Retort salad and its production process

Retort salad having improved emulsification heat stability can be produced by using mayonnaise-like food containing a partial hydrolyzate of an alcohol-denatured soybean protein with protease as an emulsifier.

BACKGROUND OF THE INVENTION 
This invention relates to a process for producing retort salad using 
mayonnaise-like foods containing soybean protein as an emulsifier. 
Because of the advantages of long shelf life and convenience, a variety of 
so-called "retort foods", that are prepared by charging foods into heat 
and pressure resistant containers such as cans, jars, bottles, bags, and 
tha like and then heat sterilizing the foods at 100.degree. C. or above, 
usually 120.degree. C. or above, have been put on the market in the past. 
Though potato salad and tuna salad have been commercially available as the 
retort salad foods using mayonnaise, they fail to make the best of the 
good body inherent in mayonnaise because the emulsion breaks down and the 
vinegar and oil separate from each other. 
On the other hand, mayonnaise-like foods using soybean protein as an 
emulsifier instead of eggs has been proposed, for example, in U.S. Pat. 
No. 4,163,808 and a retort salad can be produced using such 
mayonnaise-like foods. It is recommended to use soybean proteins which 
have been partially hydrolyzed with an enzyme in the production of retort 
salad because the proteins can considerably improve the emulsifying power. 
In this case too, however, the emulsifying power is in sufficient and the 
emulsion is likely to be broken down by heating during the retort salad 
production process. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to obtain a high quality 
retort salad whose emulsification stability is not lost even during 
heating. 
The abovementioned object can be accomplished by a process which comprises 
reacting protease with an aqueous dispersion of an alcohol-denatured 
soybean protein to cause partial hydrolysis and then concentrating or 
drying the reaction product, preparing a mayonnaise-like food using the 
resulting partial hydrolyzate of the soybean protein as an emulsifier, 
mixing the food with salad raw materials, packing the mixture into a 
container capable of withstanding heat and pressure, and finally 
heat-sterilizing the contents at an elevated temperature. 
The present invention has been made on the basis of the finding that when a 
soybean protein whose water-solubility has been lowered by 
alcohol-denaturation is freated with an enzyme the emulsifying power as 
well as emulsification stability, in particular the emulsification 
stability during heating, can be remarkably improved within the range from 
neutral to acid. 
In accordance with the present invention, a retort salad is provided which 
has extremely high emulsification heat stability. The term "emulsification 
haeat stability" used herein means the property that the emulsion is not 
broken down when a salad using the mayonnaise-like food is packed into a 
heat and pressure resistant container and heat-sterilized at a temperature 
of 100.degree. C. or above after deairing and sealing. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The soybean proteins to be used in the process of the present invention may 
be any of the ordinary skimmed soybeans, extracted soybean protein, 
concentrated soybean protein and isolated soybean protein. These starting 
soybean proteins are washed with an alcohol, preferably with aqueous 
ethanol. Washing with the alcohol is carried out in the customary manner 
wherein the alcohol concentration is medium ranging from about 50 to 80%, 
the solvent ratio is about 5 to 20 times by weight and the washing 
temperature is preferably below the boiling point at the alcohol 
concentration employed. It is of course possible to use alcohol steam. By 
this washing, the soybean protein is denatured to an ordinary nitrogen 
solubility index of below 20 and its solubility is lowered. The higher the 
degree of denaturation, the more favourable the influence upon the 
emulsification stability. 
The emulsifying power of the alcohol-denatured soybean protein is low, but 
when it is partially hydrolyzed with an enzyme the soybean protein 
acquires excellent emulsification heat stability, which is the 
characterizing feature of the present invention. Partial hydrolysis is 
effected in the following manner. The alcohol-denatured soybean protein is 
added to and dispersed in water and the pH is adjusted. While neutral or 
alkaline protease such as "Bioprase" (a product of Nagase Sangyo K.K.), 
papain or "Alkarase" (a product of Novo Co.) is being added and the 
dispersion is stirred, it is subjected to the enzymolysis. It is preferred 
in this case that the soybean protein is hydrolyzed at the optimal pH and 
temperature of the particular enzyme being used. 
Hydrolysis is terminated at a point where the solubility ratio of protein 
in trichloroacetic acid (hereinafter referred to as "TCA solubility 
ratio") reaches 8 to 35%, preferably 10 to 25%. If the TCA solubility 
ratio is below 8%, the emulsification heat stability of the resulting 
retort salad is poor and the oil separates out. If the ratio exceeds 35%, 
on the other hand, both emulsification stability and emulsification heat 
stability become undesirably inferior. When the protein solution 
hydrolyzed to the intended TCA solubility ratio is heated (generally to 
90.degree. C. or above), the enzyme is inactivated. After the enzyme is 
inactivated, the protein is concentrated or dried after insoluble matters 
are either removed or not. 
Using the thus obtained partially hydrolyzed soybean protein as the 
emulsifier, fats, vinegar, seasonings and the like are mixed and 
emulsified to provide a mayonnaise-like food. The amount of the soybean 
protein used is 0.5 to 10%, preferably 1 to 5%, calculated as dry weight 
over the total weight of the mayonnaise-like food. If the amount is below 
0.5%, the emulsification heat stability of the retort salad becomes 
inferior and the oil separates. If the amount exceeds 10%, the 
emulsification heat stability of the retort salad can be improved but the 
viscosity of the mayonnaise-like food itself becomes so high that it 
cannot be mixed smoothly with the salad materials. 
The mayonnaise-like food obtained in this manner is then mixed with the raw 
materials for the salad such as fishmeat, meat of poultry, tissue-like 
soybean proteins, vegetables, seasonings, condiments, and so forth. As the 
salad raw materials, fishmeat flakes such as of tuna and victorfish, 
loosened chicken white meat, tissue-like soybean flakes restored by water, 
boiled and mashed potato, sliced onion, sweetcorn powder, seasoning, 
condiments, and so forth can be used either alone or in combination. 
The mixture of salad material and mayonnaise-like food is packed into a 
can, a jar, a heat-resistant bag or a bag-like container, deaired and 
sealed, then placed in a sterilization oven and heated to be heat 
sterilized for a sufficient period at a temperature (generally 100.degree. 
C. or above) at which heat-resistant bacteria are killed. A retort salad 
having extremely high emulsification heat stability is thus obtained. 
Hereinafter, the present invention will be described in further detail with 
reference to Examples thereof which are merely illustrative and in no way 
limiting. In the Examples, the terms "part" or "parts" mean the "part by 
weight" or "parts by weight".

EXAMPLE 1 
10 Kg of lowly denatured skimmed soybean was washed with 100 Kg of 60 wt% 
aqueous ethanol solution inside a sealed tank at 50.degree. C. for 30 
minutes. Filtration of the solution provided 15.5 Kg of cake. When the 
cake was dried and then milled by a vacuum dryer, 6.8 Kg of concentrated 
soybean protein was obtained in a powder form having 6.3% moisture 
content, 68% of crude protein (calculated as an anhydride) and 10.5 of 
NSI. The protein was placed into a tank conteining 75 l of hot water at 
60.degree. C. and equipped with a jacket, and was stirred and dispersed. 
The pH was adjusted to 8.5 with sodium hydroxide. After 25 g of "Bioprase 
SP-4" (a product of Nagase Sangyo K.K.) was added, the protein was 
hydrolyzed at 56.degree. C. for 60 minutes. The pH was further adjusted to 
9.5 with sodium hydroxide, and the enzyme was inactivated by heating to 
92.degree. C. for 15 minutes. The TCA solubility ratio of the product was 
12.8%. The TCA solubility ratio was determined in the following manner: 
10% aqueous solution of the sample and the same amount of 20% aqueous 
trichloroacetic acid solution were mixed and were left to stand for 60 
minutes. After insoluble matter was removed, nitrogen in the supernatant 
was determined by the Kjeldahl method and the solubility of the sample 
with respect to the total nitrogen was expressed as a percentage. 
The partially hydrolyzed protein solution was neutralized with hydrochloric 
acid, vacuum concentrated, and spray-dried to give 6.1 Kg of the intended 
soybean protein. 
600 parts of soybean salad oil, 150 parts of vineger, 180 parts of water 
and 45 parts of seasoning were added to 25 parts of the soybean protein 
obtained above and were homogeneously mixed and emulsified to give a 
mayonnaise-like food. Both the soybean protein and the seasoning were 
dispersed and dissolved in the water in advance, and while they were being 
stirred with a Homomixer the soybean salad oil was added dropwise to be 
emulsified, followed by addition of the vinegar. 
130 parts of the mayonnaise-like food were mixed with 660 parts of potato 
that had been boiled, peeled, and then mashed, 70 parts of sweetcorn 
grains, 60 parts of carrot that was cut in about 5 mm cubes and boiled, 70 
parts of onion sliced in a 3 cm-length and 10 parts of seasoning including 
salt, sugar, white pepper, etc. to make a potato salad. 
About 100 g of this potato salad was packed in a heat-resistant plastic 
film bag, which was then deaired and sealed by a heat-fusion bonding 
device equipped with a vacuum pump. The bag was then placed in a 
sterilizing oven, heated and sterilized for 4 minutes after the 
temperature of the center of the bag reached 120.degree. C., and cooled to 
give the retort potato salad. 
This product had a stable emulsion and the mayonnaise had good body as the 
vinegar and oil did not separate from each other. The emulsion remained 
stable even after the product was stored at 37.degree. C. for 3 months. 
For comparison, three kinds of mayonnaise or mayonnaise-like foods were 
prepared in the same way as in Example 1 using, as the emulsifier, the 
soybean protein of Example 1, the egg yolk as a Comparative Example 1 and 
a partially hydrolyzed soybean protein not denatured with alcohol (TCA 
solubility ratio 13.5%) prepared by reacting the lowly denatured skimmed 
soybean with "Bioprase" in the same way as Example 1, respectively. Using 
the mayonnaise or mayonnaise-like foods, three kinds of retort salads were 
prepared, their emulsification heat stabilities were compared, and the 
results are shown in Table 1. 
TABLE 1 
______________________________________ 
Emulsification heat stability of emulsifiers 
(% by weight of total weight of isolated oil) 
Mayonnaise or 
mayonnaise-like 
Retort potato 
Emulsifier food salad.sup.(1) 
______________________________________ 
Soybean protein of 
O O (O) 
this Invention 
Egg yolk 11.3 2.1 (16.2) 
Partially hydro- 
4.6 1.4 (9.6) 
lyzed soybean 
protein not alcohol- 
denatured 
______________________________________ 
Remarks .sup.(1) The value in the parenthesis represents weight percentag 
based on the amount of mayonnaise or mayonnaiselike food the isolated oil 
was added to. 
The same amount of egg yolk, calculated as dry, as of the soybean protein 
was added, and the water content was adjusted. 
The emulsification heat stability of the mayonnaise or mayonnaise-like 
foods was expressed by the weight percentage of the total amount of 
separated oil when about 200 g of mayonnaise or mayonnaise-like food was 
packed into a vinylidene chloride casing having a bent diameter of 55 mm, 
sealed and heated to 80.degree. C. for 30 minutes and thereafter 
centrifuged at 2,000 rpm after cooling to isolate the oil. The 
emulsification heat stability of each retort potato salad was also 
measured in the same way. 
The mayonnaise-like food and retort salad using the soybean protein in 
accordance with the present invention exhibited better emulsification heat 
stability during heating than those which used the egg yolk and the 
partially hydrolyzed soybean protein not denatured with alcohol. 
EXAMPLE 2 
A tuna salad was prepared by mixing 300 parts of the mayonnaise-like food 
of Example 1 with 300 parts of tuna flakes, 140 parts of onion sliced in a 
length of 3 cm and 10 parts of seasonings such as mustard powder, sodium 
glutaminate, white pepper, and so forth. 
About 200 g of this tuna salad was packed in a heat-resistant plastic film 
bag and deaired and sealed in the same way as in Example 1. The bag was 
then placed into a high temperature quick sterilization oven. After the 
temperature at the center of the bag reached 135.degree. C., 
heat-sterilization was effected for 20 seconds. The bag was cooled to give 
the retort tuna salad. 
The salad had stable emulsification and the vinegar and oil did not 
separate. The salad had the good body and taste inherent in the mayonnaise 
and its emulsion remained stable even after storage at 37.degree. C. for 3 
months.