The age-thickening of sweetened condensed milk: I. Rheological properties

1962 ◽  
Vol 29 (3) ◽  
pp. 249-258 ◽  
Author(s):  
Ruth Samel ◽  
M. M. Muers

SummaryThe viscosity and flow properties of fresh and age-thickened separated sweetened condensed milk have been investigated using a rotating cylinder viscometer. The rheological properties, and the changes therein brought about by stirring, were studied by means of flow curves obtained by plotting stress against the corresponding rate of shear. Condensed milk during age-thickening is shown to develop thixotropic properties, with a high initial viscosity or ‘gel strength’ which, however, is easily reduced by stirring. On prolonged stirring the viscosity reaches a minimum (the ‘permanent’ viscosity), which becomes progressively higher as the age-thickening process continues. On storage subsequent to stirring there is only an insignificant recovery of the lost viscosity at room temperature, but a substantial recovery at higher temperature.Sweetened condensed whey shows no age-thickening, and the casein-free supernatant liquids obtained by centrifuging dilutions of both fresh and age-thickened condensed milk have the same nitrogen contents and the same viscosities. This is interpreted to mean that the whey proteins play no significant part, directly or in association with casein, in the age-thickening process. Also the sedimentation rate of the casein during centrifuging is greater with age-thickened than with fresh condensed milk, which indicates an increase in casein particle size during agethickening.The following working hypothesis is suggested to account for this behaviour. During age-thickening a slow irreversible change occurs in the size or shape of the casein micelles, probably by aggregation, which produces the rise in ‘permanent’ viscosity. These micelles orient themselves in such a way as to form a loose network enclosing some of the dispersion medium, thereby producing the large increase in the initial viscosity. On stirring, this network is broken down, and although it can reform in time at high temperatures, such recovery is greatly retarded at room temperature by the high viscosity of the aqueous phase containing the soluble constituents of the condensed milk.

1991 ◽  
Vol 58 (3) ◽  
pp. 337-344 ◽  
Author(s):  
A. I. Alvarez de Felipe ◽  
Berta Melcón ◽  
Julián Zapico

SummaryThis work aimed to relate changes in rheological behaviour with structural details of different sweetened condensed milk samples as well as to clarify the role of additives in delaying gelation. To study the process of gelation, rheological techniques were applied to fresh and stored condensed milk samples for times and at temperatures known to favour gelation (45 d at 37 °C). Electron microscopy techniques were used to relate the rheological changes with observed structural details. Examination by electron microscopy showed that micellar aggregation and association of casein micelles with fat globules increased during storage. The whey proteins appeared to play an important role in the thickening process.


1998 ◽  
Vol 65 (4) ◽  
pp. 555-567 ◽  
Author(s):  
JOHN A. LUCEY ◽  
MICHELLE TAMEHANA ◽  
HARJINDER SINGH ◽  
PETER A. MUNRO

The effect of interactions of denatured whey proteins with casein micelles on the rheological properties of acid milk gels was investigated. Gels were made by acidification of skim milk with glucono-δ-lactone at 30°C using reconstituted skim milk powders (SMP; both low- and ultra-low-heat) and fresh skim milk (FSM). The final pH of the gels was ∼4·6. Milks containing associated or ‘bound’ denatured whey proteins (BDWP) with casein micelles were made by resuspending the ultracentrifugal pellet of heated milk in ultrafiltration permeate. Milks containing ‘soluble’ denatured whey protein (SDWP) aggregates were formed by heat treatment of an ultracentrifugal supernatant which was then resuspended with the pellet. Acid gels made from unheated milks had low storage moduli, G′, of <20 Pa. Heating milks at 80°C for 30 min resulted in acid gels with G′ in the range 390–430 Pa. The loss tangent (tan δ) of gels made from heated milk increased after gelation to attain a maximum at pH ∼5·1, but no maximum was observed in gels made from unheated milk. Acid gels made from milks containing BDWP that were made from low-heat SMP, ultra-low-heat SMP and FSM had G′ of about 250, 270 and 310 Pa respectively. Acid gels made from milks containing SDWP that were made from ultra-low-heat SMP or FSM had G′ values in the range 17–30 Pa, but gels made from low-heat SMP had G′ of ∼140 Pa. It was concluded that BDWP were important for the increased G′ of acid gels made from heated milk. Addition of N-ethylmaleimide (NEM) to low-heat reconstituted milk, to block the —SH groups, resulted in a reduction of the G′ of gels formed from heated milk but did not reduce G′ to the value of unheated milk. Addition of 20 mm-NEM to FSM, prior to heat treatment, resulted in gels with a lower G′ value than gels made from reconstituted low-heat SMP. It was suggested that small amounts of denatured whey proteins associated with casein micelles during low-heat SMP manufacture were probably responsible for the higher G′ of gels made from milk containing SDWP and from milk heated in the presence of 20 mm-NEM, compared with gels made from FSM.


2018 ◽  
pp. 64-66
Author(s):  
E.D. Kalinina ◽  
◽  
E.Yu. Porotova ◽  
R.A. Filonov ◽  
A.D. Lodygin ◽  
...  

1962 ◽  
Vol 29 (3) ◽  
pp. 269-277 ◽  
Author(s):  
Ruth Samel ◽  
M. M. Muers

SummaryAge-thickening of sweetened condensed separated milk can be greatly reduced by removing part of the diffusible salts by dialysis of the milk before processing. This treatment is still effective after high-temperature preheating, which indicates that denaturation of the whey proteins does not influence age-thickening independently of the milk salts.When added at the level of 20 m-equiv./kg of condensed milk to the milk before condensing, those polyvalent anions (phosphate, oxalate, citrate, ethylenediaminetetra- acetate) which have the property of forming undissociated or insoluble calcium salts greatly increase the rate of age-thickening. Fluoride, which forms an insoluble calcium salt but is monovalent, has the opposite effect.It is suggested that both natural and added polyvalent anions destabilize the condensed milk by forming linkages between the calcium atoms of the calcium caseinate micelles, producing the larger, more asymmetric units which were postulated in Part I to account for the viscosity changes during age-thickening. Fluoride would have the opposite effect by combining with the calcium, and so preventing such linkage formation by the polyvalent anions.When added to the milk at the same level as the anions, divalent cations, especially calcium and zinc, reduce the rate of age-thickening. It is suggested that this is due to removal of milk-serum anions, particularly phosphate and citrate, as insoluble or undissociated salts.The effect of both anions and cations added after condensing is almost the opposite of that produced by the same ion before condensing, with the exception that phosphate increases age-thickening in both cases. There is as yet no clear explanation for this observation.


2013 ◽  
Vol 2013 ◽  
pp. 1-5 ◽  
Author(s):  
John Yew Huat Tang ◽  
Bariah Ibrahim Izenty ◽  
Ahmad Juanda Nur’ Izzati ◽  
Siti Rahmah Masran ◽  
Chew Chieng Yeo ◽  
...  

This study aimed to investigate the survival ofVibrio choleraeO1 in 3 types of preparation for cooked rice,Oryza sativaL., (plain rice, rice with coconut milk, and rice with ginger); coffee,Coffea canephora, (plain coffee, coffee with sugar, and coffee with sweetened condensed milk); and tea,Camellia sinensis, (plain tea, tea with sugar, and tea with sweetened condensed milk) held at room temperature (27°C). The survival ofV. choleraeO1 was determined by spread plate method on TCBS agar. Initial cultures of 8.00 log CFU/mL were inoculated into each food sample. After 6 h incubation, significant growth was only detected in rice with coconut milk (9.67 log CFU/mL;P<0.05). However, all 3 types of rice preparation showed significant growth ofV. choleraeafter 24 h (P<0.05). For coffee and tea preparations,V. choleraesurvived up to 6 h in tea with condensed milk (4.72 log CFU/mL) but not in similar preparation of coffee. This study showed evidence for the survivability ofV. choleraein rice, coffee, and tea. Thus, holding these food and beverages for an extended period of time at room temperature should be avoided.


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