J. Inst. Brew., January-February, 1986, Vol. 92,pp. 73-80 73

NITROSAMINES IN MALT AND BEER

By T. Wainwright

(Brewing Research Foundation, Lyttel Hall, Nutfield. Surrey RHl 4HY)

Received 2J March 1985

A brief account is given of the co-operation between the malting and brewing industries and theMinistry of Agriculture, Fisheries and Food in solving the NDMA problem in beer. The formation and

removal of NDMA during processing is discussed with emphasis on methods used commerciallyto limit

NDMA in the products.

Key words: Beer brewing, dimethylamine, malt, malting,

nitrite, nitrosamine.

Introduction

In the last 20 years, and more particularly within the last

decade, there has been great concern about the role of

nitrosamines in causing cancer. Although there is still no

conclusive proof that nitrosamines do cause cancers in

man, a large number of nitrosamines have been shown to be

carcinogenic in other animal species, including primates,

and they cause cancer-like changes in human tissues in

laboratory experiments. Moreover, there is circumstantial

evidence strongly suggesting that, under certain conditions,

nitrosamincs formed in the digestive tract cause human

cancers. Obviously, brewers do not want their products to

increase the risk ofcancer and, as soon as it was known that

beer did sometimes contain nitrosamines—particularlynitrosodimethylamine (NDMA), a great deal of effort and

money was spent in finding out where the nitrosamines

came from and how to minimise their occurrence in beer.

The situation is now well under control and the NDMA

content of beer is lower than the normal NDMA content of

human blood.13.In Britain, the brewing industry became aware of the

nitrosamine problem late in 1978 when it learnt the German

analytical results subsequently published by Preussmann.24Almost immediately the Brewer's Society, in co-operation

with the Maltsters' Association of Great Britain (MAGB),

set up a Nitrosamine Working Party to examine the situa

tion. The Ministry of Agriculture, Fisheries and Food

(MAFF) agreed to be represented on the Working Party

and the Brewing Research Foundation obtained equipment

and allocated staff to study NDMA in malt and beer and to

find ways of minimising its occurrence.

In the USA and Canada working parties were similarly

formed in order to get rapid solutions to the problem. Help

from these sources and from other contacts, especially inAustralia and Germany, as well as from the Scotch Whisky

Association, is gratefully acknowledged.

Until the BRF was able to measure NDMA, analyses

were kindly done either by MAFF or the Laboratory ofThe

Government Chemist. BRF received help and advice from

them, and from the Leathcrhead Food Research Associ

ation before purchasing a Thermo Electron Analyser

(TEA).9The results of the investigations were not publicised but

the MAGB and Brewers' Society were at all times keptinformed ofdevelpments, and in 1980 a comprehensive con

fidential report was prepared by the Nitrosamine WorkingParty and circulated to maltsters and brewers. Only brief

reports of certain aspects have been published from

BRF.23i28>3° Now that there is no longer any serious concern that NDMA in beer is a health hazard, it is appropriate

to publish more ofthe results, and it is intended to do this in

a series of articles in this Journal. It seems, however, also

appropriate to publish this more general article highlightingsome of the conclusions which can be drawn from the work

and pointing out that, whilst the problem has been solved in

a practical sense, scientific understanding of the reaction

mechanisms and controls is far from complete and much

remains reasonable conjecture rather than proven fact.

These conjectures, nevertheless, have led to and are still

leading to alternative approaches for controlling NDMA.

Origin ofNDMA in beer

The amounts of NDMA in malts and beers are very low

and very sensitive analytical techniques are required to

measure them. The concentrations arc expressed as parts

per billion (ppb)—i.e. parts per 10.9 This is equivalent to mgper tonne or mg per 10 hectolitre.

It is now well-established that the NDMA in beer comes

from the malt. This was in fact thought to be the case almost

as soon as it was known that beer contained NDMA."Barley contains little NDMA and that present in malt is

formed during kilning. Systematic study of other beer

ingredients showed that NDMA was not normally present

in significant quantities in any processing aid, additive

or ingredient other than malted barley although small

amounts have been found in hops. Because the NDMA in

hops becomes diluted about 1 in S00 during brewing this

contribution is negligible. It is worth observing that BRF

have established that unsulphured hops, like sulphured

hops, contain little NDMA (e.g. 4-0 ppb and <0-l respec

tively). The NDMA content of water is quite variable but

can reach 1 ppb. It is not readily removed by boiling or

distillation. Ion-exchangers arc more likely to contribute

NDMA than to remove it6 but charcoal filters shouldadsorb it.7 At BRF, supply water is used in preference todeionized water for NDMA analysis of malts, and from

time to time trouble is experienced with the 'blank' analyses

due to contamination of the water. However, experience

suggests that NDMA in supply water is not a common

source of NDMA in beer since most beers nowadays

contain less than 1 ppb.

When the study began it was clear that malt would be the

major source ofNDMA in beer, but it was not clear- to what

extent NDMA woud be formed or removed during beer

production. In short it can be said that there is negligible

production, and relatively little removal—perhaps 10 to

20%—and this occurs mainly in the wort boiling stage.

The most convincing evidence that NDMA is not nor

mally formed during beer fermentation, or during mashing

or wort boiling, is the fact that most beers now contain less

than 1 ppb, and many, less than 0-1 ppb. However, before

this situation was reached, several laboratories, including

BRF, compared the amount ofNDMA in malt with that in

the wort or beer produced from it. In some cases11*25NDMA has been measured at various stages in the brewing

process and these clearly show no NDMA formation after

mashing.

It is known that malt contains a few ppm dimethylamine

(DMA)10-22 but there is usually no formation of NDMAfrom this during malt storage. It had been thought that

NDMA might be formed during mashing or wort boiling

by reaction between DMA, or other nitrogen-containing

organic compounds in the malt, and nitrate or nitrite in the

74 t. wainwright: nitrosamines in malt and beer [J. Inst. Brew.

NH

CH3'

CH,

CHV

N2O3andN2O4

(active forms of NOx)

in air passed through

the malt during kilning

Oimethylamine

(DMA)

Nitrosodimethylamine

(NDMA)

Intermediatesrequires active forms of NOx or nitrites

CH,

CH,

NCH2CH2

Hordenine in germinated barley

Fig. 1. Formation ofNDMA from hordeninc.

brewing liquor. Indeed such Formation does occur if nitrite

is added. For instance, when 3 x 10~4 M sodium nitrite wasadded (about 14ppm nitrite ions) an additional 80ppb

NDMA was formed along with six other volatile nitro

samines. However, the amounts of nitrite in a mash are

normally far too low (about 0-5 ppm) to have an effect. It

should be remembered that some wort infections can pro

duce nitrite from nitrate. Nitrate must be avoided, since

amine precursors for at least seven volatile nitrosamines,

and for many more non-volatile nitroso-compounds, are

present in wort.

It is worth stressing, however, that under normal circum

stances mashing with malt does not lead to the formation of

volatile nitrosamines, and one can estimate the NDMA

content of a malt by making a standard ASBC, EBC, or

JOB wort and sampling the supernatant. It is not necessary

to add reagents to prevent nitrosation.

Formation of NDMA during fermentation might have

been expected, since nitrosamines have been found in wines,

ciders and other beverages which are not made from malt,

and since micro-organisms26 or indeed, inert particles, areknown to catalyse NDMA formation from DMA.19 This isthought to be a surface effect rather than an enzymic reac

tion. Although wort and beer contain DMA, there is no

evidence of NDMA formation during beer fermentation—

presumably the nitrite concentration is too low, even for the

catalysed reaction.

Removal ofNDMA during brewing

NDMA is volatile in steam and some is removed during

wort boiling. The extent to which this can occur will

obviously vary from brewery to brewery. NDMA is

adsorbed on certain materials, particularly charcoal, but

little is removed on breaks, on yeast, or by filter materials.

Otherwise the concentration in beer would not be approxi

mately equal to that expected from the amount ofmalt used

and its NDMA content. There is usually a dilution of 5 to

10-fold. Some NDMA is removed on spent grains31 and it ispossible to discard some with water if wet milling is used,5but extract is then also sacrificed.

Control ofNDMA in malt

In 1979 it was possible that some of the NDMA and the

few detected occurrences of nitrosodiethylamine (NDEA) in

malt were due to contamination with chemicals used on

growing or stored barley or malt. Pesticide manufacturers

and suppliers have had to take steps to limit nitrosamine

formation by their products. Nowadays the occurrence of

NDMA on malt can all be accounted for by reaction of

N2O3 and N2O4 with amines naturally present in germin

ated barley. The two amines most likely to be involved are

hordenine and dimethylamine (DMA), some of the latter

being formed from hordenine. This is shown in very brief

outline in Fig. I. Various avenues to control NDMA are

indicated in Table I.

A voidance ofN2O3 and N2O4

The malting and brewing industries have become very

familiar with the term NO, which is used to include all the

oxides of nitrogen. These are formed in combustion reac

tions involving oxygen in air and nitrogen, either as gaseous

nitrogen in air or in nitrogenous compounds in fuel. Nitro

gen in the air is the only source of NO, when certain oil and

gas fuels are used. Clearly this provides sufficient NO, to

give high NDMA values. NO, is also formed in the air at

hot (ca 800°C) metal surfaces even without a flame. The

actual composition of NO, is very variable because it

inlcudes N2O, NO, NO2, N2O3 (i.e. NO+NO2) and N2O4

(i.e. 2NO2). Only the latter two are very active in forming

nitrosamincs during malting so there are two alternative

ways to minimise NDMA formation. One is to use heating

systems which allow very little contact with NO,. This

involves the use ofindirect heating or low NO, burners. The

other is to use heating systems which, whilst producing

NO,, ensure that it is in a relatively harmless form. This is

the case when fuels rich in sulphur are used—such as heavy

oil, anthracite or coke—or when the sulphur dioxide (SO2)

content of the combustion gas is increased by burning

elemental sulphur or organic sulphur compounds, or by

injecting SO2 into the kiln gas. Measurements in oil fired

Vol.92, 1986] t. wainwright: nitrosamines in malt and beer 75

TABLE I

2.

5.

6.

7.

8.

Stages of Control

Avoidance ofN2O3 a

N2O4 during kilning

Decreased formation of

hordeninc or DMA during

germination

Minimal transfer beforekilning of liordcnine from

roots to husk

Nitrosation of hordenine

prevented

Competition for theNO, present

Minimal breakdown or

nitrosatcd hordeninc loDMA

Removal or DMA andNDMA encouraged

NDMA removal from malt

Methods

Indirect heating

Low—NO, burners

Use ofS-rich fuels, S or SO2(Collectively called

sulphuring)

Restrict growth by use of(1) control of temperature

or moisture

(2) GA and shortergermination time

(3) bromatc or octanoatc

(4) CO2 build up and/or O2deficit

(5) restceping

(6) physical or chemical

damage to the chitting

barley

Care with turning andtransfer

Make the surface of the

malt acidic by spraying withacid, sulphuring, anaerobic

conditions, or spraying with

sugars. Dehydrate or cause

root withering. Wash out

the hordenine

Spray with ascorbic acid or

sugars. Sulphuring

Use low kilning

temperatures.

Abrasion before

germination. Damage to the

seedling before kilning.

Control of air flow in kiln

Removal by steam during

kilning. Removal duringwet milling in brewery

kilns show NO, levels of 3 ppm but all except 01-0-2 ppm is

NO.2 Experiments at BRF have used various methods toconvert NO, entirely to NO and, without decreasing the

total NO, passing through a kiln, these treatments invari

ably suppress NDMA formation. Because, as discussed

later, the presence of S in fuel or use of additional S or SO2

can prevent NDMA formation in other ways, it is import

ant to stress this role of S in making NO, less active, since

this also restricts the formation of other nitrosamines which

occur, particularly in experimental malts. Use of indirect

heating or low NO, burners, ofcourse, will also decrease the

formation of all nitrosated compounds.

At the time maltsters became aware of the NDMA prob

lem, some kilns in Britain were still using anthracite or

heavy oil, but there was a tendency, both because of fuel

costs and because of problems with SO2 pollution, to

convert to natural gas. Since then, for legal reasons, oil

companies have decreased the sulphur content of heating

oils. Maltsters have not been allowed by law to use oil with a

high sulphur content and, in order to control NDMA, many

maltsters have had to burn additional S or use added SO2.

Fortunately ways have now been learnt to get low NDMA

levels without using, or with minimal use of, SO2.

The major breakthrough was in the development of new

kilning systems. It was shown that, in the absence of any

other palliative treatment, NDMA levels below 2-5 ppb

could not be obtained if the NO, in the drying air from a gas

burner exceeded 015 ppm.2 Use of indirect heating does notintroduce NO, into the drying air other than that present in

ambient air—provided suitable surfaces are used and those

in direct contact with the drying air do not get too hot.

Internationally, there has been a massive investment in

replacing existing burners with new indirect heating systems

and this has been very beneficial. However, in 1979 indirect

heating systems were not readily available and they

appeared very expensive before the fuel-saving advantages

of replacement were fully realised. Consequently, there was

also considerable development work on low NO, burners

and many of these have been installed.

In Britain a Burner Working Party was set up under the

chairmanship of Mr O. P. Hudson, representing the

MAGB, and a series of burners were intensively examined,

with and without sulphuring, by using them successively in

one commercial mailings. This work provided much useful

information. It showed that low NO, conditions could

be achieved with some burners and that the NO, concen

trations did not rise steeply with temperature later in kilning

as is the case with burners previously used. As with results

published from Germany2 it was possible to show that NOis not converted to NO2 during passage through the kiln

and that both NO and NO2 are removed by the malt, even

during the first hours of kilning when no NDMA is present

in the malt. The amounts removed are vastly in excess of

that required to account for the NDMA formed. This early

removal of NO, is consistent with the formation during this

period of nitrosated derivatives of hordenine (Fig. 3) which

can give NDMA later in kilning. Practical experience shows

that if this nitrosation early in kilning is prevented (e.g. by

short periods of treatment with SO2 or use of indirect heat

ing only at this stage) the malt contains relatively little

NDMA even if no special precautions are taken later in

kilning.

These burner studies, along with many results from

elsewhere, show that the levels of NO2 produced by the

new burners are sufficiently low to give, under favourable

conditions, less than 2-5 ppb NDMA in malt. However,

ambient NO, is often much higher than that produced by

the burner. For instance, measurements on one UK malting

site showed levels up to 1 ppm.8 The ambient NO, comesfrom many sources, but very high levels can be caused by

local traffic such as lorries or fork-lift trucks in or near the

maltings. A car exhaust can produce 400 ppm. Even short

periods of exposure to NO, can cause high NDMA levels in

malt, and the resiting of air intakes to the kiln can by itself

bring a great improvement. Nevertheless, it is unrealistic to

expect that levels ofNDMA less than 2-5 ppb in malt will be

consistently achieved simply by using low NO, burners—

especially with ale malts. In Canada low NO, burners were

installed, but these have been replaced so that all the malt

ings now use indirect heating. Internationally it is normal to

use some degree of sulphuring when using low NO, burners

and even in many cases, when using indirect heating.

Decreasedformation ofhordenine

It is still impossible to prove unambiguously that

hordenine is the most important source of NDMA in malt,

but there is a large amount of circumstantial evidence that

this is the case. It is possible that dimethylamine (DMA),

which is present in green malt in ppm quantitities, does

make some contribution, but the evidence suggests that

any NDMA formed from this DMA is removed during

germination and malt kilning. On the other hand, hordenine

is decomposed during kilning with NO, to give DMA at a

stage when the NDMA formed is not all removed. Other

nitrogenous compounds, such as gramine, may also pro

duce some NDMA but they are likely to be less important

than hordenine.29Whether or not hordenine is the only source of NDMA,

the treatments discussed in this section decrease the for

mation of the amine precursors of NDMA, and analyses

showed that both the hordenine and DMA concentrations

in germinated barley were decreased. The quantititcs of

gramine were too small to measure.

76 t. wainwright: nitrosamines in malt and beer [J. Inst. Brew.

Hordcninc and DMA are formed in the growing seedling

and are present in high concentrations in both the roots

and acrospire, and at a lower concentration in the rest of

the developing embryo. BRF results show that, in general,

any condition which restricts the growth of the seedling

decreases the synthesis of the precursors. German results

show that there is not a strict correlation between growth

and precursor formation. In some malting techniques the

precursors (or the amino acids from which they are formed)

appear to be partially removed to support further growth1'and the concentrations of DMA, hordenine and graminc

perhaps correlate more with modification than with

growth.27 However, more NDMA is usually formed if thegermination period is longer or the germination tempera

ture or moisture is higher. Barley varieties or samples which

grow more vigorously would be expected to produce more

precursor but, on the other hand, to make good malt it is

often necessary to allow more growth of varieties which are

difficult to modify.

Maltsters often use additions of gibbcrcllic acid, or use

abrasion, to achieve modification in a shorter germination

time and with lesr growth. The NDMA content of a kilned

malt, and the hordenine content of the green malt from

which it was made, are both higher if the barley has been

abraded and treated with GA, if a fixed germination period

is used. However, barley processed in this way, is usually

germinated one day less and this produces much lower

values for NDMA.

Bromate, often used in malting, decreases growth and

malting loss and also decreases NDMA formation.8Octanoate has similar effects on malting performance and,

whilst not used commercially, inhibits NDMA formation. It

was noticed that some drum makings gave low NDMA

figures and experiments have shown that the restriction of

oxygen supply during malting gives low precursor and

NDMA levels. The anaerobic conditions restrict growth

only slightly but greatly inhibit hordenine formation.17

There is a marked inhibition of NDMA formation.8Resteeping, particularly with warm water, with acid, or

with mctabisulphite, also restricts growth and precursor

formation and gives less NDMA in the malt.30All the above treatments, on the laboratory scale, can

produce malts with less than 2-5 ppb NDMA even under

high NO, conditions. The treatments, however, affect other

malt properties and it is not practicable to use restriction

of precursor formation commercially, to produce malts

with little NDMA unless some combination of treatments

is used (e.g. to balance excess proteolysis with inhi

bition of proteolysis). However, less extreme forms of the

treatments can be, and are used—for instance low bromate

concentrations—to help give low NDMA levels when other

approaches to NDMA control are used.

With otherwise reproducible conditions during kilning,

the amount of NDMA in the finished malt increases with

the NO, concentration. [Some workers who believe that the

precursor in the green malt is DMA rather than hordenine,

have calculated a regression equation

NDMA (ppb) = 14-7 x DMA (ppm) x NO2 (ppm)072refs 12,22]

Ensuring very low NO2 conditions—possibly with the help

of SO2—guarantees low NDMA levels. The concentration

of SO2, when required, is at least ten times that of the NO2

and often very much more.

Other treatments currently being studied involve damage

to the barley at the onset of germination by physical or

chemical means. For instance, application of pressure

decreases subsequent growth and the formation of

hordenine and NDMA. It has been observed that this type

of pressure can be applied in the act of pumping steeped

grain to a germination vessel. It is too early to be sure that

Acrospire

Roots

ndosperm

Aleurone

Husk Pericarp-Testa

GERMINATED BARLEY

Precursors mainly in roots and acrospire

KILNED DE-ROOTED MALT

NDMA most concentrated in roots and acrospire

but also present in husk, endosperm etc.

KEY

Highest concentration of precursor/NDMA

Intermediate concentration of precursor/NDMA

Lowest concentration of NDMA

Fig. 2(a). Distribution of precursors in gcrmiantcd barley. They aremost concentrated in the seedling where they are formed, butbecome transferred to other regions, (b) NDMA in kilned malt.

The concentration is highest in the roots (removed by screening)

and acrospirc, but NDMA is also present in other parts.

any of these techniques will find commercial application but

some look promising.

Before ending this discussion on the role ofhordenine it is

important to point out that hordenine formation is not

completely suppressed by any of the treatments and there isusually enough to form, theoretically, ppm rather than ppb

NDMA. The hordenine is not completely removed during

kilning30 and, as pointed out later, it can undergo severalreactions without necessarily forming NDMA. A factor of

crucial importance, however, is the actual location of the

hordenine (Fig. 2).

There is a high concentration of precursors in the rootlets

and the NDMA content of kilned roots is much higher than

that of screened malt. Most of the precursor is, however, in

the rest of the seedling and in the husk fraction. The amount

in the husk fraction depends on its transfer from the seed

ling (see below). The precursors in the acrospire will be

more accessible if the husk is thin, broken or abraded, and

even more so if the acrospire has emerged. The fact that

hordenine (and DMA) is not uniformly distributed partly

explains the lack of correlation between NDMA formation

and precursor concentration. A diminution in precursor

concentration in certain regions of the grain can cause a

disproportionately large decrease in NDMA production.

Minimal transfer ofhordenine

Although hordenine and DMA are formed in the seed

ling, they are easily extracted in water and can be trans

ferred from the seedling to the husk and endosperm. Indeed

Vol. 92. 1986] t. wainwright: nitrosamines in malt and beer

NDMA

77

DMA

NO

CHjCHj N.

CH2CH2N

CH3

CH,

SCH3

Fig. 3. Alternative routes from hordenine to DMA and NDMA.

it has been shown that mixing green malt with cellulose

powder transfers precursors to the cellulose so that when

kilned this gives nitrosamines.

There is some evidence that either turning or transfer of

green malt can increase the NDMA level of kilned malt,

presumably by transfer of precursor to the husk e.g. by

smearing from the rootlets.

Nitrosation ofhordenine prevented

It is probable that there are at least two routes from

hordenine to NDMA as shown in Fig. 3. In one pathway,

the hordenine is nitrated in the phenolic ring before (or

simultaneously with) nitrosation of the amine. In the other

pathway, this ring nitration does not occur. Both pathways

are likely to occur together, but the relative extents will

depend on the NO2 concentration and the opportunity for

oxidative reactions. In both pathways DMA is liberated

and requires more NO, for nitrosation. Closely related

suggested pathways, involving formation of NDMA from

nitrosated hordenine without intermediate release of DMA,

are described in other publications11'29 but, since theyappear less likely to be important, arc not discussed here.

The nitrosation of the amine part of hordenine is pre

vented under acidic conditions because it forms a saltCH,

R5NH+ X." A wide variety of acids, including HNO3, canCHj

prevent NDMA formation. American work showed thathigh NO2 concentrations can prevent NDMA formation inmalt—presumably by forming HNO3. Trials in commercialkilns show that acid treatments are effective but not con

venient.8'14 One American company has found it possibleto avoid sulphuring by use of phosphoric acid in combi

nation with sugars (sec later) although the acid alone did

not give sufficient control.20Sulphuric acid sprayed onto malt prior to kilning can pre

vent NDMA formation, and one of the advantages of using

fuel containing sulphur, or of sulphuring, is that sulphuric

acid is formed and the outer regions of the malt where the

hordenine is located become acid. Indeed, in some mailings

using sulphuring to control NDMA, the surface pH'8(not the ground malt pH) gave an indication of whether or

not the NDMA content should be low. Surface pHs below 4

almost always give low NDMA values. However, for acidi

fication to be effective it must be achieved before nitroso-

hordenine can be formed. This means that sulphuring must

be used from the outset of, or even before, heating during

kilning. The SO2, both in the kiln gas and after adsorption

on the malt, reduces NO, to NO whilst being converted to

H2SO4. Thus early application of SO2 can provide all the

protection which is required, and some maltsters manage to

get sufficient control by sulphuring (250 g S/tonne) just for

the first 8 hours of kilning with the concentration of SO2 in

the air entering the kiln being SO ppm in the first hour but

only 10 ppm towards the end of sulphuring.23 (NB NO,would be not more than 1 ppm). Unpublished work ofother

maltsters shows that even less SO2 is required and shorter

times ofapplication can be used, but it must be applied early

and distributed over all the malt. With indirect kilning,

injection of SO2, if required, is effective when applied well

before the break point and with the highest concentration

(e.g. 40 ppm) at the start of kilning." Experiments at BRFshow that, even with a high-NO, burner, NDMA can be

suppressed by 125 g S/tonne applied immediately before

kilning as a sodium bisulphite spray buffered to give a green

malt suface pH of 4-5. Because of the success in controllingNDMA by sulphuring many maltsters have favoured this

approach and have been improving techniques so that less

sulphur is used and the finished malt docs not have a high

residual SO2 content.

The surface pH, and therefore NDMA formation, is

lowered by some other malting treatments. One is the

restriction of oxygen supply and build up of CO2 concen

tration, used either as a deliberate technique by reducing

ventilation or simply because it occurs, as in certain drum

makings which do not use a high air flow.

It was noted that one kiln, not using low NO, conditions,

consistently gave low NDMA values. Although there

turned out to be other possible contributory reasons, it was

discovered that the makings kept the germinated barley forsome hours in a pre-kiln vessel before kilning. Amongst

other consequences, this caused the roots to dry out and

wither. BRF experiments and examination of commercial

malts suggested such withering was always beneficial. Con

versely, moistening the malt just before kilning often giveshigher NDMA concentration in the finished malt. Presum

ably all the surface regions, and not just the roots becomemore desiccated and so pick up less NO,, but it is possible

that the effect depends on structural damage leading to

increased loss of NDMA (see later). In any event some

maltsters noticed improvements in NDMA levels when the

circulation of humidified air was stopped before kilning to

encourage withering.8

78 t. wainwright: nitrosamines in malt and beer [J. Inst. Brew.

Another approach, which has been tested commerci

ally,14 is to use a spray treatment before kilning to wash outhordenine which is near the surface and surprisingly easy to

remove. Obviously when using this treatment enough water

must be used to remove the hordenine and not just dis

tribute it where it is more readily nitrosated. As mentioned

above the increased moisture level will increase the uptake

of NO,. Such treatments would also wash out other

constituents including some useful extract.

Competitionfor the NO'„ present

As mentioned earlier NO, in the form of NO without

NO2 is not an effective nitrosating agent. The various forms

ofsulphuring work partly because they convert NO2 to NO.

Other substances such as ascorbic acid, sorbic acid, certain

polyphenols and metallic catalysts are also able to do this.

Some of these chemicals are used in certain food systems

to reduce nitrosamine formation. When sprayed onto

green malt or used to treat the hot air passing to the malt

they reduce nitrosamine formation. However, none of the

above substances are cheap or effective enough to use in

commercial malting.

A process has been patented by Heineken Ltd for inhibit

ing nitrosamine formation by spraying the green malt with

sugar before kilning.1 BRF experiments and commercialtrials confirm that it is effective although simultaneous use

of acid treatments is required to guarantee low NDMA

levels.14-20 Indeed a large number of similar treatmentsusing sugars and alcohols are effective and, contrary to first

indications, there is usually no advantage in spraying the

sugar onto the malt some hours before kilning. The reasons

for the inhibition have not been established. It seems most

likely that a major effect is a competition for NO, to form

nitrite esters of the alcohol groupings in the sugars and

polyols.29 There are, however, other contributory factors.Spraying with sugars, especially if there is a delay before

kilning, decreases the surface pH of the green malt. There is

also the possibility that the osmotic effect dehydrates the

seedling (see previous section).

Many other compounds naturally present in germinated

barley will remove some of the NO, and become nitrosated,

nitrated or oxidised. These include phenols, amines, amino

acids and peptides, unsaturated lipids and sulphydryl com

pounds. These are not deliberately used to control NDMA

formation in malt, but their presence and location will effect

the reactions leading to NDMA and make it more difficult

to understand what does happen.

Minimal breakdown ofnitrosated hordenine to DMA

The reactions leading to nitroso-hordenine and the

nitro-nitroso hordenine, shown in Fig. 3, occur at low

temperatures, but the reactions leading to release of DMA,

and thus formation of NDMA, from these compounds are

relatively slow below 60°C. With increasing temperature,

and with increasing time at a temperature above 60°C, the

yield of NDMA increases. In laboratory experiments at

90°C about 90% yield can be obtained but 64 hours are

required.30 The great increase in NDMA concentrationobtained by curing at 100°C instead of 85°C, whilst main

taining the same NO2 concentration,14 supports the idea

that thermal breakdown of nitrosated hordenine is ratelimiting.

One obvious way to decrease the NDMA content of malt

is to use lower curing temperatures and shorter times. This

is being done with beneficial results, but it should be noted

that even when the kilning temperature does not exceed

65°C, some commercially produced malts contain more

than Sppb NDMA.

Removal ofDMA andNDMA encouraged

Both DMA and NDMA are relatively volatile but, like

dimethyl sulphide in malt, they are not readily removed

once the malt is dry. Germinated barley contains several

ppm DMA and, contrary to expectation, only about half of

this is removed during the several hours of kilning before

the break when the temperature rises. After that time it is

difficult to know if the DMA originally present is removed

since more is formed by degradation of hordenine and,

possibly, other compounds. The fact is that DMA is present

in the malt throughout kilning.

Nevertheless, there are good reasons for believing that

any DMA which is located where it could react with NO, is

removed during the initial stages of kilning along with any

NDMA formed from it. Little NDMA is present in the malt

up to the break point but there is some in the exhaust gas.

Acidification of the green malt should encourage, not pre

vent, nitrosation of free DMA, but green malt treated with

acid either forms no NDMA or that formed is driven off.

Another good reason for believing that the DMA in the

germinated barley is not a significant source of NDMA in

malt is that unkilned malt contains very little NDMA even

when it contains much DMA and the ambient NO, con

centration is high. Reaction between DMA and NO, or

nitrite occurs readily at ambient temperatures.

Although some DMA is retained in the malt during kiln

ing, both DMA10 and NDMA are present in the kiln

exhaust. There is some transfer ofNDMA from malt at the

bottom of a kiln to the malt above it, but relatively little

is adsorbed and most passes through. Similarly NDMA

in recirculated kiln air is not taken up by the malt.

The volatilisation of DMA and/or NDMA presumably

accounts for the gradient of NDMA in the malt bed. In

unsulphurcd kilns the highest levels are at the top14 whereasthere has been more heating with NO, at the bottom.

Whilst the concentration ofNDMA is highest in the roots

and acrospires of the kilned malt, most of the NDMA is in

the husk and endospermu#18>3° since the roots and acro-spire form only a small proportion of the total weight. The

obvious explanation is that one or more of the compounds

hordenine, DMA or NDMA is transferred during germi

nation and kilning to the husk and endosperm. As men

tioned above, hordenine can be transferred, but it also

seems that DMA and/or NDMA is also transferred during

kilning—probably by diffusion.

There are several means by which the removal of DMA

and NDMA can be enhanced. Use ofdehusked barley gives

low levels ofNDMA and very minor dehusking by abrasion

also seems beneficial. In experiments with green malts col

lected from commercial kilns, for study at BRF, it became

apparent that freezing the green malt resulted in much lower

NDMA levels after kilning. It is now clear that damage to

the seedling, or perhaps to the lipid layers in the husk,

before kilning facilitates escape of DMA or NDMA. One

easy way to cause this damage experimentally is to subject

the green malt to a short treatment with hot air or steamjust

before kilning. Several of the treatments such as withering,

acidification or spraying on sugars may be partially effective

by damaging barrier layers.Another factor of great importance is the rate of air flow

through the kiln. Many of the malts with high NDMA

levels were produced in kilns with a low air flow. Although

not proven, it is likely that this reduced the rate of removal

of DMA and NDMA but, of course, the malt would have

picked up more NO, because of slower drying and longer

exposure times. High air flows appear to remove more

NDMA during kilning.14

NDMA removalfrom mall

This approach should not be required but, before the prob

lem was solved, it was necessary to consider ways to remove

NDMA. One technique, used experimentally, is to treat themalt briefly with steam. Whilst NDMA is not readily

removed from the dry malt it is readily volatilised in thesteam. In theory, steam could be injected into malt on a kiln

Vol.92,1986] T. WAINWR1GHT: NITROSAMINES IN MALT AND BEER 79

without causing much condensation or temperature drop sothat the extra energy requirement would be small.

Analyses of malt forNDMA can be done without grindingthe malt since the NDMA is readily extracted by water. This

ease of extraction means that the NDMA can be removed

by the water used in wet milling. Tests in a commercialbrewery showed this was one way to use malts with high

NDMA contents but (he water discarded also contained

considerable amounts or extract. If it had been necessary to

adopt such a process it would have been essential to find a

way to add this extract back to the wort whilst eliminating

the NDMA. The possibility of reducing the NDMA content

of wort by at least 50% through discarding water used in

wet milling has also been reported.3

Comments on NDMA analysis

There is still some disquiet about the accuracy and reli

ability ofNDMA analysis of malt. In many cases the discus

sion is now somewhat academic since the levels are so low

(1 ppb or less) that accuracy is not really necessary. There

are, however, commercial problems with values around

2-5 ppb or 5 ppb, because some customers apply specific

limits and employ penalty or rejection clauses.

There is at present no perfect method for analysis of

NDMA in malt and if there were it could be misleading

for the brewer. As explained, the NDMA is distributed

unevenly in the malt. Reproducible analyses can be

obtained by making aqueous extracts of intact malt but it

is unreasonable to expect this procedure to extract all

the NDMA, especially from the endosperm. Indeed some

experiments show more NDMA can be extracted by repeat

ing the extraction. On the other hand, grinding the malt

often gives lower values, presumably because the NDMA is

either volatilized or becomes adsorbed on surfaces. In any

case, the degree of milling of the malt is unlikely to equal

that occurring in a brewery and it is known that NDMA

is present in spent grains. Extraction by blending under

water in theory should give reproducible results but several

analysts have encountered difficulties with this technique.

Very stable emulsions can be formed. Nevertheless such a

method has been proposed as a reference method by the

IOB Analysis Committee.15The method recommended by the ASBC involves

vacuum distillation using an oil/water mixture and ground

malt.4 This is also favoured by some Government Agenciessince it allows confirmation of the identity of NDMA by

mass spectrometry. (Many early results were erroneous due

to false identification ofa peak on a chromatograph). How

ever, there is a great danger of artifactual formation of

NDMA in the vacuum distillation of malt. Moreover, it is

too slow and requires too much supervision to be a con

venient routine analysis. Consequently, analysts normally

use their own preferred extraction method. Usually this has

been compared with a reference method. The EBC Analysis

Committee has concluded that it is not necessary to

standardise the analysis method.21Comparison of several methods at BRF suggests that no

one method consistently gives the highest values for all

malts. This is not surprising since the distribution, and

therefore extractability of the NDMA, will depend on

exactly how the malt was made. The malting technique will

also affect the possibility ofartifactual formation ofNDMA

from nitrosated hordenine, or DMA, and bound NO,, andthe occurrence of nitrosating inhibitors or competitors, such

as SO2 or sugars or phenols. Analysis of beers is less of a

problem.3-21

Outlook

The NDMA content of malt can be limited by various

techniques. Many of these, e.g. the use of indirect heating,

will decrease the formation of all nitroso-compounds during

kilning although there may be some formation from

ambient NO, during germination. It would be useful to

discover if DMA in barley forms NDMA which is removed

during germination. If it does not, presumably few other

nitrosamines will be formed during germination.

Other nitrosamines do occur in malt, for instance

nitrosated hordeninc. Study of one of the other volatile

nitrosamines present, nitrosoprolinc, shows that some of

the techniques to control NDMA do not prevent, and mayindeed increase, formation of other nitrosamincs. Study of

nitrosoproline gives support to the hypotheses of how

various palliative treatments work and clearly show theadvantage of minimising the concentration of active forms

of NO,.Little is yet known about the non-volatile nitrosamines

in beer. The amounts greatly exceed the NDMA concen

tration16 but, at present, it is held that they do not

constitute a health hazard.

Conclusions

Although the detailed reactions, and even reactants,

responsible for NDMA in beer are not established with cer

tainty, enough has been learnt to minimise its occurrence.

This has been achieved solely by reducing the NDMA con

tent of malt. This has been done mainly by the introduction

of indirectly fired kilns, or the use of low NO, burners, plus

sulphuring. Various other minor modifications to malting

have been made as a result of studies of alternative means of

control. In many cases without these modifications either

the amount of sulphur used would need to be higher, or the

NDMA levels would not be sufficiently controlled. Higher

levels of sulphuring add to environmental pollution besides

affecting malt properties. It is possible that some other

approaches which arc still only tested at the laboratory scale

may eventually be adopted because of the disadvantages in

sulphuring. But there is no urgent need for change.

Above all, however, the transient existence of the NDMA

problem showed how responsibly and effectively the malt

ing and brewing industries can act to protect their

customers.

Acknowledgements.—Besides the individuals and organis

ations referred to above, I would especially like to thank my

colleagues, past and present, at BRF for the major role they

have played in these studies. Much of the work was done

under pressure and ingenuity was called for in devising

experimental systems.

I also wish to thank the Director of BRF for permission

to publish this paper.

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