Journal of Science of Food and Agriculture J Sci Food Agric 79 :253–256 (1999)

Insolubilisation and gelation of heat–frozensoymilkMakoto Shimoyamada,* Kayoko Toü mats u and Kenji WatanabeLaboratory of Food Material Engineering, Faculty of Agriculture,Gifu Univers ity , 1-1 Yanagido,Gifu 501-1193, Japan

Abstract : Soymilk prepared from soybean seed was heated and frozen. After thawing, precipitation

was shown to occur in soymilk. Precipitation from heated and frozen soymilk increased with increase

in heating time. Precooling treatment (Ô5ÄC) before freeze-storage of heated soymilk resulted in gel-

like solidiücation of soymilk. The gel strength of the freeze-gel formed from soymilk was related to

the heating time and the viscosity of the soymilk before freezing.

1999 Society of Chemical Industry(

Keywords: soymilk ; freeze-storage; gelation; freeze-gel

INTRODUCTION

Soybeans have been widely utilised for foodstuþs aswell as a source of oil. They contain a large amountof protein and oil which are very excellent qualitiesin term of nutrients and functionality. Soymilk,which is a kind of emulsion constituted of soybeanprotein and lipids, is used not only as a beverage butalso to make tofu (soybean curd). Soymilk containsvery high levels of protein, lipid and other nutrients.

In processing tofu, heated soymilk is mixed withcoagulants such as magnesium chloride and/orcalcium sulphate etc.1,2 Soybean protein whichsuþers partial unfolding by heat treatment is cross-linked through polypeptide chains to form a three-dimensional network. Generally speaking, gelation isexplained as the formation of a three-dimensionalnetwork of high molecular weight components, suchas proteins and polysaccharides. Lipids also contrib-ute to the formation of tofu, as well as protein. Thephysical properties of tofu gels have been reportedby many workers.3h5

Hashizume6 reported that soybean protein wasprecipitated during freeze-storage after heating ofwater extracts from defatted soybeans. In thisprocess, the precipitation was reported to increasewith the increase in protein concentration or thedecrease of freeze-storage temperature, reachingabout 70% at maximum. He suggested that this pre-cipitation of soybean protein might be utilised foradditives to hams, sausages and similar products.Lately, Soeda7 reported that the cold-gel was formedduring refrigeration storage at 5¡C of soybeanprotein paste.

Therefore we have applied this freeze-precipitation to soymilk, or the mixture of protein

and lipid. The degree of precipitation was deter-mined and the morphology of freeze-storage soymilkwas observed.

MATERIALS AND METHODS

Preparation of soymilk

Soymilk was manually prepared from commerciallyavailable soybean seed (Glycine max L Merrill,Tsurunoko). Soybean seeds were imbibed in 5 partsdeionised water (by weight) to one part seeds afterwashing with tap water, and milled and squeezed atabout 70¡C. The resulting soymilk contains 65g kg~1of protein.

Processing of soymilk

Raw soymilk preparations were heated at 100¡C in ascrew-capped test tube, or at 110¡C in an autoclave,for various times. The heated soymilk was put intoindividual plastic tubes (30ml each) when necessary,and stored at a certain temperature in a temperature-controlled refrigerator or a freezer. Freeze-storedsoymilk was thawed in warm water at 30¡C.

Estimation of freezing curves of soymilk

The temperature of the soymilk during cooling andfreezing was recorded with a LR4210 or LR4220 penrecorder (Yokogawa Electric Co) using a thermocou-ple (Type T ; Japan Industrial Standard).

Determination of protein

Protein content was measured by Lowry’s method.8A sample was diluted with 0.1M phosphate buþer(pH 7.6) and bovine serum albumin was used forstandard.

* Corres pondence to : Makoto Shimoyamada, Laboratory of FoodMaterial Engineering, Faculty of Agriculture, Gifu Univers ity, 1-1

Yanagido, Gifu 501-1193, Japan

Contract/grant s pons or : Nakano Vinegar Co Ltd(Received 9 September 1997; revis ed vers ion received 2 March

1998; accepted 27 May 1998)

( 1999 Society of Chemical Industry. J Sci Food Agric 0022–5142/99/$17.50 253

M Shimoyamada, K K WatanabeToü matsu,

Sodium dodecylsulphate-polyacrylamide gel

electrophoresis (SDS-PAGE)

SDS-PAGE of proteins in soymilk was carried outby Laemmli’s method9 using 12.5% acrylamide gels.Electrophoresis gels were stained by coomassie bril-liant blue R-250.

Measuring viscosity and gel strength

Viscosity was measured using a cone and plate typeviscometer (Visconic ELD, Tokimec Co Ltd). Theshear rate used was 190s~1. Gel strength was deter-mined using a rheometer (Rheoner RE3305,Yamaden Co). Gels, in a plastic vessel (50mmheight ; 30mm id), were compressed with a cylin-drical plunger (16mm od) to 60% of the originalsample height at a rate of 1mm s~1.

RESULTS AND DISCUSSION

Precipitation during freeze-storage of heated

soymilk

Previously, Hashizume6 showed that heating andsubsequent freeze-storage led to some precipitationfrom a soybean protein solution prepared from defat-ted soybean. In this study, using soymilk from wholesoybean, which is considered to be an emulsion ofsoybean proteins and lipids, the precipitation afterheat–freeze treatment of soymilk was determined,after thawing of freeze-treated samples.

After 7 days of freeze-storage of soymilk heated at100¡C, precipitation, which was calculated from theamount of protein in the supernatant after thawingand centrifugation (1000]g, 30min), increased withincrease in heating time and reached a maximum at3min of heating, where 75% of the initial amount ofprotein was precipitated. Further heating of soymilkled to no further increase of precipitation in freeze-storage soymilk (Fig 1). With 48 days freeze-storageof heated soymilk, similar trends were shown, andthe precipitation was slightly higher than that for thesample freeze-stored for 7 days.

The supernatant was also subjected to SDS-PAGE (Fig 2). From this SDS-PAGE pattern, thebands from each of the subunits of conglycinin and

Figure 1. Precipitation of heated s oymilk during freeze-s torage.

7 days freeze-s torage; 48 days freeze-s torage. Soymilk…, L,was heated at 100¡C.

Figure 2. SDS-PAGE patterns of the s upernatant of thawed

s oymilk after heating and s ubs equent freeze-s torage. MW,

molecular marker (1) 1 day; (2) 2 days ; (3) 3 days (4) 7 days ; (5)

14 days ; (6) 21 days ; (7) 28 days of freeze s torage. Each s ample

was heated for 3min at 110¡C. a, a-s ubunit of conglycinin ; b,b-s ubunit of conglycinin ; A, acidic polypeptides of glycinin ; B,bas ic polypeptides of glycinin.

the acidic polypeptides from glycinin were main-tained during freeze-storage. On the other hand,basic polypeptides of glycinin disappeared in thesupernatant during freeze-storage. These datashowed that the basic polypeptides of glycinin trans-ferred to insoluble precipitates. Heat treatment ofsoy protein or 11S solutions was reported to causecleavage of glycinin subunits to the acidic and thebasic polypeptides. The basic polypeptides disso-ciated from glycinin subunits formed insolubleaggregates.10h12 In this study on soymilk, the aggre-gates derived mainly from basic polypeptides mayhave responded to initiation of gel formation. Theband from the soluble aggregates, detected at the topof the gel, decreased in intensity with increase infreeze-storage time, and almost disappeared after 14days, when soluble aggregates from the soybean pro-teins were thought to be totally changed to insolubleprecipitate.

Effects of cooling and/or freezing conditions oninsolubilisation of heated soymilk

The eþects of cooling and/or freezing conditions forfreeze-storage of heated soymilk were studied. Ini-tially soymilk samples heated at 110¡C, which con-tained 65g kg~1 protein, were cooled in air to roomtemperature. Then the samples were put into arefrigerator which was controlled at [20¡C or[80¡C. After storage for 14 days, thawed sampleswere either partially gel-like or übrous precipitates(Figs 3(A) and (B)). Next, heated soymilk was cooledto room temperature, cooled to [5¡C for 2h, andthen stored at [20¡C for 14 days. This sample was agel-like mass resembling yogurt, or soft tofu, afterthawing (Fig 3(C)). On the other hand, soymilkwhich was stored at [5¡C for 14 days showed noprecipitation (Fig 3(D)). These results showed thatthe precooling caused heated soymilk to form a gel-like coagulant after freeze-storage. After centrifu-

254 J Sci Food Agric 79 :253–256 (1999)

Soymilk freeze-gel

Figure 3. Morphology of heated

s oymilk after freezing and

thawing under various

conditions : (A) frozen at

[20¡C for 14 days ; (B) frozenat[80¡C for 14 days ; (C)cooled at[5¡C for 2 hand then frozen at[20¡Cfor 14 days ; (D) cooled at

[5¡C for 14 days . Each s amplewas previous ly heated for

3min at 110¡C.

gation (1000]g, 30min) of the freeze–thawedsample with precooling, the precipitation was esti-mated to be about 78% of total protein, which isalmost the same level as for the freeze–thawedsample without precooling (Fig 1). Gel-like coagu-lation of soymilk may be related not only to insolubi-lisation of soybean protein but also to other factors.

In order to characterise the precooling process,freezing curves were determined during the coolingand freezing processes (Fig 4). In the case of directfreezing at [20¡C, the temperature of the sampledecreased to about [0.4¡C, at which ice crystalsbegan to form with little supercooling, then the tem-perature remained constant for about two hourswhilst the ice crystals grew larger. In the case of a2-step freezing, during precooling the sample was

Figure 4. Freezing curves of heated s oymilk s ubjected to certain

cooling conditions . Solid line, directly frozen at[20¡C; brokenline, cooled at[5¡C for 2 h and then frozen at[20¡C. Eachs ample was previous ly heated for 3min at 110¡C and cooled toroom temperature.

maintained in the supercooled state without icecrystal formation. After changing the freezer tem-perature to [20¡C, the sample temperature grad-ually decreased, after a much shorter ýat region, thanthat observed for direct freezing.

During freezing of heated soymilk, the surfacetemperature of the sample would decrease faster thanin its centre. Ice crystals were considered to form atthe surface initially, and gradually grew into theinner part of soymilk over a relatively long period oftime. This growing phase of the ice crystals resultedin the large size of the ice crystals ranging from thesurface to the centre of the sample. The large spaces,which were derived from large ice crystals afterthawing of ice crystals, are thought to disturb the gelformation on a macro scale. On the other hand,during the percooling stage, the soymilk was main-tained at supercooling and the temperatures invarious parts of the sample were also same (about[5¡C). When the temperature in the freezer waschanged to [20¡C, the destruction of supercoolingand ice formation rapidly began everywhere invarious parts of the sample. A larger quantity of verysmall ice crystals appeared to be formed uniformlywithin a very short time. These ice crystals mayresult in the apparently uniform gelation of soymilkduring freezing storage.

Gel strength and water-holding capacity of

freeze-gel from soymilk

Heating, precooling and freezing treatments wereshown to lead to gel-like coagulation of soymilk. Thestrain–stress curves of gels made from soymilk were

J Sci Food Agric 79 :253–256 (1999) 255

M Shimoyamada, K K WatanabeToü matsu,

Figure 5. Typical s tres s –s train curves of s oymilk freeze-gels .

Sample, freeze-gel made from s oymilk heated for 10min at

110¡C.

measured by a rheometer (Fig 5). Each freeze-gel hasa breaking point. The gel strength of the freeze-gelsincreased with the increase in heating time (Fig 6).Gel strengths reached a maximum at 3min ofheating time, which coincided with the precipitationduring the freeze-storage of the soymilk. The vis-cosity of heated soymilk before freezing alsoincreased in a similar way to the gel strength. Anincrease in gel strength appears closely related to theincrease in the prefreezing viscosity of soymilk. Thepresent ündings suggest the possibility that the gelstrength of certain products might be controlled.

Further, separated water was lower in gel-likecoagulants with precooling (28%) than in freeze pre-

Figure 6. Vis cos ity of heated s oymilk and gel s trength of the

res ulting freeze-gels . Solid line, vis cos ity ; broken line, gel

s trength. Each s ample was heated at 110¡C.

cipitates without precooling (35%). Precooling wasalso considered to be important for the water-holding capacity of freeze-gel from heated soymilk.

ACKNOWLEDGEMENTS

Support form Nakano Vinegar Co, Ltd is gratefullyacknowledged. We thank Ms H Shimazu and Ms YKato of Nakano Vinegar Co, Ltd for their technicalassistance.

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