Dietary proteins are known to possess a variety of nutritional, functional and biological properties. Nutritionally, the proteins are a source of energy and amino acids, which are essential for growth and maintenance. Functionally, the proteins contribute to the physicochemical and sensory properties of various protein-rich foods. Furthermore, many dietary proteins possess specific biological properties that make these components potential ingredients of functional or health-promoting foods. Many of these properties are attributed to physiologically active peptides encrypted in protein molecules. Particularly rich sources of such peptides are milk and eggs, but they are also found in meat and many plants. These peptides are inactive within the sequence of the parent protein but can be released during gastrointestinal digestion or food processing (Korhonen and Pihlanto, 2003).
An angiotensin converting enzyme (ACE) inhibitory peptide has been isolated and purified from the hydrolysates of irradiated bovine blood plasma protein. Blood plasma protein was irradiated to eliminate microbial contamination and was enzymatically hydrolyzed using the commercial proteases Alcalase, Esperase and Flavourzyme. An ACE inhibitory peptide was isolated using membrane filtration, gel permeation chromatography, and normal phase and reverse phase high-performance liquid chromatog-raphy. The purified ACE inhibitory peptide was identified to be a tripep-tide, His-Pro-Tyr (Lee and Song, 2003).
The physiological functions of enzymatic hydrolysates of collagen or keratin contained in livestock and fish waste have been studied and results demonstrated that the enzymatic hydrolysate of meat meal, a collagen waste, showed strong ACE inhibitory activity (IC50 = 0.6-2.8 mg/ml). In contrast, the enzymatic hydrolysate of a mixture of horn and hoof, a keratin waste, showed high antioxidative activity. Thus, collagen or keratin contained in livestock and fish waste may be convertible to useful products by enzymatic hydrolysis, thereby providing new physiologically functional food materials (Ohba et al., 2003).
Inhibitory activities against ACE of enzymatic hydrolysates of porcine skeletal muscle proteins have been investigated. Myosin B, myosin, actin, tropomyosin, troponin and water-soluble proteins extracted from pork loin were digested by proteases - including pepsin, alpha-chymotrypsin and trypsin. After digestion, hydrolysates produced from all proteins showed ACE inhibitory activities, and the peptic hydrolysate showed the strongest activity. In the case of myosin B, the molar concentration of peptic hydro-lysate required to inhibit 50% of the activity increased gradually as digestion proceeded. The hydrolysates produced by sequential digestion with pepsin and cc-chymotrypsin, pepsin and trypsin, or pepsin and pancreatin showed weaker activities than those digested by pepsin alone, suggesting that ACE inhibitory peptides from peptic digestion might lose their active sequences after digestion by the second protease. However, the hydro-lysates produced by sequential digestion showed stronger activities than those by chymotrypsin, trypsin or pancreatin alone. These results suggested that the hydrolysates of porcine meat were able to show ACE inhibitory activity, even if they were digested in vivo, and that pork might be a useful source of physiologically functional factors (Katayama et al., 2003).
Carnosine is a natural antioxidant present in skeletal muscle. A number of patents describe the potential use of carnosine as a therapeutic agent in applications such as: wound healing; treatment of hypertension and trauma; treatment of cataracts and stomach and duodenal ulcers; and also as a bactericidal, anti-inflammatory and immunomodulatory agent. A number of cosmetic applications for carnosine (e.g. with respect to skin aging) have also been described. Food applications for carnosine include use in food flavourings (beef/brothy) and as an additive to decrease deterioration in foods and discolouration rate of meat (reviewed by James et al., 1995).
Recent research has revealed that the degradation products of dietary sphingolipids are biologically active and have the capacity to inhibit the development of colon cancer in mice. An investigation of the content of sphingomyelin and neutral glycosphingolipids in commonly consumed meat and fish products reported generally lower amounts of sphingolipids in fish meat than in red meat and poultry, with poultry being the richest source of this class of lipids. In fish, the sphingomyelin/neutral glycolipids ratio varied from 1 to 2.9, while in poultry the ratio varied between 5.2 and 19.2, and in red meat it varied from 1.6 to 8.3 (Hellgren, 2001). If there is future demand for these bioactive ingredients, meat/poultry processing may be a source of these compounds.
The polyamines putrescine, spermidine and spermine occur in the cells of living organisms where they fulfil an array of physiological roles, including a role in human cell growth and proliferation which has been of great interest in studies on tumour growth. However, polyamines could be useful for post-operation patients, during wound healing, and for growth and development of the neonate digestive system. Both endogenous and dietary polyamines participate in such processes. Data on polyamine contents in foods are limited in the literature. While putrescine content increases as a result of bacterial activity during inappropriate storage and processing of foods of animal origin, spermidine and spermine originate mainly from raw materials. Higher contents of spermidine, compared with spermine, are typical for foods of plant origin, while an opposite relation is characteristic of foods of animal origin. Legumes, cauliflower and broccoli are foods with high spermidine content; while meat and meat products are high in sper-mine (Kalac and Krausova, 2005).
Immediately after slaughter, spermine and spermidine have been detected in red and white meats. Spermine was the prevalent amine, ~70% of total, while low levels of histamine were also detected in chicken thighs. During storage at 4 °C, there was a decrease in spermine, spermidine levels remained constant, and putrescine, cadaverine, histamine and tyramine were formed. At 15 days, higher levels of amines were found in breast compared with thigh. An index based on the ratio of the polyamines sper-midine and spermine was considered appropriate for the evaluation of chicken meat quality. Chicken-based meat products (mortadella, frankfurters, sausage, meatballs, hamburger and nuggets) were analyzed for bioac-tive amines. Nuggets were the only products with amine profiles similar to fresh chicken meat. There was a prevalence of spermidine over spermine for most of the products, suggesting the incorporation of significant amounts of vegetable protein in the formulations (Silva and Gloria, 2002).
Meat and meat products have effects on appetite and have shown highly satiating characteristics. If meat-based products could be designed to be have less energy (calorific) density, while remaining satiating and having sensory appeal, they could form the basis of functional foods to perhaps address the growing incidence of obesity (Fernandez-Gines et al., 2005).
During the last two decades, many food protein fragments have been demonstrated to elicit biological effects in various in vitro or in vivo test systems. A considerable number of these bioactive peptides are opioid receptor ligands, which may be regarded as exogenous supplements to the endogenous opioidergic systems in humans. Most of these food-derived opioid receptor ligands are fragments of milk proteins; however, bovine serum albumin and haemoglobin, i.e. constituents of meat, have been demonstrated to contain fragments behaving like opioid receptor ligands. Practically all of these compounds display opioid agonist activity, but only very few of them behave like opioid antagonists. However, in terms of evidence-based dietary supplementation, more studies are needed to prove that oral administration of food protein-derived opioid receptor ligands or their precursors have any beneficial effects in humans in order to support a benefit for the consumer (Teschemacher, 2003).
Bioactive peptides, biogenic peptides, opioid peptides, immuno-stimu-lating peptides, mineral-soluble peptides, antihypertensive peptides and antimicrobial peptides can therefore originate from food materials and enzymatic hydrolysis of proteins. These peptides - which are produced in the enzymatic hydrolysate of treated food materials such as milk, animal and fish meat, and also from other products - have been recently reviewed (Yamamoto et al., 2003).
While meat ingredients have been added to a variety of foods, several functional (i.e. healthy) food ingredients have also been added to meat and meat products. For example, omega-3 fatty acids from fish oils, olive oil, soy proteins, antioxidants such as tea catechins, green tea extracts, phenolic compounds from rosemary, and dietary fibre (bran, oats, inulin) have all been added to meat products to enhance their nutritional profile (Fernandez-Gines et al., 2005). Furthermore, modifications to the feed an animal receives can modify the lipid, fatty acid and vitamin E content in meat (Aharoni et al., 2005).
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