Joseph J. Rackis

Flavor problems of vegetable food proteins

Flavor is a major factor that limits the use of many vegetable proteins in foods. In high quality whole cereal grains, flavor and flavor stability present little or no problem; but when some cereals are further processed into protein concentrates and isolates, objectionable flavors can arise from oxidative deteri-oriation of unsaturated fatty esters in protein-bound lipids. However, degermed wheat and corn flours (endosperm products) have little or no flavor. Raw legumes and oilseeds enriched with respect to lipoxygenases and other metallo-proteins possess lipid-derived, objectionable flavor compounds. Lipoxygenase-mediated conversion of lipids to lipohydroperoxides and their subsequent degradation form volatile and nonvolatile constituents responsible for off-flavors. n-Hexanal, 3-cis-hexenal, n-pentylfuran, 2(1-pentenyl)furan, and ethyl vinyl ketone are major contributors to grassy-beany and green flavors. Higher 2,4-alkadienals have oxidized painty, rancid flavors sometimes noted in residual lipids. Geosmin, an oxygenated hydrocarbon, is responsible for the musty, moldy, earthy flavor of dry beans. This compound may contribute to similar flavors noted in soy and corn protein isolates. Thermally degraded phenolic acids account for some of the objectionable cooked odors of soy products that have been subjected to high temperature treatment such as retorting, autoclaving, and sterilization. Oxidized phosphatidylcholine most likely accounts for the bitter taste of soy products. Oxygenated fatty acids, including the bittertasting trihydroxy octadecenoic acids, have been identified in the bitter phosphatidylcholines isolated from soybeans. Oxidized lipids appear to be associatted with the bitter, astringent, and rancid flavors of protein isolates prepared from wet-milled corn germ flour. Grassy-beany, bitter flavor compounds preexist in the maturing soybean and are also generated during processing. In some legumes development of off-flavors can be readily controlled by rapid inactivation of lipoxygenase with heat, alcohol, or acid treatment. Legume powders of acceptable flavor quality can be prepared by wet-milling whole seeds in aqueous alcohols. Extraction of meals with hydrogen bond-breaking solvents, such as alcohols or azeotropic mixtures of hexane and alcohol, effectively removes protein-bound lipids to yield concentrates with greatly improved flavors. Soy protein concentrates approaching the blandness of wheat flour have been prepared by a combination of azeotrope extraction and steaming. Similar processes can also be used to greatly improve flavor scores of corn germ protein isolates. Based on our present knowledge about the identity of off-flavor constituents and how they are derived, much progress has been made to effectively remove or modify them. These developments should result in new emerging technology that would be applicable to the manufacture of highly acceptable protein products from various vegetable sources.

Protease Inhibitors in Plant Foods: Content and Inactivation

To understand the present widespread interest in protease inhibitors (PI), one must look back to the vast amount of research that has been conducted for over 50 years. Intensive investigations, like wave motions on the water, followed a highly cyclical pathway depending upon the interest and support for research in various disciplines. Nutritionists were concerned with the potential adverse effects of the inhibitors of trypsin and chymotrypsin in the intestinal tract of man and animals (Liener and Kakade, 1980; Liener, 1981; Rackis and Gumbmann, 1981); biochemists recognized that the peculiar properties of PI’s may yield fundamental information concerning protein-protein interactions especially the mechanism of proteolytic hydrolysis (Richardson, 1981; Ryan, 1979); medical scientists envisioned an important role for PI’s in the treatment of various metabolic disorders (Katunuma et al., 1983; Vogel et al., 1968; Weyer, 1968), and biologists foresaw an important physiological role of PI’s to regulate plant growth, protein synthesis and resistance to microbial and insect pests (Richardson, 1977, 1981; Ryan, 1973, 1979).

Flatulence Caused. by Soya and Its Control through Processing

Elimination of flatulence is a challenging practical problem associated with the comsumption of soybeans as well as other food legumes and other selected foodstuffs. The problem is compounded by the variability in susceptibility among individuals. Research has established that the oligosaccharides—verbascose, stachyose, and raffinose—are the major cause of soybean flatulence. They escape digestion and are fermented by intestinal microflora to form excessive amounts of carbon dioxide and hydrogen. Hot water treatment, aqueous alcohol extraction, and isoelectric protein precipitation processes have been adapted to produce flatus-free products commercially. At the household level, soaking combined with germination appears to be a practical means of producing soybean sprouts having low flatus activity. Food legumes, which include some oil-seeds, peas, and beans, as well as selected vegetables, contain enough of the oligosaccharides—verbascose, stachyose, raffinose—to be a major cause of flatulence in humans and animals. In the absence of alpha-galactosidases in the mammalian intestinal mucosa, these oligosaccharides escape digestion and are not absorbed. As a consequence, the active microflora in the ileum, colon, and fecal matter of the large intestine ferment them to form excessive levels of rectal gas, primarily carbon dioxide and hydrogen. In some instances, undigested starch and other carbohydrates contribute to the flatulent effect of diets. With 70% of the world’s population being lactase-deficient (hypolactasia), susceptibility to flatulence would be more widespread with diets containing both food legumes and milk products.Use of food additives, antibiotics, and phenolic compounds to inhibit flatulence is not a practical approach. However, soya processing technology used to manufacture protein concentrates and isolates can be adapted to produce flatus-free products from other food legumes. Hot water treatment, aqueous alcohol extraction, or isoelectric protein precipitation insolubilizes most of the protein and removes the oligosaccharides. Tempeh and tofu are two other soya products that exhibit little or no flatus activity. Soaking, fermentation, enzymatic hydrolysis, and germination can also be used to eliminate oligosaccharides. Tests with humans and rats indicate that a combination of such processes can be used to reduce flatus activity. The beneficial effects of germination on flatulence, often conflicting and contradictory, have been attributed to failure to control conditions that ensure removal of most of the oligossaccharides. Whether the high-molecular-weight soybean polysaccharides (dietary fiber), which normally do not cause flatulence, can be modified during germination to become substrates for flatus production by the intestinal microflora is not known. Such an effect could compensate for the loss of stachyose and raffinose.

Lipid-Derived flavors of legume protein products

Abstract and SummaryLegumes contain unsaturated lipids that are susceptible to oxidative deterioration. Enzymic and non-enzymic deterioration of these lipids results in the development of off-flavors. The primary objective of this review is to summarize what is currently known about lipid-derived flavors of soybeans and underblanched pea seeds(Pisum sativum). Identifying the numerous volatile compounds arising from breakdown of lipid hydroperoxides coupled with organoleptic evaluation defines the flavor problem. Major contributors to the green-beaniness of soybeans were found to be 3-cis-hexenal, 2-pentyl furan, and ethyl vinyl ketone. Oxidized phosphatidylchohnes cause some of the bitter taste. The interaction of lipid breakdown products with proteins, carbohydrates, and other constituents can affect flavor characteristics and also increase the problems of their removal from soy protein products. To prepare bland products, it will be necessary to develop processes that effectively remove bound flavor components and prevent formation of derived flavors. Solvent systems based on alcohol have been used to extract flavor principles from soybeans; aqueous alcohol treatment of the intact seed or blanching with hot water or steam inhibits formation of off-flavors in peas and soybeans. A new approach involving infusion of antioxidants into the intact seed to control lipid deterioration during processing and storage is proposed to minimize flavor formation without subsequent undesirable changes in protein which occur with alcohol treatments.

Significance of soya trypsin inhibitors in nutrition

Although recent evidence clearly indicates that trypsin inhibitors (TI) and low protein digestibility are the major factors responsible for the pancreatic hypertrophic and growth inhibitory effects of raw soybeans, there was uncertainty regarding the biological threshold level of TI at which these biological effects occur. To obtain such data, dehulled defatted flakes (10% dietary protein) containing graded levels of TI were fed to weanling rats for 4 weeks in two feeding trials. Normal pancreas weights were obtained in rats fed samples in which only 54 to 68% of the original TI of raw soya flour was inactivated. In partially toasted flakes with a nitrogen digestibility value of 77%, the average tolerance level of dietary TI activity that did not cause pancreatic hypertrophy was calculated to be 385 mg TI/100 g diet. TI tolerance level at maximum nitrogen digestibility of 85%, which did not significantly lower weight gain and reduce protein efficiencey ratios, was 260 mg TI/100 g diet. Continuous ingestion of high levels of TI (459 mg TI/100 g diet) in a 20% protein diet for 215 days did not inhibit growth nor cause pancreatic hypertrophy when compared to rats fed toasted soya flour diets. Pancreatic hypertrophy that occurs in rats fed raw soya diets containing up to about 1300 mg TI/100 g diet for 35 days was reversed by switching the rats to control diets or to 30% toasted flour. In long-term feeding studies, no pancreatic hypertrophy occurred in rats fed commercial edible-grade soya flour, concentrate, or isolate from time of weaning to adulthood (ca. 300 to 330 days). TI content of the diets ranged from 178 to 310 mg/100 g diet. Microscopic examination of the pancreas revealed no abnormalities. Gross appearances of heart, kidney, spleen and liver were normal. In long-term feeding, vitamin B-12 supplements were needed to provide optimum growth and to maintain body weight. Results of numerous chemical analyses, relatively short-term human tests and long-term animal feeding studies indicate that with proper control of manufacturing processes, soya protein products can be produced that, in mixed diets, have protein nutritional value approaching that of animal protein.

Development of a pilot plant process for the preparation of a soy trypsin inhibitor concentrate

A pilot-plant procedure was developed to prepare a soy trypsin inhibitor (TI) concentrate in sufficient quantities to support a lifetime (2-yr) feeding trial in which diets containing varying amounts of TI would be fed to rats to assess the physiological effects on the pancreas and other organs. Starting with water dispersions of commercial defatted soy flour, separation of TI (MW<21,500) from non-TI protein (MW 180,000–350,000) by virtue of their MW difference was attempted using ultrafiltration techniques but was not successful. However, good separation was obtained when selective acid precipitation coupled with “salting in” of the TI with 0.1 N sodium chloride was employed. Low MW components were separated successfully by ultrafiltration using a 1,000 MW cutoff membrane. The final soy TI concentrate obtained by freeze drying exhibited a 9-fold increase in TI activity.