Safety and sensory aspects of nitrite alternatives in meat curing.
Nitrite is an important meat preservative. Its incidental use dates back to ancient times and as early as 3000 BC in Mesopotamia. Rock salt was used for meat preservation and nitrate impurities present were reduced to nitrite by the post-mortem reducing activity of the muscle tissues. Regulated use of nitrite, as such, has been practiced since the mid-1920s to ensure the safety of products, and prevent food poisoning. Nitrite is a very effective antimicrobial agent and retards the germination of spores and formation of deadly toxin of Clostridium botulinum. In addition, nitrite is responsible for a reddening effect and development of the characteristic and well-loved flavour of cured meats. Sodium chloride is always incorporated with nitrite in the curing system as well as ascorbates or erythorbates, used as cure accelerators, and also to provide other beneficial effects. Furthermore, polyphosphates may be used in certain products as a curing adjunct.
Despite all of its desirable effects, nitrite has also been the culprit in the formation of carcinogenic N-nitorsamines in certain cured products under high temperatures of panfrying. N-nitrosopyrrolidine and N-nitrosodimethylamine have been detected at <100 ppb in fried bacon.[2,13] Free, unbound nitrite reacts with amines and amino acids in meat, leading to the production of these and other N-nitrosamines. The rate of N-nitrosamine formation in meats is proportional to the concentration of the available nitrite squared. Consequently, methods of cooking, nitrite concentration, salt concentration, pH and presence of reductants are factors determining the potential production of N-nitrosamines. In addition, nitrite in meats upon ingestion may also produce N-nitrosamines in the stomach or may act as a cocarcinogen. Carcinogenity of N-nitrosamines in a variety of experimental animals such as rodents, fish, etc. has been confirmed. Thus, it is prudent to develop alternatives to nitrite. The National Academy of Sciences has recommended that exposure to N-nitroso compounds to humans from nitrite-cured meats be reduced and strategies for long-term research on alternatives to nitrite usage be developed.[14,15,18,21] This is in line with the stated policy of the Canadian government.
Since the likelihood of finding a single compound to mimic all functions of nitrite is infinitely remote, Sweet in a US patent provided formulations of composite non-nitrite curing agents. His alternative curing mixtures included an antimicrobial agent to ensure the safety of the products and an antioxidant/sequestrant system to prevent rapid oxidation and development of warmed-over flavour (WOF). To reproduce the pleasant colour of cured products, he used erythrosine, a potent plant-colourant extract.
To inhibit the possibility of N-nitrosamine formation in cured products, other methods have also been proposed. These include the use of N- nitrosamine-blocking agents such as ascorbates and [alpha]-tocopherol, or a lowering of the addition level of nitrite in order to prevent overcuring with its undesirable consequences. In the latter case, addition of an antimicrobial agent may be required if the nitrite level was less than 100 parts per million. The N-nitrosamine problem has also led to the removal of nitrate from most curing processes in North America, eliminating concerns over controlling the reduction rate of nitrate to nitrite.
Among the methods described above, total elimination of nitrite from the curing mixture, such as that proposed by Sweet, is most attractive. Our own work has concentrated on developing composite curing mixtures.
Antimicrobial Role of Nitrite and its
Nitrite exerts a concentration-dependent antimicrobial effect in cured meat products. The degree of protection provided to meats depends on the concentration of the residual nitrite, duration of temperature abuse, and extent of contamination. The mechanism(s) by which nitrite inhibits/retards the outgrowth of spores and vegetative cells has not yet been fully elucidated. However, it appears that a reaction with iron-containing enzymes is involved. A better understanding of the exact mechanism(s) of the antimicrobial role of nitrite is still needed.
To take advantage of all positive aspects of nitrite yet to eliminate/reduce the chance of N-nitrosamine formation, application of low levels of nitrite (10-40ppm) has been studied. These, with or without several alternative antimicrobial agents in meat systems, have been tested by different investigators. The compound of choice must be at least as effective as nitrite; be safe; be heat stable; be flavourless, and; be preferably effective at low concentrations.
Among the antimicrobial agents investigated, parahydroxybenzoic acid esters (parabens) were found to be good candidates as inhibitors in microbiological media. However, their effectiveness in meat against C. botulinum was questionable. Moreover, potassium sorbate at a level of 2600ppm exhibited antibotulinal activity equivalent to that of nitrite at a concentration of 156ppm. In combination with 40ppm of sodium nitrite, potassium sorbate was found to reduce the extent of N-nitrosamine formation from nearly 100 parts per billion to less that 5ppb. Sensory data on the use of 120ppm of sodium nitrite or 40ppm of sodium nitrite in combination with 2600ppm of potassium sorbate has shown no product difference between the colour and panel scores of the two sets of experiments. Potassium sorbate is a white crystalline compound and has GRAS (generally recognized as safe) status.
Sodium hypophosphite, another candidate, was found to be quite effective. At 3000ppm or at 1000ppm in combination with 40ppm of sodium nitrite, sodium hypophosphite provided antibotulinal protection to meat products which was similar to that provided by 120ppm of nitrite. It has been reported that bacon processed with 3000ppm of sodium hypophosphite has a flavour as desirable as conventionally-processed bacon.
Methyl and ethyl esters of fumaric acid at 1250 to 2500ppm levels of addition have been reported to display antibotulinal activity similar to that provided by sodium nitrite. Methyl fumarate-treated samples were sensorially indistinguishable from that of their nitrite-cured counterparts.
Lactic acid, its sodium or potassium sales, or lactic acid-producing microorganisms may also be used in providing microbial stability to muscle foods. Incorporation of lactic acid, preferably in the encapsulated form, or lactic acid-producing bacteria together with fermentable carbohydrates in cured-meat formulations is permitted to reduce the pH.[2,26] Excellent protection against the formation of botulinum toxin has been achieved in bacon using these bacteria and sucrose without the presence of nitrite. While lactic acid, as such, may also be used for surface treatment, use of lactate salts as a component of muscle foods may prove beneficial.
Finally, the use of radiation sterilization is an established method of microbial inactivation and has been studied as a method of preserving many foodstuffs. It is currently used for sterilization of spices and herbs and sometimes to inhibit sprouting of potatoes. It has also been found effective against C. botulinum in cured meat and has been tested in combination with low concentrations (25-40ppm) of sodium nitrite in bacon and ham. Thus, irradiation may be used either to substitute for the antimicrobial action of nitrite or to reduce the addition level of nitrite required for its antimicrobial activity.
Of the above antimicrobial agents/processes, lactates and radiation sterilization perhaps offer the best safe alternatives to nitrite. Our body produces approximately 130 grams of lactic acid on a daily basis, thus, its use at levels of up to 2-3% may be regarded as safe. Low-dose radiation sterilization, especially at reduced temperatures, is also an attractive option.
Sensory Effects of Nitrite and its Alternatives
Effects on Colour
The colour of raw meat comes from its hemoproteins, principally myoglobin. Myoglobin is composed of a protein, globin, and an iron-porphyrin prosthetic group. The colour of heat-processed cured meats is due to (di)nitrosyl ferrohemochrome in the products. The exact chemical nature of the cooked cured-meat pigment (CCMP) has remained controversial. Figure 1 represents possible reactions and intermediates involved in the production of CCMP in muscle tissues or in model systems. A report of our findings in this area will be communicated elsewhere. This pigment may also be produced from hemoglobin prepared from bovine or porcine blood, after plasma separation and as outlined in Figure 2. Only as little as 3ppm of sodium nitrite is theoretically required for 50% conversion of myoglobin to CCMP.
Preparation of CCMP from hemoglobin may be achieved directly or through a hemin intermediate. Hemin may be extracted from red blood cells by acidified salt solutions, among other methods. We have also produced it as a by-product in the preparation of seal protein hydrolyzates. Direct preparation of CCMP from red blood cells has been achieved in a novel one-step, one-pot reaction involving heat processing of blood at elevated temperatures, controlled pH conditions, and in the presence of a reductant. The preformed cooked cured-meat pigment, similar to the pigment present in nitrite-cured meats, undergoes decomposition. Its stabilization/protection is crucial.[7,8]
Stabilization of the CCMP by its storage under a modified headspace gas or by microencapsulation which was first proposed in 1983 has now been achieved.[7,21] Stability of CCMP in each case was better than about 90% after up to 18 months of storage (see Table 1). Carbohydrate-based polymers were found to protect and perhaps physically entrap the pigment molecules. The helical structure of starch molecules may provide a large enough cavity to host the pigment molecules fully or partially.
Table : Table 1 Stability of the Protected CCMP (%) of Initial Colour Activity)
Storage Time Protection Method
Modified Headspace Encapsulated 2.5 - 3.0 99.1 - 99.7 91.5 - 96.1 6 99.1 - 99.4 - 9 98.2 - 98.9 90.7 - 95.3 18 - 89.4 - 93.0
Application of this protected pigment to a variety of emulsion-type muscle foods ranging from cod surimi to seal meat containing up to 9% hemoproteins, has been accomplished.[9,21] Table 2 summarizes the quantity of CCMP required to mimic the color effects of muscles treated with sodium nitrite. Failure in production of a cured colour in cod surimi, devoid of hemoproteins, suggests that interaction between the added CCMP and myoglobin in meats might be involved. Results of our detailed studies in this area will be communicated elsewhere.
Table : Table 2 Effect of Muscle Species on the Required Concentration of CCMP to Obtain a Cured Colour
Species CCMP, ppm Hunter Value L a Cod Surimi 0 71.2 -2.1 Chicken Breast 3-6 74.6 5.0 Pork 6-12 58.6 12.1 Beef 24-36 48.0 18.1 Seal 48-60 24.1 21.8 Seal Surimi 18-30 28.8 14.4
The true chemical structure of the pre-formed CCMP with regard to the number of nitric oxide moieties present in each molecule has recently been elucidated. Due to the proprietory nature of these results, details will be reported at a latter date.
In addition to the pre-formed CCMP, a number of other colourants as nitrite-substitutes have been tested. These included the use of other chemicals to replace the NO groups in the protoporphyrin ring of the hemin or the use of colourants from plant-origin. Of these, only the latter option involving betalins or erythrosine presents a practical alternative for colour formation in meats. Elimination of the iron atom from the iron-poyhyrin ring prevented the appearance of a cured-colour in the products.
The overall colour difference ([DELTA]E) of treated meat samples, expressed as:
[Mathematical Expression Omitted]
was calculated using 156ppm nitrite-treated meats as reference. The colour difference values of all nitrite-cured and pigment-treated pork, with the exception of that prepared by the addition of 24ppm of CCMP, were not significantly (P< 0.05) different.
Effects on Flavour
Only as little as 10-40ppm of sodium nitrite was found necessary to attain a nitrite-cured colour and flavour in meat products. Lipids make an important contribution towards the overall flavour of meat and are primarily responsible for species differentiation. They are responsible for both desirable and undesirable odour and flavour effects in muscle foods. Their undesirable role results from autoxidation of polyunsaturated fatty acids.
Nitrite, due to its strong antioxidant properties, retards the development of warmed-over flavour (WOF) in cured meats and eliminates the formation of overtone carbonyl compounds which obscure the true-to-nature flavour of cooked meats without being influenced by their lipid components. Based on these observations, it was suggested that the true nature of meat flavour may indeed be that of cured meats. Use of alternative curing systems involving an antioxidant/a sequestrant to duplicate the action of nitrite was attempted. Results summarized in Table 3 indicate a sharp decrease in the concentration of higher aldehydes in both nitrite-cured and nitrite-free processed meats. This is consistent with the early findings of Cross and Zigler who reported that nitrite-cured meats had a very simplified flavour volatile spectrum than their uncured counterparts. Futhermore, volatiles of uncured pork and chicken, when passed through a 2,4-dinitrophenylhydrazine solution had a similar and a cured flavour note.
Table : Table 3
[Tabular Data Omitted.]
Curing adjuncts such as phosphates and polyphosphates have been found to be effective in retarding lipid oxidation and providing a cured flavour when used in conjunction with low levels of an antioxidant such as TBHQ or together with ascorbates. However, the use of polyphosphates is not allowed in all types of cured products in Canada. Therefore, we have formulated several nitrite-free, phosphate-free curing mixtures which have reproduced a similar flavour effect in nitrite-free cured meats as compared with their nitrite-cured counterparts. Amongst these we have found that C and E vitamins in specific combinations provided an excellent alternative for nitrite from a flavour viewpoint, as it was revealed earlier in the popular press.
Cumulative Effects of Nitrite-Free Curing
Mistures: Safety and Sensory Considerations
While additions of 10-40ppm of nitrite is sufficient to attain a cured colour and flavour in processed meats, addition of 100-200ppm of it ensures microbial stability of the products. It is the residual, free nitrite that has a dual effect; while it acts as a potent antibotulinal agent, it is also involved in possible formation of carcinogenic N-nitrosamines. The bound nitrite (10-40ppm) which confers colour and flavour in cured meats is relatively safe and produces very little N-nitrosamines. Nonetheless, it would be preferred to eliminate the possibility of N-nitrosamine formation entirely.
Our pre-formed CCMP together with curing adjuncts successfully reproduced both the colour and flavour attributes of cured meats. Moreover, the first 10-40ppm of sodium nitrite may be eliminated from the curing mixture. Thus, use of antimicrobial agents already at hand, together with the above ingredients can produce safe products as well. Several antimicrobal agents in composite non-nitrite curing mixtures were used by Sweet. Recent studies on systems containing the pre-formed CCMP indicated that sodium hypophosphite at 3000ppm level of addition was somewhat preferred. Nonetheless, all agents tested were effective only at relatively higher concentrations as compared with 100-200ppm of sodium nitrite. Although some of these compounds have a GRAS status, same is not true for all. Thus, we have recently examined the use of low-dose (0-10 kGy), low-temperature radiation sterilization of the CCMP-treated meat products. In addition to ensure product safety, nitrite-free cured meats exhibited sensory properties indistinguishable from those of their nitrite-cured counterparts. Similar studies using the pre-formed CCMP with starter cultures and lactates have been underway in our laboratories.
Taking Another Look at Nitrite
We must look objectively at nitrite again. Canada's limits on the level of nitrite usage, coupled with the responsible nitrite reductions taken within the meat processing industry, have reduced the possibility of over-curing which was the greatest cause for concern. Thus, benefits from the controlled and responsible use of nitrite overwhelms the possible risks from outbreak of botulism food poisoning. Nonetheless, because of nitrite's connection with cancer-causing N-nitrosamines, its use is a potential trouble area. Hence, if nitrite is ever banned, we have a practical solution at hand which could address the issue.
Financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada is acknowledged.
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PHOTO : Figure 1 Formation of cooked cured-meat pigment (CCMP) from myoglobin and nitrite. Symbol P denotes protein. Hemoglobin may replace myoglobin. (Adapted from Killday et al., 1988.)
PHOTO : Figure 2 Reactions of myoglobin and hemoglobin with a nitrosating agent
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|Author:||Shahidi, Fereidoon; Pegg, Ronald B.|
|Publication:||Canadian Chemical News|
|Date:||Feb 1, 1991|
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