216.
U T l L l Z A T l O N O f H I C R O E A V E COOKERY
IN M E A T P R O C E S S I N G
R.
V.
DECAREAU
Microwave cooking i s cooking by means of electromagnetic
energy i n t h e high frequency range. The e n e r a i s a c t u a l l y converted
i n t o heat by a c t i o n i n t h e product a t t h e atomic and molecular l e v e l .
T i m e does not permit e l a b o r a t i o n of t h i s mechanism, however f o r those
i n t e r e s t e d , a number of references which cover t h e subject i n some
d e t a i l a r e l i s t e d i n t h e t a b l e of references. For t h e purpose of t h i s
p r e s e n t a t i o n it i s s u f f i c i e n t t o say t h a t microwave energy penetrates
deeply i n t o food m a t e r i a l s and gives up energy i n t h e form of heat a s
it p e n e t r a t e s ; t h a t t h e p e n e t r a t i o n i s a function of frequency and
temperature; and t h a t of t h e microwave frequencies a l l o c a t e d by t h e
Federal Communications Commission only two--915 mHz and 2450 mHz -- a r e of
p r a c t i c a l significance f o r food processing a t t h e current s t a t e of
the art.
Since t h e generating equipment i s e l e c t r o n i c it has no
thermal i n e r t i a ; t h a t i s , t h e r e i s no heating l a g . One does not have
t o w a i t f o r t h e equipment warm up o r cool down. It i s a very cont r o l l a b l e form of energy, and very e f f i c i e n t because it i s not wasted
i n heating t h e product environment. I n other words a l l of t h e
c a l o r i e s generated i n t h i s way a r e put where they can do t h e most good
- - i n t h e product, Thus even though t h i s i s a more c o s t l y form of
energy, none i s wasted i n a properly designed system, whereas i n conv e n t i o n a l systems which use steam, hot a i r , hot o i l , or e l e c t r i c i t y
much energy "goes up t h e stack" a.nd i s l o s t .
Microwave heating i s a l s o c a l l e d volume heating t o d i s t i n g u i s h
it from conduction heating because i t s e f f e c t i s throughout t h e product
r a t h e r t h a n j u s t a t t h e surface. This i s not t o say t h a t conduction
heating does not p l a y a p a r t . Temperature gradients are established i n
products heated by microwave energy and t h e magnitude of t h e gradients
depend on t h e s i z e of t h e product being heated--the t h i c k e r t h e product
t h e steeper t h e gradient. An example of t h i s i s shown i n Figure 1.
Figure 1 i s a graph of t h e temperature p a t t e r n i n a 7-rib, ovenprepared r o a s t of beef. One curve represents t h e r e s u l t s obtained a t
915 mHz and t h e other a t 2450 mHz. Both r o a s t s received t h e same t o t a l
heat input. The temperature i n t h e center can be c a l l e d t h e "microwave
reference temperature" and i s t h e temperature t o which a l a r g e r o a s t
should be cooked wjith microwave energy t o obtain t h e b e s t y i e l d and a
r a r e degree o f doneness. It i s obvious a l s o t h a t a t t h i s temperature
t h e r o a s t i s not r a r e but r a t h e r raw. However when t h e r o a s t i s allowed
t o stand a t room temperature f o r about 45 minutes t h e heat located about
one inch below t h e surface w i l l be conducted inward and outward, and a
f i n a l cross s e c t i o n temperature of 135-145OF. w i l l be reached. Attempts
t o cook d i r e c t l y t o t h i s temperature with microwave energy would r e s u l t
217.
i n severe overcooking of t h e outer l a y e r s and f i n a l y i e l d s of usable
meat might be a s low a s 50 per cent. Indeed s e v e r a l s t u d i e s reported
i n t h e l i t e r a t u r e have penalized microwave cooking by attempting t o
apply conventional cooking reference standard-s. (Pollack, G. A. e t
a l , 1966; Marshall, Nancy, 1959). Properly applied microwave cooking
has given y i e l d s of b e t t e r than 80 p e r cent for r o a s t s of t h i s s i z e .
It might a l s o be pointed out t h a t i n t h e cooking of such
l a r g e r o a s t s , advantage can be taken of t h e f a c t t h a t microwave energy
i s r e f l e c t e d from metal. Thus a f t e r an i n i t i a l cooking period of perhaps 20 minutes, t h e ends of t h e r o a s t can be wrapped i n aluminum f o i l
t o prevent f u r t h e r cooking of t h e ends while permitting t h e t o t a l
energy t o be d i r e c t e d t o t h e heating of t h e t h i c k e r center portion.
The f o i l i s removed a f t e r 2 0 minutes of cooking. I n terms of cooking
rates such l a r g e r o a s t s a r e cooked a t a r a t e of aboxt 2.0 minutes per
pound or about 44 minutes f o r a 22 pound r o a s t , The center temperature
w i l l t h e n coast t o r a r e doneness a t 1.0 t o 1.5 degrees per minute. The
surface appearance w i l l be comparable t o t h a t obtained i n a hot oven.
It is possible i n many instances t o o b t a i n adequate browning
with microwave cooking. Small domestic s i z e r o a s t s cooked a t t h e lower
power l e v e l s of domestic microwave ovens (0.5 t o 1.0 k i l o w a t t s ) w i l l be
adequately browned though t h e time t o cook may be 20 minutes or l e s s .
The reference temperature i s increased t o 100 t o 110OF. f o r r o a s t s i n
t h e neighborhood o f f i v e pounds. A piece of s t e a k or a hamburger p a t t y
however would have a gray e x t e r i o r though t h e i n t e r i o r appearance would
be a s desired. It i s necessary t o pre-sear or post-sear such small
items as t h e very short cook time--two t o t h r e e minutes--for an 8-10
ounce s t e a k i s too short f o r much browning t o take place.
The r a t e a t which meat i s cooked i s a l s o important. A t very
high power l e v e l s neat pieces such a s cut-up p o u l t r y p a r t s can be
cooked i n a few seconds t o give an extremely tough product. When t h e
r a t e i s reduced t o give a cooking time of 1 0 t o 1 2 minutes, shear p r e s s
values a s an index of toughness were equivalent t o values obtained by
a l l steam cooking. A s a r e s u l t of cooking s t u d i e s with steam and microwave energy, s i n g l y a s w e l l a s simultaneously, a system was designed,
b u i l t , and i n s t a l l e d i n t h e p l a n t of a major p o u l t r y yrocessor (Anon, 1966).
Steam cooking s t u d i e s alone indicated t h a t t h e outer
inch of meat could
be cooked t o doneness i n about six minutes, but t h a t t h e heating curve for
t h e meat a t t h e bone tapered o f f r a p i d l y requiring about 40 minutes t o
reach a done condition, and f u r t h e r time t o insure cooking of t h e marrow.
It made economic sense t o use steam t o accomplish cooking of t h e outer
l a y e r s of meat. Microwave energy made sense t o insure adequate cooking t o
t h e bone. A s a r e s u l t t h e microwave content o f t h e process was minimized
t o t h e point where t h e equipment c o s t s were not p r o h i b i t i v e ; t h e y i e l d s
were s u b s t a n t i a l l y increased over water or steam cooking; t h e q u a l i t y was
a c t u a l l y enhanced because of t h e g r e a t e r r e t e n t i o n of n a t u r a l j u i c e s ; and
a short enough cook cycle was obtained which permitted t h e e n t i r e operation
t o be conveyorized, thus r e s u l t i n g i n a reduction i n t h e labor requirement.
I n p r a c t i c e two p a r a l l e l conveyors a r e used; one i n which b r e a s t s and thighs
a r e cooked, and t h e other f o r wings and drum s t i c k s . This i s necessary
because it takes less energy t o cook t h e smaller p a r t s . If a l l p a r t s were
cooked together u n t i l t h e b r e a s t s and thighs were done t h e n t h e smaller
p a r t s would be overcooked.
218.
I n summary, a number of general r u l e s f o r t h e microwave
cooking of meats can be formulated:
1). Each type of meat has i t s own optimum cooking r a t e .
A reference temperature needs t o be established for
2).
each meat item, and t h i s temperature w i l l vary with
t h e s i z e of t h e item.
Roasting
should not be c a r r i e d t o completion with
3).
microwave energy, r a t h e r advantage should be taken
of t h e r e s e r v o i r of heat b u i l t up below t h e meat
surface t o complete t h e cooking.
Wrapping
t h e ends of l a r g e r o a s t s with aluminum
4).
f o i l during a p o r t i o n of t h e cooking cycle w i l l
f a c i l i t a t e cooking of t h e generally t h i c k e r center
portion.
5 ) . For production cooking it i s d e s i r a b l e t o employ a
combination of microwave and o t h e r forms of energy.
It cannot be overemphasized t h a t conventional cooking
standards and techniques a r e not always applicable t o microwave cookery,
b u t t h a t when t h e product heating performance has been properly analyzed,
a system involving microvave energy may give b e t t e r y i e l d s and f i n e r
q u a l i t y than t h e conventional methods which it replaces.
References
Anon. ( 1966a )
Microwave oven with steam atmosphere produces higher q u a l i t y , more
p r o f i t a b l e precooked chicken.
Food Processing and Marketing 27 ( 4 ) 92-96, 100.
-
Anon (1966b)
Microwave cooking c u t s labor c o s t s i n h a l f .
Broiler Industry 29 (1)58-62.
Brown, G. H., C. N. Hoyler, and R . A. Bierwirth (1947).
Theory and Application of Radio Frequency Heating.
D. Van Nostrand Co. Inc. New York. 37Op.
Cable, J. W. (1954)
Induction and D i e l e c t r i c Heating
Reinhold Publishing Corp. New York, 576 p.
Copson, D. A. (1962 )
M i crowave Heat ing
A V I Publishing Co. Westport, Conn. 433p.
Marshall, Nancy (1959)
Electronic Cookery of Top Round Beef
Jour. Home Ec. 60 (1)31-34.
Moore, R. E., Mandigo, R. W., and Henrickson, R. L. (1966)
The Effect of Cutting, Chilling, and Cooking Method on t h e Q u a l i t y of
Pork Loin.
20 ( 7 , 107-109).
Food Technol. -
219.
Pollack, G. A . and Louise C. Foin (1960)
Comparative Heating E f f i c i e n c i e s of a Microwave and a Conventional
E l e c t r i c Oven
Food Technol. 14 ( 9 ) 454.
-
Figure 1. Temperature p a t t e r n i n 7-rib r o a s t of beef
cooked by means of microwave energy.
P
2oc
K
/
19c
lac
17C
915 mHz
'\$
\
\
,'
"i,
\
16C
15C
14(
13(
12(
\
\,
'
2450 mHz-
\
\
After 45 minutes
I
'8
11(
10(
9(
\
. _-
Microwave cooking
reference temperature
Inches through t h e diameter
, /
I
i
,
'
Y
X
220.
DR. WANDERSTOCK: Thank you, Dr. Decareau f o r bringing t h i s
most i n t e r e s t i n g information t o us. We now move on t o an e n t i r e l y
d i f f e r e n t aspect of meat cookery r e l a t i v e t o what a c t u a l l y happens i n
t h e muscle f i b e r s when meat i s cooked. D r . R. L. Hostetler of Texas
A & M University w i l l present t h e paper.
###########