Developing alternative meat-curing systems

Review

Developing Over the past three decades there has been increasing research interest in the development of nitrite-free meatcuring systems. This has resulted in alternatives to the use of nitrite in cured meats for colour, flavour or antimicrobial effects. This review examines the development of nitrite-free meat-curing systems.

A particular concern with the use of nitrite for the curing of meat has been the formation of carcinogenic N-nitrosamines I in some cured products, under certain processing conditions, or in the stomach. Such concern has led to efforts to develop alternatives to the use of nitrite in meat curing. To develop such alternatives, the various roles of nitrite in the curing process have had to be considered: reproduction of the characteristic pink colour of nitrite-cured meats; prevention of the formation of off-flavours in prepared products; development of the delicate flavour and texture generally associated with cured meal~; and protection of the meat against the germination of Ciostridium botulinum, the growth of vegetative cells, and the production of toxin. Researchers have considered several ways of formulating safer cure mixtures. These include reducing the level of nitrite added to meats, the use of compounds that inhibit the formation of N-nitrosamines, and the complete elimination of nitrite from the curing process. Present-day meat-curing practices use only 120mg nitrite per kilogram of meat in the manufacture of bacon. Lower levels of 100mg nitrite per kilogram of bacon, or 40 mg nitrite per kilogram of bacon combined with the use of starter cultures have been recommended by the US Department of Agriculture 2. In addition, the use of mixtures of ascorbic acid or its derivatives and (x-tocopherol to inhibit the formation of N-nitrosamines has been considered. Research on the elimination of the use of nitrite has concentrated on formulating multi-component alternatives, as it was early recognized that the multifunctional properties of nitrite could not he duplicated by a single compound. Sweep formulated nitritefree curing mixtures consisting of a colorant, an antioxidant/sequestrant and an antimicrobial agent. The synthetic pigment erythrosine was used as the colorant; antioxidant/sequestrant systems used included phosphates/polyphosphates and tert-butylhydroquinone; and an~.imicrobi',d agents included alkyl esters of p-hydroxybenzoic acid and of sorbic acid (mid its sodium and potassium salts).

alternative meat-curing systems Fereidoon Shahidi on the other hand, appears to be due to the formation of nitrosyl ferrohaemochrome. Although the exact chemical nature of cooked, cured meat pigment (CCMP) has not yet been fully established, possible reaction intermediates in its formation have been proposed 4 (Fig. 1). CCMP may be added to meat to mimic the colour changes produced by nitrite. CCMP may be manufactured from haemoglobin, an abundant by-product of slaughterhouses, or from the haemin extracted from it; it has also been produced in good yields from a seal protein hydrolysate preparation (Shahidi, F., unpu01ished). The use of synthetic pigments and plant extracts as meat colorants has also been studied. Erythrosine and the betalains have shown potential in this respecP.5. The use of other colorants, such as heterocyclic compounds and a number of organic and inorganic chemicals, has been the subject of much research interest. Unfortunately, none of these substitutes is yet in commercial use, for a variety of reasons. The use of S-nitrosocysteine as a colorant in nitrite-free curing systems has also been studied6; however, the use of S-nitrosocysteine instead of nitrite does not appear to reduce the risk of N-nitrosamine formation. Powdered cooked, cured meat pigment and its applications in meat processing

There are two key steps in the preparation of powdered cooked, cured meat pigment (PCCMP) 7. The first step is the preparation of CCMP from bovine or porcine haemoglobin, either directly or from haemin, which may be extracted from red blood cells according to established proceduress. The addition of a nitrosating agent to haemin or red blood cells in the presence of a reducing agent results in the fo, mation of CCMP 9'~°. The second key step is the stabilization of t_heCCMP. Various methods, such as microencapsulation or storage under a modified gas atmosphere have been considered". A reducing ag~at may also be added to the mixture to prevent oxidative degradation of the pigmy,it during and after the encapsulation process. MicroRole of nitrite and alternatives to nitrite in colour encapsulation has been achieved by suspending the pigdevelopment in meats The colour of raw meat depends primarily on its myo- ment in a solution or paste of carbohydrate polymers, globin content. The colour of heat-processed cured meats, followed by dehydration (e.g. spray drying) to obtain PCCMP"; PCCMP can be stored in the dry form for 18 FereidoonShahidi is wilh the Food Science Program, Departmentof m~mths. When dissolved in water or in a pickle, it can Biochemistry,MemorialUniversityof Newfoundland,St John's,NF, Canada be used as a colorant in nitrite-free meat-curing mixtotes. Colour stability of the pigment in meats under A1B 3X9. Trends in Food Science& TechnologySeptembert 991

o1991,ElsevierSciencePublishersLtd,(UK) 0924-2244/91/$02.00

2] 9

fluorescent

lighting is similar to that of nitrite-cured

products7.

The effects of nitrite and alternativesto nitrite on the flavour of cured meats Almost !000 volatile compounds, representative of most classes of organic compound, have been identified in cooked red meats and poultry ~2. Many such compounds are regarded as unimportant to the flavour of meat, and some may have been artifacts ~3. Lipid-derived products constitute a large proportion of the volatile components of cooked meat' 4'~s. The profile of secondary products of lipid oxidation depends, of course, on the fatty acid composition of the lipids, which varies from species to species. Generally, hexanal is one of the major oxidation produetsl4; hexanal content depends on the amount of linoleic acid in the muscle tissue. Because it has strong antioxidant effects, nitrite inhibits the breakdown of unsaturated fatty acids and the formation of secondary oxidation products in cured meats ~6.Thus, the formation of higher aldehydes such as hexanal is almost eliminated by the presence of nitrite .5. Other chemical changes that are responsible for the unique flavour of cured meats are not clearly understood. Cross and Ziegler ~5 reported that, after oxidized carbonyl flavour components were removed by passing the volatiles of uncured chicken or beef through a solution of 2,4-dinitrophenylhydrazine, an aroma similar to H20

that of cured ham was revealed, suggesting that flavour differences between cured and uncured meats are mainly quantitative, rather than qualitative. Analysis by gas chromatography has shown that the spectrum of volatiles is much simpler in cured meat than in an uncured control (Table I)*L Since curing with nitrite inhibits the formation of oxidation products, it may be justifiable to assume that the flavour of nitrite-cured meat of a given species is actually the 'natural' flavour of meat from that species, without the overtones caused by the oxidation of lipid components ~7. There is some evidence that any other agent, or combination of agents, that prevents lipid oxidation may be used to duplicate the antioxidant role of nitrite in meat curing, and will, thus, prevent the deterioration of meat flavour 3,~8. However, while it has been suggested that as little as 10mg/kg nitrite might be sufficient to produce a satisfactory flavour in cured meats ~9, the above hypothesis does not explain why the intensity of currd meat flavour is proportional to the logarithm of nitrite concentration 2°. For a comprehensive review of cured-meat flavour, see Ref. 21. Combinations of sodium ascorbate, sodium tripolyphosphate and low levels of an antioxidant, with or without added pigment (CCMP), have been shown to reduce the concentration in cured-meat systems of selected aldehydes and substances reactive with 2-thiobarbimric acid ~2. CCMP also possesses antioxidant

B

OH

+

NO

--

NO

ol bi,,

t ol

llobio

AutoReduc t i o n

Gto[bin

Metmyoglobin

Nitrosylmetmyoglobin

NO

-- +.

b Gl[obin

w

m

HyoBlobln' HaO

NO

w

Nitrosylmyoglobin Radical Cation

NO

I.

"

T

I

Olobin (or Olobin Radical) Nitrosylmyoglobin

Nitrosyt Hemochrome

~?

[NO]/Heat NO

I

NO Dinitcosy!

Hemochrome

Fig. I Possiblereaction intermediatesin the formation of pigmentin cooked, cured meat(adaptedfrom Ref.4). 220

Trendsin FoodScience& TechnologySeptember1991

Table1. Effectof nitrite curing on the levels of selected volatile aldekydes in extracts of cooked porka activity, and has been shown to act synergistically with ascorbates; the addition of 12mg CCMP and 55Omg sodium ascorbate per kilogram of meat resulted in antioxidant activity similar to that of 50mg/kg sodium nitrite. The addition of sodium tripolyphosphate in combination with CCMP and sodium ascorbate further enhances oxidative

stability23.

Antimicrobial activity of nitrite and alternatives to nitrite Nitrite has a concentration-dependent antimicrobial effect in meats. However, while the colour and flavour of cured meats can be duplicated by the use of as little as lo-40 mg sodium nitrite per kilogram of meat, nearly all nitrite molecules remain bound in the meat matrix when such low levels are added. Since the presence of unbound free nitrite is essential to ensure adequate antimicrobial activity, 120-156 mg/kg nitrite is generally used in the curing process, Early nitrite substitutes employed alternative antimicrobial agenSs. Sometimes low levels of nitrite were also added24,resulting in an effective curing mixture: the low levels of nitrite resulted in the desired colour and flavour attributes, while antimicrobial agents ensured the safety of the products. Such approaches reduced the levels of IV-nitrosamines formed from -100 ng/kg to less than Snglkg. However, no other food additive is as effective as nitrite a: preventing the growth of CT;botulinum in muscle foods5. Alternative antimicrobial agents that have been studied in detail include: potassium sorbate16;lactic acid, its salts, and cultures of lactic acid bacteria?‘; nisin”; antioxidants2q; fumarate esters’!‘; sodium hypophosphitti’; ethylenediaminetetraacetic acid and polyphosphates32; and sterilization by radiation”“. Nitrite was generally used as a control in these studies, or was used at low levels for colour and flavour development”. Of all of the potential substnutes for nitrite, potassium sorbate, a *generally recognized as safe’ (GRAS) food additive, has been the most extensively studiedz6. The inhibitory effect on C. botulinum of adding 2600mg potassium sorbate per kilogram of meat was equivalent to that of adding 156 mglkg nitrite. The taste and aroma acceptability scores of meat samples treated with sorbate and low levels of nitrite were equivalent to those of nitrite-cured meats. Pierson et aL3’ investigated the use of sodium hypophosphite to inhibit the growth of C. botulinurn in nitrite-free bacon. At a level of 3OOOmg/kg, sodium hypophosphite was as effective as 120mglkg nitrite. The antimicrobial effect of sodium hypophosphite is enhanced at low pH (Ref. 34) and in the presence of salt35. Sodium hypop hos phite is well suited to use in nitrite-free curing systems, as it is bland in taste, readily soluble, and has GRAS status. At levels of 125035OOmg/kg, methyl and ethyl fumarates have been reported to be as effective as 120mgIkg nitrite at inhibiting toxigenesis in C. botufinum spores3’. However, the sensory acceptability of fumarate esters has not been studied thoroughly. Trends in Food Science & Technology September 1991

Relative concentrationb Uncured meat Cured meatC

Aldehyde Pentanal

31.3

Hexanal

100.0

0.5 -1

7.0

Heptanal

3.8

Octanal

3.6

0.3

2-Octenal

2.6

ND

Nonanal

8.8

0.5

2-Nonenal

1.o

ND

Decanal

1.1

ND

2,CDecadienal

1.1

ND

0.3

aAdaptedfrom Ref. 17 bAssuming the concentration of hexanal in uncured meat is I 00.0 ‘Samplescontained 150 mg sodium nitrite and 550 mg sodium ascorbate per kilogram ND, Not detectable

Another approach to controlling microbial activity in meats has been the use of low to medium doses of y-irradiation at low temperatures3”.It has been reported that y-irradiation enables the complete elimination of nitrite from the cure; in practice, however, low concentrations of nitrite were added in many studies to ensure the development of the characteristic colour and flavour of the cured products36.In a recent study, the use of low to medium doses of y-irradiation at low temperatures was studied in meat systems treated with CCMP”‘. The treatment limited microbial growth, and the colour and flavour of the products were similar to those of products cured using 156mg/kg nitrite. The addition to bacon of lactic acid, or of lactic acid bacteria and a fermentable carbohydrate, has been investigated as a possible alternative to the use of nitrite”‘qt6.The addition of a combination of sucrose and a Luctobacillus culture provided excellent microbial stability similar

to ham

products2’.

which

were judged

to be

in flavour to conventionally cured ham2’. The addition to meat of nisin (an antibacterial product of lactic acid bacteria) at a level of 75mg/kg was more effective than !50mg/kg nitrite at inhibiting Cfostridium sporogenes28. However, during refrigerated storage, the level of nisin decreased to a point where the growth of Clostridium spp. would no longer be inhibited during temperature abuse. Acidulation with compounds other than lactic acid, such as citric acid and g!uconoB-lactone, has also been studied as e means of controlling microbial growth in meat systems. At a concentration of 1.4% (w/w) or higher, glucono-b-lactone protected meat against the growth of C. botulinum38. Chelators, such as polyphosphates, also have antimicrobial effects in meats. Sodium acid pyrophosphate is as effective as nitrite at pH 6.0; the antimicrobial activity of sodium acid pyrophosphate at pH 6.0 was better than that at pH 5.7 or 6.3 (Madril, M.T. and 221

Sofos, J.H., unpublished). Similarly, other researchers have shown fllat sodium acid pyrophosphate has antimicrobial activity in pork slurries and beef formulations39,4°. There is also evidence that sodium tripolyphosphate41 and ethylenediaminetetraacetic acid42 may have antimicrobial activity. Several phenolic antioxidants have antimicrobial activity43. The most effective appears to be butylated hydroxyanisole, which has been reported to inhibit the growth of C bolulinum when used at a level of 50 mg per kilogram of meat 44. Other phenolic antioxidants, such as butylated hydroxytoluene, tert-butylhydrecluinone and prowl gallate, appear to be less effective4s. The migration of phenolic antioxidants to the fat phase of foods and their consequent loss of antimicrobial activity is a major disadvantage. Recently, Wood et al. 46 examined the effects of a number of antimicrebiai agents in nitrite-free meatcuring systems to which CCMP had been added. The antibotulinai activity of sodium hypophosphite at a level of 3000mg/kg closely resembled that of nitrite at a level of 150 mg/kg.

Conclusions Although limits on the level of nitrite used for the curing of meats have reduced concerns over the use of nitrite, it is still necessary to examine alternatives to the nitrite-curing process in order to reduce further the risk of N-nitrosamine formation in meats. In recent years, empirical approaches have been successful in finding alternatives to the use of nitrite in curing. Thus, it is now possible to replace nitrite curing with alternative processes without adversely affecting the microbiological safety or sensory acceptability of the products.

14 15 16 17

18 19

20

21

22 23 24 25 26 27 2B 29 30 31

32 33

Acknowledgement Fina,,lcial support from the Natural Sciences and 34 Engineering Research Council (NSERC) of Canada is

gratefully acknowledged.

35

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Trends in Food Science & Technology September 1991