mole- Total citrate 9.2 mM cules and proteins able to bind minerals in the dialysat or ultrafiltrate. For ultracentrif- Soluble citrate 8.2 mM ugation, different conditions of centrifuga- % soluble citrate 89% tion can be used. Two typical conditions are Soluble Na 24.2 mM 80 000 g, 2 h and 100 000 g, 1 h. However Soluble K 34.7 mM for those concentrations determined in the dialysat, rennet whey, ultrafiltrate or ultra- Soluble Cl 30.2 mM centrifugal supernatant correspond rigor- ously to the real concentration in the milk; it is therefore necessary to use a correction 3. DISTRIBUTION OF MACRO- factor. Thus, the mineral concentrations ELEMENTS IN COW MILK found in these samples must be converted into diffusible mineral concentrations by In milk, all of these macro-elements are multiplying by a 0.96 correcting factor which distributed differently into diffusible and takes into account the excluded volume non diffusible fractions (essentially casein effect. Moreover, some slight differences of micelles). Potassium, sodium and chloride results can exist depending on the type of ions are essentially diffusible although cal- sample preparation. Thus, the concentra- cium, inorganic phosphate and magnesium tions found in the ultracentrifugal superna- are partly bound to the casein micelles tant are generally more important than those (Tab. III). About one-third of calcium, half found in the ultrafiltrate. This difference is the inorganic phosphate, two-thirds of mag- related to the presence of whey proteins and nesium and over 90% of citrate are in the soluble caseins in the ultracentrifugal super- aqueous phase of milk. A small proportion natant (and not in the ultrafiltrate) which are of calcium is also bound to α-lactalbumin able to bind some ions such as calcium and (there is one atom of calcium per protein) magnesium. 476 F. Gaucheron Table IV. Concentrations (mM) of free and associated ions in the aqueous phase of milk [7]. Anion Free ion Complexed ion Ca2+ Mg2+ Na+ K+ – 1 H2Cit + ++++ HCit2– 0.04 0.01 + + + Cit3– 0.26 6.96 2.02 0.03 0.04 – H2PO4 7.50 0.07 0.04 0.10 0.18 2– HPO4 2.65 0.59 0.34 0.39 0.52 3– PO4 +0.01+ + + Glc 1-PH– 0.50 + + 0.01 0.01 Glc 1-P2– 1.59 0.17 0.07 0.10 0.14 H2CO3 0.11–––– – HCO3 0.32 0.01 + + + 2– CO3 +++++ Cl– 30.9 0.26 0.07 0.39 0.68 – HSO4 +++++ 2– SO4 0.96 0.07 0.03 0.04 0.10 RCOOH 0.02 – – – – RCOO– 2.98 0.03 0.02 0.02 0.04 Free ion 2.00 0.81 20.92 36.29 3.1.2.