Survival diets for specific populations: Technological innovations in low-phenylalanine peptide foods and low-sodium diets

Phenylketonuria (PKU) is a genetic disorder. People with this condition lack phenylalanine hydroxylase, an enzyme that converts the essential amino acid phenylalanine into tyrosine. Therefore, by the age of 15, when brain development is complete, if a phenylalanine-restricted diet is not initiated, phenylalanine will accumulate abnormally in the body, leading to symptoms such as intellectual disability. Therefore, these patients must be given foods containing phenylalanine, which is essential for development, but at low concentrations.

Previously, patients with this condition were treated with special formula containing a mixture of amino acids but with a low concentration of phenylalanine. However, this formula tasted unpleasant, with a peculiar fishy smell. Infants disliked it, leading to developmental delays. Furthermore, long-term consumption of this unpleasant formula was also distressing. Another drawback was the high osmotic pressure of the amino acid mixture, which significantly irritated the intestines and easily caused diarrhea. Given these drawbacks, a more natural food ingredient with a better taste than the amino acid mixture, namely low-phenylalanine peptides (LPP), was developed.

Whey protein is denatured by heating, then partially hydrolyzed by the proteolytic enzyme pepsin, and finally freed by streptomycin. This hydrolysate is then processed to obtain LPP, which has a low concentration of phenylalanine and other aromatic amino acids. Industrially produced LPP has been provided to the pediatric department of a university hospital for clinical trials on children with phenylketonuria. The trials showed that LPP can lower the concentration of phenylalanine in the blood, and no adverse effects were observed with long-term consumption.

Professor Teruo Kitagawa of the Department of Pediatrics at Surugadai Hospital, Faculty of Medicine, Nihon University, conducted an experiment using LPP on 10 children aged 0-3 years with phenylketonuria. He said, "The children really enjoy eating it. The earlier the treatment for this disease, the better the outcome. If a diet based primarily on this formula is continued from about 20 days after birth until the age of 15, when brain development is complete, brain disorders can be prevented." Some infants born to mothers with this disease do not inherit the disease factor but still develop heart defects, microcephaly, intellectual disabilities, and other conditions.

To avoid this, women with this condition should undergo dietary therapy before pregnancy. There are case records at the Surugadai Hospital's pediatrics department of women whose infants developed this condition underwent dietary therapy with LPP formula under a doctor's guidance after becoming pregnant, and successfully gave birth to healthy babies. In such cases, the mothers had to undergo special dietary therapy before pregnancy, which was previously extremely painful and difficult to maintain long-term. Professor Kitagawa said, "If the LPP diet is adopted, then all patients can adhere to it long-term."

LPP has become the most advanced food for preventing congenital metabolic disorders, and more foods for preventing other metabolic disorders are being developed. To date, salt intake remains high in Japan, especially in the Tohoku region. The knowledge that excessive salt intake easily leads to high blood pressure and stomach cancer is quite widespread, and therefore salt intake has been decreasing year by year. However, dietary habits are not so easily changed, and current salt intake has not yet reached the ideal level of less than 10 grams per day.

After food enters the stomach, it is digested by the secretion of gastric acid and enzymes that digest proteins. However, the stomach itself, being composed of protein, is generally not digested. Sometimes, the stomach wall is digested by the body's own digestive enzymes, leading to stomach ulcers. The stomach wall is not digested because it is protected by a mucous membrane. However, foods high in salt can easily damage this protective mucous membrane, potentially increasing the risk of stomach cancer due to the influence of digestive juices. To reduce salt intake, various low-salt (or reduced-salt) processed foods have been developed.

However, salt enhances the preservation of food, so certain treatments are necessary to maintain food shelf life in low-salt conditions. Low-salt soy sauce is made by processing regular raw soy sauce through ion exchange resins, dialysis membranes, and vacuum concentration, and is now commercially available. While this method is suitable for producing low-salt soy sauce and other special-purpose foods, some important flavor components, such as organic acids and nitrogenous substances, are also removed during desalting. Low-salt soy sauce is mostly made from defatted soybeans, with water added at the end.

While soy sauce made this way may have a decent umami flavor, it lacks depth of flavor. Some methods involve adding ethanol during brewing to inhibit the growth of harmful microorganisms under low-salt conditions. A common method for low-salt brewing is to add a high concentration (around 100,000 yeast cells per gram) of yeast to suppress harmful microorganisms. However, since lactic acid bacteria are also inhibited, lactic acid is insufficient, necessitating the addition of lactic acid before final flavoring. The manufacturing method for low-salt bean paste is similar to that of low-salt soy sauce; it also requires the addition of a high concentration of yeast and ethanol to inhibit the growth of harmful microorganisms.

There are many types of pickled vegetables, including stored pickled vegetables with a salt concentration of 17%–20%, and ready-to-eat pickled vegetables with a salt concentration of 2%–3%. Recently, the proportion of low-salt pickled vegetables (made by desalting high-salt pickled vegetables and then seasoning them) and quick-cured pickled vegetables (made with a small amount of salt) has gradually increased. Salt enhances the preservation of food, therefore, low-salt foods have poorer shelf life. Since the use of dehydroacetic acid as a preservative has been banned, the primary method for preservation is the addition of sorbic acid and heat sterilization.

Recently, the method of adding about 3% ethanol (a natural additive) in combination with other methods to improve food preservation has become increasingly common. Improving food preservation without relying on salt and preservatives, and instead by ensuring a clean and uncontaminated food manufacturing and distribution process, as well as through methods such as low temperatures, should be considered an important future research topic for the food industry.