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The display of retail packs of ground beef after their storage in master packages under various atmospheres
Meat Science 37 (1994) 281-295
T h e D i s p l a y o f R e t a i l P a c k s o f G r o u n d B e e f after their S t o r a g e in M a s t e r P a c k a g e s under V a r i o u s A t m o s p h e r e s C. O. Gill & T. Jones Agriculture C a n a d a Research Station, Bag Service 5000, Lacombe Alberta, C a n a d a T0C 1SO (Received 28 January 1993; r e v i s e d v e r s i o n r e c e i v e d 3 April 1993; accepted 16 April 1993)
ABSTRA CT B a t c h e s o f coarsely g r o u n d b e e f t r i m m i n g s were each divided into f o u r portions. One portion j k o m each batch was vacuum p a c k a g e d , then stored at --1-5°C. The other portions were f i n e l y g r o u n d a n d distributed in retail packs. T h e retail p a c k s were m a s t e r p a c k a g e d under a t m o s p h e r e s o f N2, COz or 02 + C O 2 (2:1, v/v), then s t o r e d at 2°C. The p r o d u c t was assessed after storage times o f up to 32 days. F o r each assessment, a vacuum p a c k a n d a m a s t e r p a c k o f each type, each containing p r o d u c t s f r o m the s a m e batch, were withdrawn f r o m storage. The vacuum p a c k a g e d p r o d u c t was f i n e l y g r o u n d a n d distributed into retail packs. The n e w l y p r e p a r e d retail p a c k s a n d those f r o m the m a s t e r p a c k s were displayed in a retail cabinet in which air t e m p e r a t u r e s a v e r a g e d 4 +- 2°C. The appearance o f the displayed p r o d u c t f r o m each storage p a c k a g i n g was assessed twice daily until it was j u d g e d to be unacceptable. A t the beginning a n d e n d o f the display, the p r o d u c t in each p a c k was assessed f o r discoloration a n d offodours, the chemical states o f the m u s c l e p i g m e n t at the e x p o s e d surface were estimated, a n d the surface microflora was characterized. The appearance o f the p r o d u c t displayed after storage in a vacuum p a c k , f o r times up to 32 days, b e c a m e unacceptable within 48 h. A p r o d u c t s t o r e d in a n y o f the m a s t e r p a c k s f o r 1 d a y appeared unacceptable after 6 h o f display. The d i s p l a y life o f p r o d u c t s s t o r e d under Nz or C 0 2 was similar to that o f the vacuum p a c k a g e d p r o d u c t s when storage times were b e t w e e n 2 a n d 24 days but the display life was shorter when the storage times were 28 or 32 days. The display life o f products stored under 0 z + COz was similar to that o f the vacuum p a c k a g e d p r o d u c t when storage times 281
Meat Science 0309-1740/94/$07.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain
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C. O. Gill, T. Jones were 2, 4 or 6 days, but the appearance o f products stored under 02 + COz .for 8 days or longer was unacceptable when master packs w e r e opened. Apart f r o m those latter packs, a product was not discoloured when storage packs were opened. However, all products were discoloured, and the metmyoglobin fractions o f the surface pigments had increased, when the product was withdrawn f r om display. The products in all storage paekagings developed flora dominated by lactic acid bacteria. The spoilage flora on products stored in vacuum p a c k or under 02 + C02 did not attain the m a x i m u m numbers o f 107/g during either storage or display. Those m a x i m u m numbers were attained on products stored under N2 and CO 2 after 16 and 28 days storage respectively. Some products stored under N2 f o r 16 days or longer developed moderate or strong off-odours during display that were ascribable to microbial action. Other products developed only slight, non-microbial off-odours during display. Retail-ready packs or ground beef master-packaged under an oxygen-depleted atmosphere could then have a useful storage life o f about 30 days in commercial circumstances.
INTRODUCTION C o n s i d e r a b l e e c o n o m i c benefits w o u l d accrue to s u p e r m a r k e t o p e r a t o r s were t h e y able to o b t a i n all their fresh m e a t as finished, r e t a i l - r e a d y items (Farris et al., 1991). T h e s t o r a g e life o f m e a t in c o n v e n t i o n a l l y overw r a p p e d trays is generally t o o s h o r t to allow retail p a c k s to be centrally p r e p a r e d a n d widely d i s t r i b u t e d to retail outlets. E x t e n s i o n o f p r o d u c t s t o r a g e life has t h e r e f o r e b e e n s o u g h t t h r o u g h the use o f p r e s e r v a t i v e packagings. T h r e e types o f p r e s e r v a t i v e p a c k a g i n g are u s e d w i t h r e t a i l - r e a d y m e a t s . T h e s e are, v a c u u m p a c k a g i n g , retail p a c k a g i n g u n d e r h i g h - o x y g e n m o d i fied a t m o s p h e r e s , a n d m a s t e r p a c k a g i n g u n d e r m o d i f i e d a t m o s p h e r e s ( Y o u n g et aL, 1988). A l t h o u g h t h e r e are c o m m e r c i a l s y s t e m s f o r t h e p r e p a r a t i o n o f retail-ready fresh m e a t s t h a t e m p l o y v a c u u m a n d / o r higho x y g e n m o d i f i e d a t m o s p h e r e retail packs to preserve the p r o d u c t , neither type o f p a c k a g i n g is satisfactory for all m e a t trades. V a c u u m p a c k a g i n g is n o t a c c e p t a b l e for m o s t retail p u r p o s e s b e c a u s e c o n s u m e r s d i s c r i m i n a t e a g a i n s t t h e dull, p u r p l e c o l o u r o f anoxic red m e a t (Allen, 1989), while m o d i f i e d a t m o s p h e r e retail p a c k s m u s t be d i s p r o p o r t i o n a l l y large c o m p a r e d to the p r o d u c t they c o n t a i n if t h e p r e s e r v a t i v e a t m o s p h e r e is to be a d e q u a t e l y m a i n t a i n e d ( H o l l a n d , 1980). E v e n in c i r c u m s t a n c e s w h e r e t h e costs associated w i t h a d d i t i o n a l p a c k a g i n g a n d l o w p r o d u c t d e n s i t y are a c c e p t a b l e ( R e n e r r e , 1989) t h e stability o f s o m e p r o d u c t s in m o d i f i e d a t m o s p h e r e retail p a c k s m a y be i n a d e q u a t e ( S h a y a n d E g a n , 1990). M a s t e r p a c k a g i n g o f retail-ready p a c k s is t h e n p e r c e i v e d as a w a y o f
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avoiding s o m e o f the c o m m e r c i a l difficulties e n c o u n t e r e d with o t h e r techniques f o r preserving centrally p r e p a r e d p r o d u c t s (Scholtz e t al., 1992). A system for the central preparation o f retail-ready beef should preferably be c a p a b l e o f a d e q u a t e l y preserving g r o u n d beef, as u p to 50% o f b e e f is retailed in t h a t f o r m ( F o s t e r et aL, 1977). Because it is largely p r e p a r e d f r o m trimmings, m u c h freshly g r o u n d b e e f is likely to c a r r y a higher microbial load, a n d have less c o l o u r stability, t h a n p o r t i o n s o f n o n c o m m i n u t e d tissue cut f r o m fresh carcasses. It is t h e n likely t h a t a p a c k a g i n g system w h i c h will a d e q u a t e l y preserve retail-ready g r o u n d b e e f w o u l d be at least equally a d e q u a t e f o r o t h e r f o r m s o f r a w beef. T o ascertain the possibilities for using m a s t e r p a c k a g i n g with beef, retailr e a d y packs o f g r o u n d b e e f were m a s t e r p a c k e d u n d e r preservative a t m o s p h e r e s o f 02 + CO2, N2 o r CO2, (Gill a n d M01in, 1991), stored, a n d displayed after r e m o v a l f r o m the m a s t e r - p a c k s , using storage a n d display c o n d i t i o n s t h a t simulate c u r r e n t c o m m e r c i a l G o o d Practice.
MATERIALS AND METHODS
Preparation and packaging of ground beef A t a local b e e f p a c k a g i n g plant, b e e f t r i m m i n g s are r o u t i n e l y collected a n d v a c u u m p a c k a g e d , in 10 kg portions, for distribution to retail outlets, w h e r e the p r o d u c t is g r o u n d . O n each o f 4 days, t h r e e freshly p r e p a r e d packages o f such products were collected f r o m the beef packaging plant, a n d stored o v e r n i g h t at - 1 . 5 ° C . E a c h 30 kg b a t c h o f b e e f t r i m m i n g s was first g r o u n d t h r o u g h a plate with holes o f 10 m m diameter. T h e coarsely g r o u n d m a t e r i a l was m i x e d by h a n d , t h e n 3 × 2 kg p o r t i o n s were v a c u u m p a c k a g e d in p o u c h s o f a polyvinylidine chloride (PVDC) laminate that h a d an oxygen transmission r a t e o f a b o u t 40 cm3/m2/24 h / a t m at 25°C a n d 1002/0 r.h. (Cryovac, Mississauga, Ontario). The remaining portion o f coarsely g r o u n d material was r e g r o u n d t h r o u g h a plate with holes o f 3-7 m m diameter. T h e finely g r o u n d m a t e r i a l was m i x e d by h a n d , t h e n distributed into 27 solid polystyrene trays. T h e t r a y dimensions were 165 × 128 × 40 m m (l × w × h). E a c h tray was filled, to a b o u t 5 m m below the edge o f the walls, with 500 g o f g r o u n d beef. E a c h filled t r a y was placed in a p o u c h o f a s h r i n k a b l e film, t h a t h a d a n t i f o g g i n g properties a n d a n o x g y e n t r a n s m i s s i o n rate o f a b o u t 11 000 cm3/m2/24 h / a t m at 25°C a n d 75% r.h. (RD-106, C r y o v a c ) . T h e p o u c h was s h r u n k to the tray, using a h o t air gun. A hole 3 m m in d i a m e t e r was m a d e in the fihn, using an electric soldering tool, at o n e u p p e r
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c o r n e r o f t h e t r a y , so t h a t t h e film c o u l d be f o r m e d as a flat lid to t h e tray. T h e filled t r a y s were d i s t r i b u t e d , in g r o u p s o f three, i n t o n i n e p o u c h e s c o m p o s e d o f a n a l u m i n i u m foil l a m i n a t e t h a t h a d n o m e a s u r a b l e o x y g e n transmission rate (American National Can Co, Chicago, IL, USA). The foil l a m i n a t e d p o u c h e s w e r e e v a c u a t e d , filled w i t h gas, a n d sealed u s i n g a m o d i f i e d c h a m b e r p l u s s n o r k e l m a c h i n e ( C a p t r o n II. R M F Inc., Grandview, M O , USA) that w o u l d reliably establish an atmosphere o f CO2 c o n t a i n i n g <200 p p m o f 02. F o r e a c h g r o u p o f n i n e p o u c h e s , t h r e e w e r e e a c h filled w i t h 4 litres o f COz, t h r e e w e r e filled w i t h 4 litres, o f N 2, a n d t h r e e w e r e filled w i t h 2 litres o f CO2. W i t h i n 5 rain o f t h a t last set o f p a c k s b e i n g filled, e a c h o f t h o s e p a c k s w a s f u r t h e r filled w i t h 4 litres o f 02. T h e O2 w a s i n t r o d u c e d t h r o u g h a h y p o d e r m i c n e e d l e c o n n e c t e d b y flexible t u b i n g t o a n o x y g e n c y l i n d e r . W i t h o x y g e n f l o w i n g f r o m t h e n e e d l e a t 1 litre/s, t h e n e e d l e w a s i n s e r t e d i n t o o n e c o r n e r o f e a c h b a g , t h r o u g h a s t i c k - o n s e p t u m , a n d r e t a i n e d in t h e b a g f o r 4 s. I m m e d i a t e l y a f t e r t h e n e e d l e w a s r e m o v e d , t h e a r e a w h i c h it h a d p e n e t r a t e d w a s sealed by m e a n s of a h o t iron. The batch identity of the g r o u n d beef and the type o f pack a t m o s p h e r e w e r e n o t e d o n e a c h p o u c h . T h e b a t c h o r i g i n o f t h e p r o d u c t w a s also noted on each v a c u u m pack.
Storage and sampling of the master-packaged ground beef T h e v a c u u m p a c k a g e d p r o d u c t w a s s t o r e d a t --1-5°C + 0.5°C. T h e p r o d u c t p a c k a g e d in foil l a m i n a t e p o u c h e s w a s s t o r e d at 2 _ 0-25°C. P r o d u c t s w e r e w i t h d r a w n f o r e x a m i n a t i o n a f t e r s t o r a g e f o r 1, 2, 4, 6 a n d 8 d a y s , a n d a t s u b s e q u e n t 4 d a y i n t e r v a l s u p t o 32 d a y s . A t e a c h s a m p l i n g time, a p o u c h o f the v a c u u m p a c k a g e d p r o d u c t a n d a m a s t e r p a c k o f e a c h a t m o s p h e r e t y p e w a s w i t h d r a w n f r o m s t o r a g e . All t h e p r o d u c t s withdrawn at each sampling time originated from the same batch of g r o u n d beef. A t each sampling time, the v a c u u m p a c k a g e d p r o d u c t was r e m o v e d first. T h a t p r o d u c t w a s finely g r o u n d a n d m i x e d , t h e n t h r e e retail p a c k s w e r e p r e p a r e d as f o r t h e m a s t e r - p a c k e d p r o d u c t . T h e t h r e e m a s t e r p a c k s w e r e t h e n w i t h d r a w n , a n d all 12 retail p a c k s w e r e p l a c e d in a d i s p l a y case.
Display and sampling of the retail packs A f t e r e a c h s t o r a g e t i m e , t h e 12 s a m p l e s w e r e p l a c e d t o o c c u p y a n a r e a o f a p p r o x i m a t e l y 600 X 6 0 0 r a m in t h e c e n t r e o f t h e d i s p l a y s h e l f o f a
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horizontal, fan-assisted display case (model L P M 1 2 T , Hill Refrigeration, B a r r i e , O n t a r i o , C a n a d a ) t h a t w a s i l l u m i n a t e d f o r 12 h / d a y w i t h i n c a n d e s c e n t l i g h t s g i v i n g a l i g h t i n t e n s i t y o f 750 l u x a t t h e d i s p l a y s h e l f s u r f a c e . T h e a r e a o f t h e d i s p l a y s h e l f w a s 3-5 x 0-8 m . T h e a i r i n l e t t e m p e r a t u r e o f t h e d i s p l a y case w a s set at - - 2 ° C . A t e m p e r a t u r e d a t a logger (Tru-Test, A u c k l a n d , N e w Zealand) was placed at the centre o f t h e d i s p l a y a r e a , t o m o n i t o r t h e air t e m p e r a t u r e s e x p e r i e n c e d b y t h e product. D u r i n g display of the g r o u n d meat, other areas o f the display case w e r e u s e d , t o v a r y i n g e x t e n t s , f o r t h e d i s p l a y o f o t h e r p r o d u c t s . T h e d i s p l a y e d p r o d u c t s w e r e a s s e s s e d f o r retail a c c e p t a b i l i t y a n d c o l o u r 1 h a f t e r o p e n i n g o f t h e m a s t e r p a c k s . T h e e n v e l o p i n g film w a s t h e n r e m o v e d f r o m e a c h t r a y , a n d t h e o d o u r w a s assessed. T h e n , a sterile c o r k b o r e r w a s u s e d t o o b t a i n f o u r s a m p l e s o f m e a t t h a t w e r e 20 m m in d i a m e t e r a n d 10 m m t h i c k . T w o s a m p l e s w e r e r e m o v e d f r o m o p p o s i t e edges o f the tray, for the estimation o f d e o x y m y o g l o b i n , o x y m y o g l o b i n a n d m e t m y o g l o b i n , a n d two samples were similarly r e m o v e d for microbiological analysis. T h e t r a y s w e r e r e p a c k a g e d in b a g s o f t h e s h r i n k a b l e film, a n d t h e s a m p l e d a r e a s w e r e o b s c u r e d b y strips o f w h i t e t a p e a t e a c h e n d o f e a c h t r a y . T h e p r o d u c t w a s s u b s e q u e n t l y a s s e s s e d f o r retail a c c e p t a b i l i t y a n d c o l o u r , a t a l t e r n a t i n g p e r i o d s o f 6 a n d 18 h. E a c h set o f r e t a i l p a c k s f r o m the same storage packaging was removed from display when the majority of a panel considered that two more of the packs had unaccetable appearances. T h e r e m o v e d p r o d u c t was assessed for odour, a n d sampled for estimation of the different forms of myoglobin and for microbiological a n a l y s i s .
Subjective evaluations of the ground beef All a s s e s s m e n t s w e r e m a d e b y a f i v e - m e m b e r p a n e l . R e t a i l a c c e p t a b i l i t y w a s s c o r e d o n a 7 - p o i n t scale w h e r e 1 -- e x t r e m e l y u n d e s i r a b l e , 2 :- undesirable, 3 = slightly undesirable n o r undesirable, 5 = slightly desirable, 6 = d e s i r a b l e a n d 7 = extremely desirable. P r o d u c t s were r e m o v e d f r o m d i s p l a y w h e n t h e m a j o r i t y o f t h e a s s e s s m e n t p a n e l r e c o r d e d a retail a c c e p t a n c e score o f 3 o r less f o r t w o o r t h r e e o f a set o f t h r e e retail p a c k s . D i s c o l o r a t i o n w a s assessed o n a 5-point scale w h e r e 1 -- n o d i s c o l o r a t i o n , 2 = 1 - 1 0 % d i s c o l o r a t i o n , 3 = 1 1 - 2 5 % d i s c o l o r a t i o n , 4 = 2 6 - 5 0 % discoloration and 5 = 50-100% discoloration. A note was made of the type of discoloration observed. O d o u r w a s a s s e s s e d o n a 4 - p o i n t scale w h e r e 1 = n o o f f = o d o u r , 2 = slight o f f - o d o u r , 3 = m o d e r a t e o f f - o d o u r a n d 4 = s t r o n g o f f - o d o u r . N o t e was made of the types of off-odour that were detected.
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Assessment o f myoglobin forms E a c h excised sample w a s w r a p p e d in the s h r i n k a b l e film, t o c o v e r the surface t h a t h a d b e e n e x p o s e d to the p a c k a t m o s p h e r e a n d / o r air b y a single l a y e r o f film. T h e s p e c t r u m f r o m each o f t h o s e surfaces w a s o b t a i n e d using a reflectance s p e c t r o p h o t o m e t e r ( M a c b e t h C o l o u r E y e 1500/Plus; K o l l m o r g e n C o r p , N e w b u r g , N Y , U S A ) . T h e values f o r reflectance at 474, 525, 572 a n d 610 n m were c o n v e r t e d to K / S values (ratio o f the a b s o r b e n c e (K) to the scattering coefficient (S)). S t a n d a r d K / S values h a d p r e v i o u s l y b e e n o b t a i n e d f o r samples o f g r o u n d b e e f in which the muscle p i g m e n t h a d b e e n w h o l e l y c o n v e r t e d to the d e o x y , o x y or m e t form. T h e p e r c e n t a g e f r a c t i o n s o f the f o r m s o f m y o g l o b i n w e r e e s t i m a t e d f r o m the m e a s u r e d a n d s t a n d a r d K / S values ( H u n t , 1980).
Microbiological analysis T h e t w o s a m p l e s o b t a i n e d at e a c h s a m p l i n g time f r o m e a c h retail p a c k were c o m b i n e d , a n d a 5 g p o r t i o n w a s w e i g h e d into a s t o m a c h e r bag. T h e w e i g h e d s a m p l e w a s diluted w i t h 45 ml o f 0-1% p e p t o n e water, a n d w a s s t o m a c h e d with the diluent f o r 2 rain. T h e h o m o g e n a t e w a s diluted 10- a n d 100-fold, then 0-1 ml v o l u m e s o f the u n d i l u t e d h o m o g e n a t e , a n d o f e a c h d i l u t i o n were s p r e a d on d u p l i c a t e plates o f Plate C o u n t A g a r (Difco). T h e plates w e r e i n c u b a t e d f o r 3 d a y s at 25°C. F l o r a n u m b e r s a n d c o m p o s i t i o n s w e r e d e t e r m i n e d f r o m plates b e a r i n g 2 0 - 2 0 0 colonies. R e p r e s e n t a t i v e colonies o f each t y p e were p i c k e d f r o m each plate a n d identified to the generic level b y the criteria o f C o w a n (1974). T h r e e colonies o f a t y p e were p i c k e d w h e n t h a t or higher n u m b e r s w e r e available. O t h e r w i s e , all the available colonies w e r e picked.
RESULTS
Evaluation o f product removed from storage P r o d u c t s t h a t w e r e freshly g r o u n d , after s t o r a g e in v a c u u m p a c k for a n y time, were generally assessed as being desirable, n o t d i s c o l o u r e d a n d w i t h o u t o b j e c t i o n a b l e o d o u r . P r o d u c t s s t o r e d f o r 1 d a y in a n y o f the three types o f m a s t e r p a c k w e r e o f p o o r a p p e a r a n c e a n d d i s c o l o u r e d . A f t e r all longer s t o r a g e times, p r o d u c t s f r o m N2 or CO2 a t m o s p h e r e s were assessed as desirable a n d n o t d i s c o l o u r e d . S u c h p r o d u c t s h a d n o o f f - o d o u r at s t o r a g e times u p t o 24 days, b u t slight s o u r or acid o d o u r s were d e t e c t e d in p r o d u c t s s t o r e d f o r longer times. P r o d u c t s s t o r e d u n d e r
Masler packaging
TABLE
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of ground beef
1
T h e P r e d o m i n a n t O r g a n o l e p t i c S c o r e s R e c o r d e d b y a f i v e - m e m b e r P a n e l a t t h e first A s s e s s m e n t s o f G r o u n d Beef, P r e p a r e d f r o m C o a r s e l y G r o u n d P r o d u c t S t o r e d i n V a c u u m P a c k a t -1.5°C~ o r S t o r e d i n R e t a i i P a c k s M a s t e r P a c k a g e d u n d e r N2, CO2 o r 0 2 + C O 2 (2:1 v/v) A t m o s p h e r e s a t 2 ° C T i m e in storage (days)
Balch
Storttge pack Vacuum RA
1
2 4 6 8 12 16 20 24 28 32
D
N2 0
RA
D
CO 2 0
RA
D
02 + CO 2 0
RA
D
0
I
7
1
1
4
3
1
3
4
1
5
2
1
II III IV I III II IV II IV lI
6 5 7 7 5 7 6 7 6 5
1 1 1 1 2 1 1 1 1 1
1 1 1 1 1 1 1 1 1 l
7 6 6 6 6 7 6 7 7 7
1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 2 2
6 6 7 6 6 7 6 7 7 7
1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 2 2
6 5 5 2 ---
1 1 1 5 ~--
1 1 1 3 ---
----
----
----
R A = R e t a i l A c c e p t a n c e : 1 - e x t r e m e l y u n d e s i r a b l e , 2 - u n d e s i r a b l e , 3 = slightly u n d e s i r a b l e , 4 = neither desirable nor undesirable, 5 = slightly desirable, 6 = desirable, 7 = extremely desirable; D - discoloration: 1 - none, 2 - 1-10%, 3 - 11-25%, 4 26-50%, 5 51 100%; O = o f f - o d o u r i n t e n s i t y : 1 = n o n e , 2 = slight, 3 = m o d e r a t e , 4 = s t r o n g . - n o t assessed. an Oz + CO2 atmosphere were assessed as only slightly desirable, but not discoloured, after 4 and 6 days of storage. After storage for 8 days, such products were undesirable and discoloured, with a moderate, stale odour (Table 1), When products had been stored for 1 day, the estimated fraction of metmyoglobin at the surface of products from the vacuum pack was less than the metmyoglobin fractions estimated for products from any of the master packs. At later times, the estimated fractions of metmyoglobin and oxymyoglobin were similar for products that had been stored under vacuum, N2 or CO2. All such products stored for 16 days or longer had very small fractions of metmyoglobin. Products stored under 0 2 + CO 2 for 8 days had a large fraction of metmyoglobin (Table 2). After 1 day of storage, the flora numbers in the product from all four types of packagings were about 103/g. The flora were composed of approximately equal numbers of Gram-positive mesophiles (micrococci and bacilli) and Gram-negative psychrotrophs (acinetobacteria, moraxellae and pseudomonads). Lactic acid bacteria were only a minor fraction of some flora, and were undetected in most. With the product that was
TABLE 2 Average Values for the Chemical States of the Muscle Pigment at the first Examination. o f G r o u n d Beef, P r e p a r e d f r o m C o a r s e l y G r o u n d P r o d u c t S t o r e d in V a c u u m P a c k s a t - - 1 - 5 ° C , o r S t o r e d i n R e t a i l P a c k s M a s t e r P a c k a g e d u n d e r N 2 , C O 2 o r O 2 + C O 2 (2:1 v / v ) Atmospheres at 2°C
T i m e in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
Storage p a c k Vacuum
I II llI IV I IlI II IV II IV II
N2
C0 2
02 + C0 2
% oxy
% met
o.4 o x y
% m et
% oxy
% met
o~ o xy
% met
63 69 32 66 54 46 96 95 79 84 92
24 19 34 17 13 27 5 1 6 6 5
53 75 65 66 60 81 97 86 82 85 85
31 11 17 15 I4 6 0 2 0 8 3
34 62 65 67 66 74 100 86 90 87 91
48 13 20 13 12 12 0 6 0 6 1
51 87 59 74 41 -------
35 19 34 22 47 -------
oxy = oxymyoglobin, met = metmyoglobin. % deoxymyoglobin = 100 - ( o x y + m e t ) . - not assessed.
TABLE 3 T o t a l n u m b e r s o f , a n d f r a c t i o n s o f L a c t i c a c i d B a c t e r i a in, t h e M i c r o f l o r a O b t a i n e d a t the first Examination of Ground Beef, Prepared from Coarsely Ground Product Stored i n V a c u u m P a c k s a t - 1 . 5 ° C , o r S t o r e d i n R e t a i l P a c k s M a s t e r P a c k a g e d u n d e r N 2, C O 2 o r 0 2 + C O 2 (2:1 v / v ) A t m o s p h e r e s a t 2 ° C
Time in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
Storage p a c k Vacuum
I [I III IV I lII II IV II IV II
N2
CO 2
0 2 + CO 2
Total count (log No./g)
Lac (%)
Total count (log No./g)
Lac (o~)
Total count (log No./g)
Lae (%)
Total count (log N o . / g )
Lac (%)
3-18 3.26 2.83 3.08 3-94 2.81 3-20 4.08 4-15 5.96 5-84
8 N N 5 2 12 23 94 65 99 77
3.24 3-32 2.60 3.64 4-43 2.83 5-77 6-51 6.78 7.38 7.32
N N 5 95 93 27 77 100 75 100 95
3.37 3-00 2-66 3.74 4-18 2-48 5-15 6.48 5-54 7-11 6-20
N
3-53 3.15 2.40 3-90 4-81 -------
N 2 2 98 56 ----
Lae = lactic acid bacteria, N = not detected. - not assessed.
3 N 97 47 8 66 100 100 100 100
---
Master packaging o f ground beef
289
s t o r e d in v a c u u m p a c k s , a n u n a m b i g u o u s i n c r e a s e i n f l o r a n u m b e r s w a s n o t e v i d e n t u n t i l 2 0 d a y s , w h e n t h e t o t a l c o u n t s e x c e e d e d 104/g a n d l a c t i c acid bacteria had become the dominant organisms. The numbers increased t o a b o u t 106/g a t 32 d a y s w i t h l a c t i c a c i d b a c t e r i a r e m a i n i n g p r e d o m i n a n t ( T a b l e 3). O t h e r o r g a n i s m s d e t e c t e d a t s t o r a g e t i m e s a f t e r 2 0 d a y s w e r e the mesophilic and strictly aerobic types that were predominant in the initial flora. W i t h p r o d u c t s t h a t w e r e s t o r e d in t h e m a s t e r p a c k a g i n g s , u n a m b i g u o u s growth of the flora was evident after 8 days of storage, when lactic acid bacteria predominated. With products stored under N 2 or CO2, lactic a c i d b a c t e r i a p r e d o m i n a t e d in f l o r a t h a t a t l a t e r t i m e s a t t a i n e d n u m b e r s o f 107/g. H o w e v e r , a f t e r e a c h o f t h e l o n g e r s t o r a g e p e r i o d s , t h e n u m b e r s o b t a i n e d f r o m t h e p r o d u c t s t o r e d u n d e r C O 2 w e r e s o m e w h a t less t h a n t h o s e r e c o v e r e d f r o m p r o d u c t s t o r e d u n d e r N2. M o r e o v e r , a f t e r s t o r a g e for 20 days or longer, only lactic acid bacteria were recovered from the product stored under CO2, but enterobacteria were also recovered from s o m e o f t h e p r o d u c t s t o r e d u n d e r N 2 ( T a b l e 3).
Display times and temperatures The temperature of the air around the retail packs was generally about 4°C, but the products stored for 1 day experienced an air temperature of
TABLE 4 The average Air Temperatures during the Display of G r o u n d Beef, and the Times at which the Displayed Product was Assessed as Undesirable. The G r o u n d Beef was Prepared from Coarscly G r o u n d Product Stored in Vacuum Packs at -1.5°C, or Stored in Retail Packs Master Packaged under N2, CO 2 or 02 + CO 2 (2:1 v/v) Atmospheres at 2°C Time in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
I II III IV I III II IV II IV i1
-- not assessed.
Average air temp. (°C)
6-4 3.3 2.4 4,9 4-3 4-2 4-2 2-6 3-0 3.1 4.6
Display time (h) Storage packs Vacuum
N2
C02
02 + CO2
24 48 48 48 48 4g 48 30 48 48 30
6 48 72 48 24 48 78 24 48 24 24
6 48 72 48 24 54 72 24 48 24 24
6 96 48 48 0 -------
290
C. O. Gill, 72 J o n e s
TABLE 5 T h e P r e d o m i n a n t O r g a n o l e p t i c Scores R e c o r d e d by ,a f i v e - m e m b e r P an el at the F i n a l Assessments o f Displayed G r o u n d Beef, Prepared f r o m Coarsely G r o u n d P r o d u c t Stored in V a c u u m P a c k at --1.5°C, o r Stored in R e t a i l Packs M a s t e r P a c k a g e d u n d e r N2, CO2 o r 02 + CO2 (2:1 v/v) A t m o s p h e r e s at 2 °C T i m e in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
Storage pack Vacuum
I II III IV I III II IV II IV II
N2
C02
02 + C02
RA
D
0
RA
D
0
RA
D
0
RM
D
0
3 3 3 3 3 3 3 3 3 3 3
3 4 2 2 2 3 3 3 4 3 3
1 1 2 1 2 2 2 2 2 2 2
3 3 3 3 3 3 3 2 3 2 3
5 3 3 3 5 2 2 5 5 5 4
1 1 2 2 1 2 3 3 2 4 2
2 3 3 3 3 3 3 2 3 2 3
5 4 3 3 5 2 4 5 3 5 3
1 1 2 1 1 2 2 2 1 2 1
3 3 3 3 --------
5 4 2 2 -------
1 2 2 2 -------
R A = Retail Acceptance: 1 = extremely undesirable, 2 = undesirable, 3 = slightly undesirable, 4 = ne i t h er desirable n o r undesirable, 5 = slightly desirable, 6 = desirable, 7 = e x t r e m e l y desirable; D = d i s c o l o r a t i o n : 1 = none, 2 = 1-10%, 3 = 11-25%, 4 = 26-50%, 5 = 5 1 100%; O - o f f - o d o u r intensity: 1 - none, 2 = slight, 3 - m o d e r a t e , 4 - strong. - n o t assessed. 6°C, while at other times the products experienced air temperatures of 3°C or less. The products that had been vacuum packaged were generally considered undesirable after they had been displayed for 48 h, but such products stored for 1 day were rejected after display for only 24 h. The products from the master packs were generally rejected at the same times, or after longer display than the products that had been vacuum packaged. Exceptionally, after storage for 1 day, all the master packaged products were rejected after display for only 6 h, while after 2 days storage the products from the 02 + CO2 atmosphere remained acceptable on display for twice as long as the other products ( T a b l e 4). Evaluation
of products
removed
from
display
When a product was withdrawn from display, it was generally assessed as slightly undesirable or undesirable, with <25% of the surface discoloured by dull or brown areas. Slight, stale off-odours were detected in most of the
TABLE 6 Average Values for the Chemical States of the Muscle Pigment at Ground Beef Surfaces at the Times the Product was Withdrawn from Display. The Ground Beef had been P r e p a r e d f r o m C o a r s e l y G r o u n d P r o d u c t S t o r e d in V a c u u m P a c k s at - 1 . 5 ° C , o r S t o r e d in R e t a i l P a c k s M a s t e r P a c k a g e d u n d e r N2, C O 2 o r O z + C O 2 (2:1 v / v ) A t m o s p h e r e s a t 2 ° C
Time in storage (days)
1
2 4 6 8 12 16 20 24 28 32
Batch
Storage p a c k Vacuum
I
II III IV I III II IV II IV II
N2
C02
02 + C02
% oxy
°.4 met
o~ o x y
o.4 met
°.4 o x y
% met
°/6 oxy
°/6 met
48 70 36 71 66 34 62 50 70 57 85
43 27 50 19 26 58 33 42 27 35 15
39 70 60 81 42 57 58 46 66 23 78
47 23 33 19 49 33 41 46 30 72 21
25 57 50 74 31 55 64 45 73 34 80
58 36 39 18 59 40 36 43 24 57 19
36 51 4I 67 --------
50 45 50 25 --------
oxy = oxymyoglobin, met = metmyoglobin. % d e o x y m y o g l o b i n = 100 - ( o x y + m e t ) . - not assessed.
TABLE
7
T o t a l n u m b e r s of, a n d f r a c t i o n s o f Lactic acid B a c t e r i a in, the M i c r o f l o r a o f G r o u n d Beef at the Times the Product was Withdrawn from Display- The Ground Beef had been p r e p a r e d f r o m C o a r s e l y G r o u n d P r o d u c t S t o r e d in V a c u u m P a c k s a t - 1 - 5 ° C , o r S t o r e d in R e t a i l P a c k s M a s t e r P a c k a g e d u n d e r N2, C O 2 o r 0 2 + C O 2 (2:1 v / v ) A t m o s p h e r e s a t 2 ° C
Time in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
Storage pack Vacuum
I II III IV I III II IV II IV II
N 2
CO 2
o2+
C02
Total count (log No./g)
Lac (%)
Total count (log No./g)
Lac (o~)
Total count (log No./g)
Lac (%)
Total count (log No/g)
Lac (%)
3-97 3.66 3-36 4-23 4-91 6-96 5-69 6-28 6-20 6.95 5-34
6 22 21 55 34 39 54 95 79 94 63
3-00 3-18 4-59 6-38 5-96 4-30 7-79 7-54 7-18 7.72 7-20
45 2 7 98 90 53 55 98 52 100 76
2-88 3-43 4.48 6-11 4-91 4.51 6-45 6-60 5-95 7.11 6.30
19 67 10 100 100 55 71 100 96 100 100
2-80 5.67 4.48 6.68 --------
20 2 N 99 --------
Lac - lactic acid bacteria, N = not detected. - not assessed.
292
C. O. Gill, T. Jones
products, and m o d e r a t e or strong, sour or acid odours were noted in some p r o d u c t s t h a t h a d been stored u n d e r N2 for 16 days or longer (Table 5). T h e values o b t a i n e d for the chemical states o f the muscle pigments varied widely w i t h o u t a n y obvious relationship to the type o f storage p a c k a g i n g or the time in storage. However, the fractions o f m e t m y o globin were generally larger t h a n w h e n the p r o d u c t was first displayed (Table 6). A f t e r the display o f a p r o d u c t t h a t h a d been stored for m o r e t h a n a few days, it generally carried a flora n u m b e r i n g a b o u t 106/g. O n l y on p r o d u c t s t h a t h a d been stored u n d e r N2 for 16 days o r longer d i d the flora g r o w d u r i n g display to n u m b e r s > 107/g. A t the times t h a t p r o d u c t s were w i t h d r a w n f r o m display, lactic acid bacteria p r e d o m i n a t e d in all flora f r o m p r o d u c t s t h a t h a d been stored in v a c u u m packs for 16 days or m o r e , or in a m a s t e r p a c k for 4 days or m o r e (Table 7).
DISCUSSION Considerable variation o f p r o d u c t quality can be expected between, a n d even within, batches o f g r o u n d beef (Gill and M c G i n n i s , 1993). T h e s t u d y was therefore designed to allow the c o m p a r i s o n o f p r o d u c t s f r o m m a s t e r packs with p r o d u c t s f r o m the same b a t c h t h a t was stored in v a c u u m packs at the o p t i m u m storage t e m p e r a t u r e , a n d p r e p a r e d for display only after opening o f the v a c u u m packs, to minimize deterioration o f the p r o d u c t d u r i n g storage. I n the event, a l t h o u g h the f o u r batches varied in their a p p e a r a n c e a n d initial microbiological load, the variation between batches was not so great as to obscure the general changes during storage that affected the display stability o f products. W h a t e v e r the previous storage conditions, the display life o f the g r o u n d beef was t e r m i n a t e d by the a p p e a r a n c e o f the p r o d u c t b e c o m i n g unacceptable. T h e process o f m a s t e r p a c k a g i n g increased the m e t m y o globin fraction o f the muscle pigment, a n d so compromised the a p p e a r a n c e o f the p r o d u c t by imparting dull, b r o w n tones (Hood, 1980). Consequently, p r o d u c t s r e m o v e d f r o m m a s t e r packs after only 1 d a y in store a p p e a r e d unattractive, a n d was u n a c c e p t a b l e after only a few h o u r s o f display. Such a result c o u l d be anticipated for p r o d u c t s p a c k a g e d u n d e r N2 or CO2, as m e t m y o g l o b i n f o r m s rapidly at the low o x y g e n c o n c e n t r a t i o n s t h a t are initially present in such packs ( L e d w a r d , 1970; O'Keeffe a n d H o o d , 1982). H o w e v e r , the similar d e t e r i o r a t i o n o f the p r o d u c t s stored u n d e r the oxygen-rich a t m o s p h e r e was unexpected, as o x y m y o g l o b i n is less readily oxidized to m e t m y o g l o b i n t h a n is d e o x y m y o g l o b i n (Taylor a n d M a c D o u g a l l , 1973), and the p e r i o d d u r i n g which the p r o d u c t s were
Master packaging of ground beef
293
exposed to low oxygen concentrations w o u l d seem t o o short for substantial a m o u n t s o f m e t m y o g l o b i n to h a v e f o r m e d . T h e initial d e g r a d a t i o n o f the m e a t c o l o u r h a d been reversed after the p r o d u c t h a d been stored in m a s t e r packs f o r 48 h. W i t h the residual oxygen largely stripped f r o m the N 2 a n d CO2 atmospheres, d e o x y m y o g l o b i n r e g e n e r a t e d f r o m m e t r n y o g l o b i n b y m e t m y o g l o b i n r e d u c i n g activities ( L e d w a r d , 1985) w o u l d persist in t h e p r o d u c t in the anoxic packs, while r e g e n e r a t e d d e o x y m y o g l o b i n w o u l d be o x y g e n a t e d in the o x y g e n - r i c h a t m o s p h e r e . In either case, the a p p e a r a n c e o f the m e a t after its e x p o s u r e to air was i m p r o v e d by the r e d u c e d c o n t e n t o f m e t m y o g l o b i n . As anoxic s t o r a g e was p r o l o n g e d , u n d e r v a c u u m , N 2 o r C O 2 , the m e t m y o g l o b i n c o n t e n t s o f the m e a t fell to a low, m i n i m u m v a l u e t h a t is p r e s u m a b l y d e t e r m i n e d by the m e t m y o g l o b i n reducing capacity o f the tissue (Echevarne et aL, 1990). Consequently, p r o d u c t s r e m o v e d f r o m the anoxic packagings was generally o f a highly desirable a p p e a r a n c e w h e n first displayed. In contrast, m e t m y o g l o b i n evidently r e f o r m e d in the p r o d u c t stored u n d e r the o x y g e n - r i c h a t m o s p h e r e , so t h a t the p r o d u c t s were o f u n a c c e p t a b l e a p p e a r a n c e after 8 d a y s at 2°C. M i c r o f l o r a d o m i n a t e d b y lactobacilli d e v e l o p e d in all stored p r o d u c t s . T h e g r o w t h o f those o r g a n i s m s is f a v o u r e d w h e n chilled m e a t is stored u n d e r a n a e r o b i c c o n d i t i o n s (Gill, 1986). As c o n d i t i o n s within a g r o u n d m e a t mass will be a n a e r o b i c even w h e n the surface is e x p o s e d t o oxygen, the lactobacilli p r e d o m i n a t e d in the p r o d u c t s stored u n d e r the oxygenrich a t m o s p h e r e as well as in p r o d u c t s stored in the anoxic packagings. Similarly, the lactic flora c o n t i n u e d to p r e d o m i n a t e d u r i n g the a e r o b i c display o f p r o d u c t s f r o m the anoxic packagings. A lactic flora generally does n o t i m p a r t o f f - o d o u r s to m e a t until s o m e time after it has r e a c h e d its m a x i m u m n u m b e r s o f > 107/cm 2 ( N e w t o n a n d Gill, 1978). T h e n u m b e r s o f b a c t e r i a o n d i s p l a y e d p r o d u c t s t h a t h a d been p a c k a g e d u n d e r v a c u u m , CO2 o r 0 2 + CO2 did n o t a t t a i n or barely exceed 107/cm2; so the flora was w i t h o u t o r g a n o l e p t i c effect. O n l y w h e n p r o d u c t s h a d b e e n s t o r e d u n d e r N2 f o r 16 d a y s o r l o n g e r did bacterial n u m b e r s in displayed p r o d u c t s substantially exceed 107/crn 2, a n d the p r o d u c t s develop a m a r k e d off-odour. In the absence o f a d e q u a t e m i c r o f l o r a , the slight o f f - o d o u r n o t e d after the display o f m o s t o f the p r o d u c t s c a n be a t t r i b u t e d to n o n - m i c r o b i a l , oxidative d e t e r i o r a t i o n . T h a t suggestion is s u p p o r t e d b y the d e v e l o p m e n t o f such a n o f f - o d o u r d u r i n g the s t o r a g e o f p r o d u c t s u n d e r the o x y g e n - r i c h a t m o s p h e r e . In c o m m e r c i a l practice, v a c u u m p a c k a g e d b e e f t r i m m i n g s t h a t are g r o u n d at retail outlets c a n be r o u t i n e l y m a i n t a i n e d at t e m p e r a t u r e s close to 2°C d u r i n g storage a n d t r a n s p o r t . T h e times b e t w e e n the p a c k a g i n g a n d g r i n d i n g o f such p r o d u c t s c a n a p p r o a c h 3 weeks (Gill a n d M c G i n n i s ,
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C. O. Gill, T. J o n e s
1993). In a w e l l - m a n a g e d display case, p r o d u c t s are likely to experience air t e m p e r a t u r e s b e t w e e n 2 a n d 5°C, with excursions to 10°C o r a b o v e d u r i n g defrosting cycles (Greer, 1981). R a d i a n t h e a t i n g b y the case lights c a n raise the t e m p e r a t u r e o f the p r o d u c t surface several degrees a b o v e the t e m p e r a t u r e o f the s u r r o u n d i n g air ( B o g h - S o r e n s e n a n d Olsson, 1990). Such storage a n d display c o n d i t i o n s , w h i c h w o u l d m e e t w i t h c u r r e n t industrial c o n c e p t s o f G o o d Practice in m e a t handling, were approxim a t e d in this study. T h u s , the findings s h o u l d indicate the likely stability o f g r o u n d b e e f in a w e l l - m a n a g e d c o m m e r c i a l s y s t e m for the d i s t r i b u t i o n of master packaged, retail-ready products. A reliable s y s t e m w o u l d clearly h a v e to assure t h a t n o p r o d u c t f r o m a master packaging was displayed until at least 2 days after it was packaged, to give the p r o d u c t time to r e c o v e r f r o m the t r a n s i e n t d e g r a d a t i o n o f its a p p e a r a n c e . T h e 4 or 5 d a y s r e m a i n i n g to the p r o d u c t in a n o x y g e n - r i c h a t m o s p h e r e w o u l d then seem u n c o m f o r t a b l y brief for convenient distribution a n d retailing o f the p r o d u c t . I n contrast, the a p p e a r a n c e o f g r o u n d beef master packaged under an N 2 or CO 2 atmosphere would become attractive, a n d c o u l d be e x p e c t e d to r e m a i n so for at least one 12 h retailing d a y , w h e n t h e p r o d u c t was e x p o s e d to air after 30 o r m o r e days o f storage. With such packagings, the microbiological stability o f d i s p l a y e d p r o d u c t s w o u l d limit the time for w h i c h p r o d u c t s c o u l d be reliably stored. T h e d e v e l o p m e n t o f the spoilage flora w o u l d be b e t t e r c o n t a i n e d b y a storage a t m o s p h e r e o f CO2 t h a n b y o n e o f N2 ( G r a u et al., 1985). H o w e v e r , use o f a n N2 a t m o s p h e r e w o u l d a v o i d possible p r o b l e m s with m a s t e r p a c k collapse t h a t c a n arise b e c a u s e o f t h e high solubility o f C O z in m e a t (Gill, 1988). M a s t e r p a c k a g i n g u n d e r 0 x y g e n - d e p l e t e d a t m o s p h e r e s seems t h e r e f o r e to offer a possible m e a n s o f a d e q u a t e l y preserving c e n t r a l l y - p r e p a r e d , r e t a i l - r e a d y p a c k s o f g r o u n d b e e f d u r i n g their d i s t r i b u t i o n t h r o u g h c u r r e n t c o m m e r c i a l systems.
ACKNOWLEDGEMENT W e t h a n k the A l b e r t a C a t t l e C o m m i s s i o n for the financial s u p p o r t o f this study. REFERENCES Allen, J. W. (1989). Nat. Provision., 201(6), 8. Bogh-Sorensen, L. & Olsson, P. (1990). In Chilled Foods: the S t a t e o f the Art, ed. T. R. Gormley. Elsevier Science Publishers, London, p. 245.
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C o w a n , S. T. (1974). M a n u a l f o r the Identification o f Medical Bacteria, 2nd edn. C a m b r i d g e University Press, Cambridge. E c h e var n e, C., Renerre, M. & Labas, R. (1990). M e a t Sci., 27, 161. Farris, D. E., Dietrich, R. A. & W ar d, J. B. (1991). M e a t Process., 30(2), 60, Foster, J. R., Fowler, J. L. & Ladiges, W. C. (1977). J. F ood Prot., 40, 790. Gill, C. O. (1986). In Advances in M e a t Research, VoL 2, ed. A. M. P e a r s o n & -T. R. D u t s o n . A V I Publishing Co., W e s t p o r t , CT, p. 49. Gill, C. O. (1988). M e a t Sci., 22, 65. Gill, C. O. & McGi nni s , C. (1993). Int. J. Food Microbiol. (in press). Gill, C. O. & Molin, G. (1991). In F ood Preservatives, ed. N. J. Russel & G. W. G o u l d . Blackie, Glasgow, p. 172. G r a u , F. H., Eustace, I. J. & Bill, B. A. (1985). J. Food Sci., 50, 482. G re e r , G. G. (1981). Dairy F o o d Sanitat., l, 460. H o l l a n d , G. C. (1980). Proc. 33rd M e a t Ind. Res. Conf., Chicago, IL., p. 21. H o o d , D. E. (1980). M e a t Sci., 4, 247. H u n t , M. C. (1980). Proc. 33rd M e a t Ind. Res. Conf., Chicago, IL., p. 41. L e d w a r d , D. A. (1970). J. F ood Sei., 35, 33. L e d w a r d , D. A. (1985). M e a t Sci., 15, 149. N e w t o n , K. G. & Gill, C. O. (1978). J. Appl. Bacteriol., 44, 91. O'Keeffe, M. & H o o d , D. E. (1982). Meat Sci., 7, 209. Renerre, M. (1989). Fleischwirtsch. Internat., 1989(1), 51. Shay, B. J. & Egan, A. F. (1990). F ood Australia, 42, 399. Sholtz, E. M., J o r d a a n , E., D r uger , J., Nortje, G. L. & N a u d e , R. T. (1992). M e a t Sci., 32, 11. T a y l o r , A. A. & M a c D o u g a l l , D. B. (1973). J. F o o d Technol., 8, 453. Y o u n g , L. L., Revier, A. D. & Cole, A. B. (1988). F o o d Technol., 42(9), 65.
T h e D i s p l a y o f R e t a i l P a c k s o f G r o u n d B e e f after their S t o r a g e in M a s t e r P a c k a g e s under V a r i o u s A t m o s p h e r e s C. O. Gill & T. Jones Agriculture C a n a d a Research Station, Bag Service 5000, Lacombe Alberta, C a n a d a T0C 1SO (Received 28 January 1993; r e v i s e d v e r s i o n r e c e i v e d 3 April 1993; accepted 16 April 1993)
ABSTRA CT B a t c h e s o f coarsely g r o u n d b e e f t r i m m i n g s were each divided into f o u r portions. One portion j k o m each batch was vacuum p a c k a g e d , then stored at --1-5°C. The other portions were f i n e l y g r o u n d a n d distributed in retail packs. T h e retail p a c k s were m a s t e r p a c k a g e d under a t m o s p h e r e s o f N2, COz or 02 + C O 2 (2:1, v/v), then s t o r e d at 2°C. The p r o d u c t was assessed after storage times o f up to 32 days. F o r each assessment, a vacuum p a c k a n d a m a s t e r p a c k o f each type, each containing p r o d u c t s f r o m the s a m e batch, were withdrawn f r o m storage. The vacuum p a c k a g e d p r o d u c t was f i n e l y g r o u n d a n d distributed into retail packs. The n e w l y p r e p a r e d retail p a c k s a n d those f r o m the m a s t e r p a c k s were displayed in a retail cabinet in which air t e m p e r a t u r e s a v e r a g e d 4 +- 2°C. The appearance o f the displayed p r o d u c t f r o m each storage p a c k a g i n g was assessed twice daily until it was j u d g e d to be unacceptable. A t the beginning a n d e n d o f the display, the p r o d u c t in each p a c k was assessed f o r discoloration a n d offodours, the chemical states o f the m u s c l e p i g m e n t at the e x p o s e d surface were estimated, a n d the surface microflora was characterized. The appearance o f the p r o d u c t displayed after storage in a vacuum p a c k , f o r times up to 32 days, b e c a m e unacceptable within 48 h. A p r o d u c t s t o r e d in a n y o f the m a s t e r p a c k s f o r 1 d a y appeared unacceptable after 6 h o f display. The d i s p l a y life o f p r o d u c t s s t o r e d under Nz or C 0 2 was similar to that o f the vacuum p a c k a g e d p r o d u c t s when storage times were b e t w e e n 2 a n d 24 days but the display life was shorter when the storage times were 28 or 32 days. The display life o f products stored under 0 z + COz was similar to that o f the vacuum p a c k a g e d p r o d u c t when storage times 281
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C. O. Gill, T. Jones were 2, 4 or 6 days, but the appearance o f products stored under 02 + COz .for 8 days or longer was unacceptable when master packs w e r e opened. Apart f r o m those latter packs, a product was not discoloured when storage packs were opened. However, all products were discoloured, and the metmyoglobin fractions o f the surface pigments had increased, when the product was withdrawn f r om display. The products in all storage paekagings developed flora dominated by lactic acid bacteria. The spoilage flora on products stored in vacuum p a c k or under 02 + C02 did not attain the m a x i m u m numbers o f 107/g during either storage or display. Those m a x i m u m numbers were attained on products stored under N2 and CO 2 after 16 and 28 days storage respectively. Some products stored under N2 f o r 16 days or longer developed moderate or strong off-odours during display that were ascribable to microbial action. Other products developed only slight, non-microbial off-odours during display. Retail-ready packs or ground beef master-packaged under an oxygen-depleted atmosphere could then have a useful storage life o f about 30 days in commercial circumstances.
INTRODUCTION C o n s i d e r a b l e e c o n o m i c benefits w o u l d accrue to s u p e r m a r k e t o p e r a t o r s were t h e y able to o b t a i n all their fresh m e a t as finished, r e t a i l - r e a d y items (Farris et al., 1991). T h e s t o r a g e life o f m e a t in c o n v e n t i o n a l l y overw r a p p e d trays is generally t o o s h o r t to allow retail p a c k s to be centrally p r e p a r e d a n d widely d i s t r i b u t e d to retail outlets. E x t e n s i o n o f p r o d u c t s t o r a g e life has t h e r e f o r e b e e n s o u g h t t h r o u g h the use o f p r e s e r v a t i v e packagings. T h r e e types o f p r e s e r v a t i v e p a c k a g i n g are u s e d w i t h r e t a i l - r e a d y m e a t s . T h e s e are, v a c u u m p a c k a g i n g , retail p a c k a g i n g u n d e r h i g h - o x y g e n m o d i fied a t m o s p h e r e s , a n d m a s t e r p a c k a g i n g u n d e r m o d i f i e d a t m o s p h e r e s ( Y o u n g et aL, 1988). A l t h o u g h t h e r e are c o m m e r c i a l s y s t e m s f o r t h e p r e p a r a t i o n o f retail-ready fresh m e a t s t h a t e m p l o y v a c u u m a n d / o r higho x y g e n m o d i f i e d a t m o s p h e r e retail packs to preserve the p r o d u c t , neither type o f p a c k a g i n g is satisfactory for all m e a t trades. V a c u u m p a c k a g i n g is n o t a c c e p t a b l e for m o s t retail p u r p o s e s b e c a u s e c o n s u m e r s d i s c r i m i n a t e a g a i n s t t h e dull, p u r p l e c o l o u r o f anoxic red m e a t (Allen, 1989), while m o d i f i e d a t m o s p h e r e retail p a c k s m u s t be d i s p r o p o r t i o n a l l y large c o m p a r e d to the p r o d u c t they c o n t a i n if t h e p r e s e r v a t i v e a t m o s p h e r e is to be a d e q u a t e l y m a i n t a i n e d ( H o l l a n d , 1980). E v e n in c i r c u m s t a n c e s w h e r e t h e costs associated w i t h a d d i t i o n a l p a c k a g i n g a n d l o w p r o d u c t d e n s i t y are a c c e p t a b l e ( R e n e r r e , 1989) t h e stability o f s o m e p r o d u c t s in m o d i f i e d a t m o s p h e r e retail p a c k s m a y be i n a d e q u a t e ( S h a y a n d E g a n , 1990). M a s t e r p a c k a g i n g o f retail-ready p a c k s is t h e n p e r c e i v e d as a w a y o f
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avoiding s o m e o f the c o m m e r c i a l difficulties e n c o u n t e r e d with o t h e r techniques f o r preserving centrally p r e p a r e d p r o d u c t s (Scholtz e t al., 1992). A system for the central preparation o f retail-ready beef should preferably be c a p a b l e o f a d e q u a t e l y preserving g r o u n d beef, as u p to 50% o f b e e f is retailed in t h a t f o r m ( F o s t e r et aL, 1977). Because it is largely p r e p a r e d f r o m trimmings, m u c h freshly g r o u n d b e e f is likely to c a r r y a higher microbial load, a n d have less c o l o u r stability, t h a n p o r t i o n s o f n o n c o m m i n u t e d tissue cut f r o m fresh carcasses. It is t h e n likely t h a t a p a c k a g i n g system w h i c h will a d e q u a t e l y preserve retail-ready g r o u n d b e e f w o u l d be at least equally a d e q u a t e f o r o t h e r f o r m s o f r a w beef. T o ascertain the possibilities for using m a s t e r p a c k a g i n g with beef, retailr e a d y packs o f g r o u n d b e e f were m a s t e r p a c k e d u n d e r preservative a t m o s p h e r e s o f 02 + CO2, N2 o r CO2, (Gill a n d M01in, 1991), stored, a n d displayed after r e m o v a l f r o m the m a s t e r - p a c k s , using storage a n d display c o n d i t i o n s t h a t simulate c u r r e n t c o m m e r c i a l G o o d Practice.
MATERIALS AND METHODS
Preparation and packaging of ground beef A t a local b e e f p a c k a g i n g plant, b e e f t r i m m i n g s are r o u t i n e l y collected a n d v a c u u m p a c k a g e d , in 10 kg portions, for distribution to retail outlets, w h e r e the p r o d u c t is g r o u n d . O n each o f 4 days, t h r e e freshly p r e p a r e d packages o f such products were collected f r o m the beef packaging plant, a n d stored o v e r n i g h t at - 1 . 5 ° C . E a c h 30 kg b a t c h o f b e e f t r i m m i n g s was first g r o u n d t h r o u g h a plate with holes o f 10 m m diameter. T h e coarsely g r o u n d m a t e r i a l was m i x e d by h a n d , t h e n 3 × 2 kg p o r t i o n s were v a c u u m p a c k a g e d in p o u c h s o f a polyvinylidine chloride (PVDC) laminate that h a d an oxygen transmission r a t e o f a b o u t 40 cm3/m2/24 h / a t m at 25°C a n d 1002/0 r.h. (Cryovac, Mississauga, Ontario). The remaining portion o f coarsely g r o u n d material was r e g r o u n d t h r o u g h a plate with holes o f 3-7 m m diameter. T h e finely g r o u n d m a t e r i a l was m i x e d by h a n d , t h e n distributed into 27 solid polystyrene trays. T h e t r a y dimensions were 165 × 128 × 40 m m (l × w × h). E a c h tray was filled, to a b o u t 5 m m below the edge o f the walls, with 500 g o f g r o u n d beef. E a c h filled t r a y was placed in a p o u c h o f a s h r i n k a b l e film, t h a t h a d a n t i f o g g i n g properties a n d a n o x g y e n t r a n s m i s s i o n rate o f a b o u t 11 000 cm3/m2/24 h / a t m at 25°C a n d 75% r.h. (RD-106, C r y o v a c ) . T h e p o u c h was s h r u n k to the tray, using a h o t air gun. A hole 3 m m in d i a m e t e r was m a d e in the fihn, using an electric soldering tool, at o n e u p p e r
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c o r n e r o f t h e t r a y , so t h a t t h e film c o u l d be f o r m e d as a flat lid to t h e tray. T h e filled t r a y s were d i s t r i b u t e d , in g r o u p s o f three, i n t o n i n e p o u c h e s c o m p o s e d o f a n a l u m i n i u m foil l a m i n a t e t h a t h a d n o m e a s u r a b l e o x y g e n transmission rate (American National Can Co, Chicago, IL, USA). The foil l a m i n a t e d p o u c h e s w e r e e v a c u a t e d , filled w i t h gas, a n d sealed u s i n g a m o d i f i e d c h a m b e r p l u s s n o r k e l m a c h i n e ( C a p t r o n II. R M F Inc., Grandview, M O , USA) that w o u l d reliably establish an atmosphere o f CO2 c o n t a i n i n g <200 p p m o f 02. F o r e a c h g r o u p o f n i n e p o u c h e s , t h r e e w e r e e a c h filled w i t h 4 litres o f COz, t h r e e w e r e filled w i t h 4 litres, o f N 2, a n d t h r e e w e r e filled w i t h 2 litres o f CO2. W i t h i n 5 rain o f t h a t last set o f p a c k s b e i n g filled, e a c h o f t h o s e p a c k s w a s f u r t h e r filled w i t h 4 litres o f 02. T h e O2 w a s i n t r o d u c e d t h r o u g h a h y p o d e r m i c n e e d l e c o n n e c t e d b y flexible t u b i n g t o a n o x y g e n c y l i n d e r . W i t h o x y g e n f l o w i n g f r o m t h e n e e d l e a t 1 litre/s, t h e n e e d l e w a s i n s e r t e d i n t o o n e c o r n e r o f e a c h b a g , t h r o u g h a s t i c k - o n s e p t u m , a n d r e t a i n e d in t h e b a g f o r 4 s. I m m e d i a t e l y a f t e r t h e n e e d l e w a s r e m o v e d , t h e a r e a w h i c h it h a d p e n e t r a t e d w a s sealed by m e a n s of a h o t iron. The batch identity of the g r o u n d beef and the type o f pack a t m o s p h e r e w e r e n o t e d o n e a c h p o u c h . T h e b a t c h o r i g i n o f t h e p r o d u c t w a s also noted on each v a c u u m pack.
Storage and sampling of the master-packaged ground beef T h e v a c u u m p a c k a g e d p r o d u c t w a s s t o r e d a t --1-5°C + 0.5°C. T h e p r o d u c t p a c k a g e d in foil l a m i n a t e p o u c h e s w a s s t o r e d at 2 _ 0-25°C. P r o d u c t s w e r e w i t h d r a w n f o r e x a m i n a t i o n a f t e r s t o r a g e f o r 1, 2, 4, 6 a n d 8 d a y s , a n d a t s u b s e q u e n t 4 d a y i n t e r v a l s u p t o 32 d a y s . A t e a c h s a m p l i n g time, a p o u c h o f the v a c u u m p a c k a g e d p r o d u c t a n d a m a s t e r p a c k o f e a c h a t m o s p h e r e t y p e w a s w i t h d r a w n f r o m s t o r a g e . All t h e p r o d u c t s withdrawn at each sampling time originated from the same batch of g r o u n d beef. A t each sampling time, the v a c u u m p a c k a g e d p r o d u c t was r e m o v e d first. T h a t p r o d u c t w a s finely g r o u n d a n d m i x e d , t h e n t h r e e retail p a c k s w e r e p r e p a r e d as f o r t h e m a s t e r - p a c k e d p r o d u c t . T h e t h r e e m a s t e r p a c k s w e r e t h e n w i t h d r a w n , a n d all 12 retail p a c k s w e r e p l a c e d in a d i s p l a y case.
Display and sampling of the retail packs A f t e r e a c h s t o r a g e t i m e , t h e 12 s a m p l e s w e r e p l a c e d t o o c c u p y a n a r e a o f a p p r o x i m a t e l y 600 X 6 0 0 r a m in t h e c e n t r e o f t h e d i s p l a y s h e l f o f a
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285
horizontal, fan-assisted display case (model L P M 1 2 T , Hill Refrigeration, B a r r i e , O n t a r i o , C a n a d a ) t h a t w a s i l l u m i n a t e d f o r 12 h / d a y w i t h i n c a n d e s c e n t l i g h t s g i v i n g a l i g h t i n t e n s i t y o f 750 l u x a t t h e d i s p l a y s h e l f s u r f a c e . T h e a r e a o f t h e d i s p l a y s h e l f w a s 3-5 x 0-8 m . T h e a i r i n l e t t e m p e r a t u r e o f t h e d i s p l a y case w a s set at - - 2 ° C . A t e m p e r a t u r e d a t a logger (Tru-Test, A u c k l a n d , N e w Zealand) was placed at the centre o f t h e d i s p l a y a r e a , t o m o n i t o r t h e air t e m p e r a t u r e s e x p e r i e n c e d b y t h e product. D u r i n g display of the g r o u n d meat, other areas o f the display case w e r e u s e d , t o v a r y i n g e x t e n t s , f o r t h e d i s p l a y o f o t h e r p r o d u c t s . T h e d i s p l a y e d p r o d u c t s w e r e a s s e s s e d f o r retail a c c e p t a b i l i t y a n d c o l o u r 1 h a f t e r o p e n i n g o f t h e m a s t e r p a c k s . T h e e n v e l o p i n g film w a s t h e n r e m o v e d f r o m e a c h t r a y , a n d t h e o d o u r w a s assessed. T h e n , a sterile c o r k b o r e r w a s u s e d t o o b t a i n f o u r s a m p l e s o f m e a t t h a t w e r e 20 m m in d i a m e t e r a n d 10 m m t h i c k . T w o s a m p l e s w e r e r e m o v e d f r o m o p p o s i t e edges o f the tray, for the estimation o f d e o x y m y o g l o b i n , o x y m y o g l o b i n a n d m e t m y o g l o b i n , a n d two samples were similarly r e m o v e d for microbiological analysis. T h e t r a y s w e r e r e p a c k a g e d in b a g s o f t h e s h r i n k a b l e film, a n d t h e s a m p l e d a r e a s w e r e o b s c u r e d b y strips o f w h i t e t a p e a t e a c h e n d o f e a c h t r a y . T h e p r o d u c t w a s s u b s e q u e n t l y a s s e s s e d f o r retail a c c e p t a b i l i t y a n d c o l o u r , a t a l t e r n a t i n g p e r i o d s o f 6 a n d 18 h. E a c h set o f r e t a i l p a c k s f r o m the same storage packaging was removed from display when the majority of a panel considered that two more of the packs had unaccetable appearances. T h e r e m o v e d p r o d u c t was assessed for odour, a n d sampled for estimation of the different forms of myoglobin and for microbiological a n a l y s i s .
Subjective evaluations of the ground beef All a s s e s s m e n t s w e r e m a d e b y a f i v e - m e m b e r p a n e l . R e t a i l a c c e p t a b i l i t y w a s s c o r e d o n a 7 - p o i n t scale w h e r e 1 -- e x t r e m e l y u n d e s i r a b l e , 2 :- undesirable, 3 = slightly undesirable n o r undesirable, 5 = slightly desirable, 6 = d e s i r a b l e a n d 7 = extremely desirable. P r o d u c t s were r e m o v e d f r o m d i s p l a y w h e n t h e m a j o r i t y o f t h e a s s e s s m e n t p a n e l r e c o r d e d a retail a c c e p t a n c e score o f 3 o r less f o r t w o o r t h r e e o f a set o f t h r e e retail p a c k s . D i s c o l o r a t i o n w a s assessed o n a 5-point scale w h e r e 1 -- n o d i s c o l o r a t i o n , 2 = 1 - 1 0 % d i s c o l o r a t i o n , 3 = 1 1 - 2 5 % d i s c o l o r a t i o n , 4 = 2 6 - 5 0 % discoloration and 5 = 50-100% discoloration. A note was made of the type of discoloration observed. O d o u r w a s a s s e s s e d o n a 4 - p o i n t scale w h e r e 1 = n o o f f = o d o u r , 2 = slight o f f - o d o u r , 3 = m o d e r a t e o f f - o d o u r a n d 4 = s t r o n g o f f - o d o u r . N o t e was made of the types of off-odour that were detected.
286
C. O. Gill, T. Jones
Assessment o f myoglobin forms E a c h excised sample w a s w r a p p e d in the s h r i n k a b l e film, t o c o v e r the surface t h a t h a d b e e n e x p o s e d to the p a c k a t m o s p h e r e a n d / o r air b y a single l a y e r o f film. T h e s p e c t r u m f r o m each o f t h o s e surfaces w a s o b t a i n e d using a reflectance s p e c t r o p h o t o m e t e r ( M a c b e t h C o l o u r E y e 1500/Plus; K o l l m o r g e n C o r p , N e w b u r g , N Y , U S A ) . T h e values f o r reflectance at 474, 525, 572 a n d 610 n m were c o n v e r t e d to K / S values (ratio o f the a b s o r b e n c e (K) to the scattering coefficient (S)). S t a n d a r d K / S values h a d p r e v i o u s l y b e e n o b t a i n e d f o r samples o f g r o u n d b e e f in which the muscle p i g m e n t h a d b e e n w h o l e l y c o n v e r t e d to the d e o x y , o x y or m e t form. T h e p e r c e n t a g e f r a c t i o n s o f the f o r m s o f m y o g l o b i n w e r e e s t i m a t e d f r o m the m e a s u r e d a n d s t a n d a r d K / S values ( H u n t , 1980).
Microbiological analysis T h e t w o s a m p l e s o b t a i n e d at e a c h s a m p l i n g time f r o m e a c h retail p a c k were c o m b i n e d , a n d a 5 g p o r t i o n w a s w e i g h e d into a s t o m a c h e r bag. T h e w e i g h e d s a m p l e w a s diluted w i t h 45 ml o f 0-1% p e p t o n e water, a n d w a s s t o m a c h e d with the diluent f o r 2 rain. T h e h o m o g e n a t e w a s diluted 10- a n d 100-fold, then 0-1 ml v o l u m e s o f the u n d i l u t e d h o m o g e n a t e , a n d o f e a c h d i l u t i o n were s p r e a d on d u p l i c a t e plates o f Plate C o u n t A g a r (Difco). T h e plates w e r e i n c u b a t e d f o r 3 d a y s at 25°C. F l o r a n u m b e r s a n d c o m p o s i t i o n s w e r e d e t e r m i n e d f r o m plates b e a r i n g 2 0 - 2 0 0 colonies. R e p r e s e n t a t i v e colonies o f each t y p e were p i c k e d f r o m each plate a n d identified to the generic level b y the criteria o f C o w a n (1974). T h r e e colonies o f a t y p e were p i c k e d w h e n t h a t or higher n u m b e r s w e r e available. O t h e r w i s e , all the available colonies w e r e picked.
RESULTS
Evaluation o f product removed from storage P r o d u c t s t h a t w e r e freshly g r o u n d , after s t o r a g e in v a c u u m p a c k for a n y time, were generally assessed as being desirable, n o t d i s c o l o u r e d a n d w i t h o u t o b j e c t i o n a b l e o d o u r . P r o d u c t s s t o r e d f o r 1 d a y in a n y o f the three types o f m a s t e r p a c k w e r e o f p o o r a p p e a r a n c e a n d d i s c o l o u r e d . A f t e r all longer s t o r a g e times, p r o d u c t s f r o m N2 or CO2 a t m o s p h e r e s were assessed as desirable a n d n o t d i s c o l o u r e d . S u c h p r o d u c t s h a d n o o f f - o d o u r at s t o r a g e times u p t o 24 days, b u t slight s o u r or acid o d o u r s were d e t e c t e d in p r o d u c t s s t o r e d f o r longer times. P r o d u c t s s t o r e d u n d e r
Masler packaging
TABLE
287
of ground beef
1
T h e P r e d o m i n a n t O r g a n o l e p t i c S c o r e s R e c o r d e d b y a f i v e - m e m b e r P a n e l a t t h e first A s s e s s m e n t s o f G r o u n d Beef, P r e p a r e d f r o m C o a r s e l y G r o u n d P r o d u c t S t o r e d i n V a c u u m P a c k a t -1.5°C~ o r S t o r e d i n R e t a i i P a c k s M a s t e r P a c k a g e d u n d e r N2, CO2 o r 0 2 + C O 2 (2:1 v/v) A t m o s p h e r e s a t 2 ° C T i m e in storage (days)
Balch
Storttge pack Vacuum RA
1
2 4 6 8 12 16 20 24 28 32
D
N2 0
RA
D
CO 2 0
RA
D
02 + CO 2 0
RA
D
0
I
7
1
1
4
3
1
3
4
1
5
2
1
II III IV I III II IV II IV lI
6 5 7 7 5 7 6 7 6 5
1 1 1 1 2 1 1 1 1 1
1 1 1 1 1 1 1 1 1 l
7 6 6 6 6 7 6 7 7 7
1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 2 2
6 6 7 6 6 7 6 7 7 7
1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 2 2
6 5 5 2 ---
1 1 1 5 ~--
1 1 1 3 ---
----
----
----
R A = R e t a i l A c c e p t a n c e : 1 - e x t r e m e l y u n d e s i r a b l e , 2 - u n d e s i r a b l e , 3 = slightly u n d e s i r a b l e , 4 = neither desirable nor undesirable, 5 = slightly desirable, 6 = desirable, 7 = extremely desirable; D - discoloration: 1 - none, 2 - 1-10%, 3 - 11-25%, 4 26-50%, 5 51 100%; O = o f f - o d o u r i n t e n s i t y : 1 = n o n e , 2 = slight, 3 = m o d e r a t e , 4 = s t r o n g . - n o t assessed. an Oz + CO2 atmosphere were assessed as only slightly desirable, but not discoloured, after 4 and 6 days of storage. After storage for 8 days, such products were undesirable and discoloured, with a moderate, stale odour (Table 1), When products had been stored for 1 day, the estimated fraction of metmyoglobin at the surface of products from the vacuum pack was less than the metmyoglobin fractions estimated for products from any of the master packs. At later times, the estimated fractions of metmyoglobin and oxymyoglobin were similar for products that had been stored under vacuum, N2 or CO2. All such products stored for 16 days or longer had very small fractions of metmyoglobin. Products stored under 0 2 + CO 2 for 8 days had a large fraction of metmyoglobin (Table 2). After 1 day of storage, the flora numbers in the product from all four types of packagings were about 103/g. The flora were composed of approximately equal numbers of Gram-positive mesophiles (micrococci and bacilli) and Gram-negative psychrotrophs (acinetobacteria, moraxellae and pseudomonads). Lactic acid bacteria were only a minor fraction of some flora, and were undetected in most. With the product that was
TABLE 2 Average Values for the Chemical States of the Muscle Pigment at the first Examination. o f G r o u n d Beef, P r e p a r e d f r o m C o a r s e l y G r o u n d P r o d u c t S t o r e d in V a c u u m P a c k s a t - - 1 - 5 ° C , o r S t o r e d i n R e t a i l P a c k s M a s t e r P a c k a g e d u n d e r N 2 , C O 2 o r O 2 + C O 2 (2:1 v / v ) Atmospheres at 2°C
T i m e in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
Storage p a c k Vacuum
I II llI IV I IlI II IV II IV II
N2
C0 2
02 + C0 2
% oxy
% met
o.4 o x y
% m et
% oxy
% met
o~ o xy
% met
63 69 32 66 54 46 96 95 79 84 92
24 19 34 17 13 27 5 1 6 6 5
53 75 65 66 60 81 97 86 82 85 85
31 11 17 15 I4 6 0 2 0 8 3
34 62 65 67 66 74 100 86 90 87 91
48 13 20 13 12 12 0 6 0 6 1
51 87 59 74 41 -------
35 19 34 22 47 -------
oxy = oxymyoglobin, met = metmyoglobin. % deoxymyoglobin = 100 - ( o x y + m e t ) . - not assessed.
TABLE 3 T o t a l n u m b e r s o f , a n d f r a c t i o n s o f L a c t i c a c i d B a c t e r i a in, t h e M i c r o f l o r a O b t a i n e d a t the first Examination of Ground Beef, Prepared from Coarsely Ground Product Stored i n V a c u u m P a c k s a t - 1 . 5 ° C , o r S t o r e d i n R e t a i l P a c k s M a s t e r P a c k a g e d u n d e r N 2, C O 2 o r 0 2 + C O 2 (2:1 v / v ) A t m o s p h e r e s a t 2 ° C
Time in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
Storage p a c k Vacuum
I [I III IV I lII II IV II IV II
N2
CO 2
0 2 + CO 2
Total count (log No./g)
Lac (%)
Total count (log No./g)
Lac (o~)
Total count (log No./g)
Lae (%)
Total count (log N o . / g )
Lac (%)
3-18 3.26 2.83 3.08 3-94 2.81 3-20 4.08 4-15 5.96 5-84
8 N N 5 2 12 23 94 65 99 77
3.24 3-32 2.60 3.64 4-43 2.83 5-77 6-51 6.78 7.38 7.32
N N 5 95 93 27 77 100 75 100 95
3.37 3-00 2-66 3.74 4-18 2-48 5-15 6.48 5-54 7-11 6-20
N
3-53 3.15 2.40 3-90 4-81 -------
N 2 2 98 56 ----
Lae = lactic acid bacteria, N = not detected. - not assessed.
3 N 97 47 8 66 100 100 100 100
---
Master packaging o f ground beef
289
s t o r e d in v a c u u m p a c k s , a n u n a m b i g u o u s i n c r e a s e i n f l o r a n u m b e r s w a s n o t e v i d e n t u n t i l 2 0 d a y s , w h e n t h e t o t a l c o u n t s e x c e e d e d 104/g a n d l a c t i c acid bacteria had become the dominant organisms. The numbers increased t o a b o u t 106/g a t 32 d a y s w i t h l a c t i c a c i d b a c t e r i a r e m a i n i n g p r e d o m i n a n t ( T a b l e 3). O t h e r o r g a n i s m s d e t e c t e d a t s t o r a g e t i m e s a f t e r 2 0 d a y s w e r e the mesophilic and strictly aerobic types that were predominant in the initial flora. W i t h p r o d u c t s t h a t w e r e s t o r e d in t h e m a s t e r p a c k a g i n g s , u n a m b i g u o u s growth of the flora was evident after 8 days of storage, when lactic acid bacteria predominated. With products stored under N 2 or CO2, lactic a c i d b a c t e r i a p r e d o m i n a t e d in f l o r a t h a t a t l a t e r t i m e s a t t a i n e d n u m b e r s o f 107/g. H o w e v e r , a f t e r e a c h o f t h e l o n g e r s t o r a g e p e r i o d s , t h e n u m b e r s o b t a i n e d f r o m t h e p r o d u c t s t o r e d u n d e r C O 2 w e r e s o m e w h a t less t h a n t h o s e r e c o v e r e d f r o m p r o d u c t s t o r e d u n d e r N2. M o r e o v e r , a f t e r s t o r a g e for 20 days or longer, only lactic acid bacteria were recovered from the product stored under CO2, but enterobacteria were also recovered from s o m e o f t h e p r o d u c t s t o r e d u n d e r N 2 ( T a b l e 3).
Display times and temperatures The temperature of the air around the retail packs was generally about 4°C, but the products stored for 1 day experienced an air temperature of
TABLE 4 The average Air Temperatures during the Display of G r o u n d Beef, and the Times at which the Displayed Product was Assessed as Undesirable. The G r o u n d Beef was Prepared from Coarscly G r o u n d Product Stored in Vacuum Packs at -1.5°C, or Stored in Retail Packs Master Packaged under N2, CO 2 or 02 + CO 2 (2:1 v/v) Atmospheres at 2°C Time in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
I II III IV I III II IV II IV i1
-- not assessed.
Average air temp. (°C)
6-4 3.3 2.4 4,9 4-3 4-2 4-2 2-6 3-0 3.1 4.6
Display time (h) Storage packs Vacuum
N2
C02
02 + CO2
24 48 48 48 48 4g 48 30 48 48 30
6 48 72 48 24 48 78 24 48 24 24
6 48 72 48 24 54 72 24 48 24 24
6 96 48 48 0 -------
290
C. O. Gill, 72 J o n e s
TABLE 5 T h e P r e d o m i n a n t O r g a n o l e p t i c Scores R e c o r d e d by ,a f i v e - m e m b e r P an el at the F i n a l Assessments o f Displayed G r o u n d Beef, Prepared f r o m Coarsely G r o u n d P r o d u c t Stored in V a c u u m P a c k at --1.5°C, o r Stored in R e t a i l Packs M a s t e r P a c k a g e d u n d e r N2, CO2 o r 02 + CO2 (2:1 v/v) A t m o s p h e r e s at 2 °C T i m e in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
Storage pack Vacuum
I II III IV I III II IV II IV II
N2
C02
02 + C02
RA
D
0
RA
D
0
RA
D
0
RM
D
0
3 3 3 3 3 3 3 3 3 3 3
3 4 2 2 2 3 3 3 4 3 3
1 1 2 1 2 2 2 2 2 2 2
3 3 3 3 3 3 3 2 3 2 3
5 3 3 3 5 2 2 5 5 5 4
1 1 2 2 1 2 3 3 2 4 2
2 3 3 3 3 3 3 2 3 2 3
5 4 3 3 5 2 4 5 3 5 3
1 1 2 1 1 2 2 2 1 2 1
3 3 3 3 --------
5 4 2 2 -------
1 2 2 2 -------
R A = Retail Acceptance: 1 = extremely undesirable, 2 = undesirable, 3 = slightly undesirable, 4 = ne i t h er desirable n o r undesirable, 5 = slightly desirable, 6 = desirable, 7 = e x t r e m e l y desirable; D = d i s c o l o r a t i o n : 1 = none, 2 = 1-10%, 3 = 11-25%, 4 = 26-50%, 5 = 5 1 100%; O - o f f - o d o u r intensity: 1 - none, 2 = slight, 3 - m o d e r a t e , 4 - strong. - n o t assessed. 6°C, while at other times the products experienced air temperatures of 3°C or less. The products that had been vacuum packaged were generally considered undesirable after they had been displayed for 48 h, but such products stored for 1 day were rejected after display for only 24 h. The products from the master packs were generally rejected at the same times, or after longer display than the products that had been vacuum packaged. Exceptionally, after storage for 1 day, all the master packaged products were rejected after display for only 6 h, while after 2 days storage the products from the 02 + CO2 atmosphere remained acceptable on display for twice as long as the other products ( T a b l e 4). Evaluation
of products
removed
from
display
When a product was withdrawn from display, it was generally assessed as slightly undesirable or undesirable, with <25% of the surface discoloured by dull or brown areas. Slight, stale off-odours were detected in most of the
TABLE 6 Average Values for the Chemical States of the Muscle Pigment at Ground Beef Surfaces at the Times the Product was Withdrawn from Display. The Ground Beef had been P r e p a r e d f r o m C o a r s e l y G r o u n d P r o d u c t S t o r e d in V a c u u m P a c k s at - 1 . 5 ° C , o r S t o r e d in R e t a i l P a c k s M a s t e r P a c k a g e d u n d e r N2, C O 2 o r O z + C O 2 (2:1 v / v ) A t m o s p h e r e s a t 2 ° C
Time in storage (days)
1
2 4 6 8 12 16 20 24 28 32
Batch
Storage p a c k Vacuum
I
II III IV I III II IV II IV II
N2
C02
02 + C02
% oxy
°.4 met
o~ o x y
o.4 met
°.4 o x y
% met
°/6 oxy
°/6 met
48 70 36 71 66 34 62 50 70 57 85
43 27 50 19 26 58 33 42 27 35 15
39 70 60 81 42 57 58 46 66 23 78
47 23 33 19 49 33 41 46 30 72 21
25 57 50 74 31 55 64 45 73 34 80
58 36 39 18 59 40 36 43 24 57 19
36 51 4I 67 --------
50 45 50 25 --------
oxy = oxymyoglobin, met = metmyoglobin. % d e o x y m y o g l o b i n = 100 - ( o x y + m e t ) . - not assessed.
TABLE
7
T o t a l n u m b e r s of, a n d f r a c t i o n s o f Lactic acid B a c t e r i a in, the M i c r o f l o r a o f G r o u n d Beef at the Times the Product was Withdrawn from Display- The Ground Beef had been p r e p a r e d f r o m C o a r s e l y G r o u n d P r o d u c t S t o r e d in V a c u u m P a c k s a t - 1 - 5 ° C , o r S t o r e d in R e t a i l P a c k s M a s t e r P a c k a g e d u n d e r N2, C O 2 o r 0 2 + C O 2 (2:1 v / v ) A t m o s p h e r e s a t 2 ° C
Time in storage (days)
1 2 4 6 8 12 16 20 24 28 32
Batch
Storage pack Vacuum
I II III IV I III II IV II IV II
N 2
CO 2
o2+
C02
Total count (log No./g)
Lac (%)
Total count (log No./g)
Lac (o~)
Total count (log No./g)
Lac (%)
Total count (log No/g)
Lac (%)
3-97 3.66 3-36 4-23 4-91 6-96 5-69 6-28 6-20 6.95 5-34
6 22 21 55 34 39 54 95 79 94 63
3-00 3-18 4-59 6-38 5-96 4-30 7-79 7-54 7-18 7.72 7-20
45 2 7 98 90 53 55 98 52 100 76
2-88 3-43 4.48 6-11 4-91 4.51 6-45 6-60 5-95 7.11 6.30
19 67 10 100 100 55 71 100 96 100 100
2-80 5.67 4.48 6.68 --------
20 2 N 99 --------
Lac - lactic acid bacteria, N = not detected. - not assessed.
292
C. O. Gill, T. Jones
products, and m o d e r a t e or strong, sour or acid odours were noted in some p r o d u c t s t h a t h a d been stored u n d e r N2 for 16 days or longer (Table 5). T h e values o b t a i n e d for the chemical states o f the muscle pigments varied widely w i t h o u t a n y obvious relationship to the type o f storage p a c k a g i n g or the time in storage. However, the fractions o f m e t m y o globin were generally larger t h a n w h e n the p r o d u c t was first displayed (Table 6). A f t e r the display o f a p r o d u c t t h a t h a d been stored for m o r e t h a n a few days, it generally carried a flora n u m b e r i n g a b o u t 106/g. O n l y on p r o d u c t s t h a t h a d been stored u n d e r N2 for 16 days o r longer d i d the flora g r o w d u r i n g display to n u m b e r s > 107/g. A t the times t h a t p r o d u c t s were w i t h d r a w n f r o m display, lactic acid bacteria p r e d o m i n a t e d in all flora f r o m p r o d u c t s t h a t h a d been stored in v a c u u m packs for 16 days or m o r e , or in a m a s t e r p a c k for 4 days or m o r e (Table 7).
DISCUSSION Considerable variation o f p r o d u c t quality can be expected between, a n d even within, batches o f g r o u n d beef (Gill and M c G i n n i s , 1993). T h e s t u d y was therefore designed to allow the c o m p a r i s o n o f p r o d u c t s f r o m m a s t e r packs with p r o d u c t s f r o m the same b a t c h t h a t was stored in v a c u u m packs at the o p t i m u m storage t e m p e r a t u r e , a n d p r e p a r e d for display only after opening o f the v a c u u m packs, to minimize deterioration o f the p r o d u c t d u r i n g storage. I n the event, a l t h o u g h the f o u r batches varied in their a p p e a r a n c e a n d initial microbiological load, the variation between batches was not so great as to obscure the general changes during storage that affected the display stability o f products. W h a t e v e r the previous storage conditions, the display life o f the g r o u n d beef was t e r m i n a t e d by the a p p e a r a n c e o f the p r o d u c t b e c o m i n g unacceptable. T h e process o f m a s t e r p a c k a g i n g increased the m e t m y o globin fraction o f the muscle pigment, a n d so compromised the a p p e a r a n c e o f the p r o d u c t by imparting dull, b r o w n tones (Hood, 1980). Consequently, p r o d u c t s r e m o v e d f r o m m a s t e r packs after only 1 d a y in store a p p e a r e d unattractive, a n d was u n a c c e p t a b l e after only a few h o u r s o f display. Such a result c o u l d be anticipated for p r o d u c t s p a c k a g e d u n d e r N2 or CO2, as m e t m y o g l o b i n f o r m s rapidly at the low o x y g e n c o n c e n t r a t i o n s t h a t are initially present in such packs ( L e d w a r d , 1970; O'Keeffe a n d H o o d , 1982). H o w e v e r , the similar d e t e r i o r a t i o n o f the p r o d u c t s stored u n d e r the oxygen-rich a t m o s p h e r e was unexpected, as o x y m y o g l o b i n is less readily oxidized to m e t m y o g l o b i n t h a n is d e o x y m y o g l o b i n (Taylor a n d M a c D o u g a l l , 1973), and the p e r i o d d u r i n g which the p r o d u c t s were
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exposed to low oxygen concentrations w o u l d seem t o o short for substantial a m o u n t s o f m e t m y o g l o b i n to h a v e f o r m e d . T h e initial d e g r a d a t i o n o f the m e a t c o l o u r h a d been reversed after the p r o d u c t h a d been stored in m a s t e r packs f o r 48 h. W i t h the residual oxygen largely stripped f r o m the N 2 a n d CO2 atmospheres, d e o x y m y o g l o b i n r e g e n e r a t e d f r o m m e t r n y o g l o b i n b y m e t m y o g l o b i n r e d u c i n g activities ( L e d w a r d , 1985) w o u l d persist in t h e p r o d u c t in the anoxic packs, while r e g e n e r a t e d d e o x y m y o g l o b i n w o u l d be o x y g e n a t e d in the o x y g e n - r i c h a t m o s p h e r e . In either case, the a p p e a r a n c e o f the m e a t after its e x p o s u r e to air was i m p r o v e d by the r e d u c e d c o n t e n t o f m e t m y o g l o b i n . As anoxic s t o r a g e was p r o l o n g e d , u n d e r v a c u u m , N 2 o r C O 2 , the m e t m y o g l o b i n c o n t e n t s o f the m e a t fell to a low, m i n i m u m v a l u e t h a t is p r e s u m a b l y d e t e r m i n e d by the m e t m y o g l o b i n reducing capacity o f the tissue (Echevarne et aL, 1990). Consequently, p r o d u c t s r e m o v e d f r o m the anoxic packagings was generally o f a highly desirable a p p e a r a n c e w h e n first displayed. In contrast, m e t m y o g l o b i n evidently r e f o r m e d in the p r o d u c t stored u n d e r the o x y g e n - r i c h a t m o s p h e r e , so t h a t the p r o d u c t s were o f u n a c c e p t a b l e a p p e a r a n c e after 8 d a y s at 2°C. M i c r o f l o r a d o m i n a t e d b y lactobacilli d e v e l o p e d in all stored p r o d u c t s . T h e g r o w t h o f those o r g a n i s m s is f a v o u r e d w h e n chilled m e a t is stored u n d e r a n a e r o b i c c o n d i t i o n s (Gill, 1986). As c o n d i t i o n s within a g r o u n d m e a t mass will be a n a e r o b i c even w h e n the surface is e x p o s e d t o oxygen, the lactobacilli p r e d o m i n a t e d in the p r o d u c t s stored u n d e r the oxygenrich a t m o s p h e r e as well as in p r o d u c t s stored in the anoxic packagings. Similarly, the lactic flora c o n t i n u e d to p r e d o m i n a t e d u r i n g the a e r o b i c display o f p r o d u c t s f r o m the anoxic packagings. A lactic flora generally does n o t i m p a r t o f f - o d o u r s to m e a t until s o m e time after it has r e a c h e d its m a x i m u m n u m b e r s o f > 107/cm 2 ( N e w t o n a n d Gill, 1978). T h e n u m b e r s o f b a c t e r i a o n d i s p l a y e d p r o d u c t s t h a t h a d been p a c k a g e d u n d e r v a c u u m , CO2 o r 0 2 + CO2 did n o t a t t a i n or barely exceed 107/cm2; so the flora was w i t h o u t o r g a n o l e p t i c effect. O n l y w h e n p r o d u c t s h a d b e e n s t o r e d u n d e r N2 f o r 16 d a y s o r l o n g e r did bacterial n u m b e r s in displayed p r o d u c t s substantially exceed 107/crn 2, a n d the p r o d u c t s develop a m a r k e d off-odour. In the absence o f a d e q u a t e m i c r o f l o r a , the slight o f f - o d o u r n o t e d after the display o f m o s t o f the p r o d u c t s c a n be a t t r i b u t e d to n o n - m i c r o b i a l , oxidative d e t e r i o r a t i o n . T h a t suggestion is s u p p o r t e d b y the d e v e l o p m e n t o f such a n o f f - o d o u r d u r i n g the s t o r a g e o f p r o d u c t s u n d e r the o x y g e n - r i c h a t m o s p h e r e . In c o m m e r c i a l practice, v a c u u m p a c k a g e d b e e f t r i m m i n g s t h a t are g r o u n d at retail outlets c a n be r o u t i n e l y m a i n t a i n e d at t e m p e r a t u r e s close to 2°C d u r i n g storage a n d t r a n s p o r t . T h e times b e t w e e n the p a c k a g i n g a n d g r i n d i n g o f such p r o d u c t s c a n a p p r o a c h 3 weeks (Gill a n d M c G i n n i s ,
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C. O. Gill, T. J o n e s
1993). In a w e l l - m a n a g e d display case, p r o d u c t s are likely to experience air t e m p e r a t u r e s b e t w e e n 2 a n d 5°C, with excursions to 10°C o r a b o v e d u r i n g defrosting cycles (Greer, 1981). R a d i a n t h e a t i n g b y the case lights c a n raise the t e m p e r a t u r e o f the p r o d u c t surface several degrees a b o v e the t e m p e r a t u r e o f the s u r r o u n d i n g air ( B o g h - S o r e n s e n a n d Olsson, 1990). Such storage a n d display c o n d i t i o n s , w h i c h w o u l d m e e t w i t h c u r r e n t industrial c o n c e p t s o f G o o d Practice in m e a t handling, were approxim a t e d in this study. T h u s , the findings s h o u l d indicate the likely stability o f g r o u n d b e e f in a w e l l - m a n a g e d c o m m e r c i a l s y s t e m for the d i s t r i b u t i o n of master packaged, retail-ready products. A reliable s y s t e m w o u l d clearly h a v e to assure t h a t n o p r o d u c t f r o m a master packaging was displayed until at least 2 days after it was packaged, to give the p r o d u c t time to r e c o v e r f r o m the t r a n s i e n t d e g r a d a t i o n o f its a p p e a r a n c e . T h e 4 or 5 d a y s r e m a i n i n g to the p r o d u c t in a n o x y g e n - r i c h a t m o s p h e r e w o u l d then seem u n c o m f o r t a b l y brief for convenient distribution a n d retailing o f the p r o d u c t . I n contrast, the a p p e a r a n c e o f g r o u n d beef master packaged under an N 2 or CO 2 atmosphere would become attractive, a n d c o u l d be e x p e c t e d to r e m a i n so for at least one 12 h retailing d a y , w h e n t h e p r o d u c t was e x p o s e d to air after 30 o r m o r e days o f storage. With such packagings, the microbiological stability o f d i s p l a y e d p r o d u c t s w o u l d limit the time for w h i c h p r o d u c t s c o u l d be reliably stored. T h e d e v e l o p m e n t o f the spoilage flora w o u l d be b e t t e r c o n t a i n e d b y a storage a t m o s p h e r e o f CO2 t h a n b y o n e o f N2 ( G r a u et al., 1985). H o w e v e r , use o f a n N2 a t m o s p h e r e w o u l d a v o i d possible p r o b l e m s with m a s t e r p a c k collapse t h a t c a n arise b e c a u s e o f t h e high solubility o f C O z in m e a t (Gill, 1988). M a s t e r p a c k a g i n g u n d e r 0 x y g e n - d e p l e t e d a t m o s p h e r e s seems t h e r e f o r e to offer a possible m e a n s o f a d e q u a t e l y preserving c e n t r a l l y - p r e p a r e d , r e t a i l - r e a d y p a c k s o f g r o u n d b e e f d u r i n g their d i s t r i b u t i o n t h r o u g h c u r r e n t c o m m e r c i a l systems.
ACKNOWLEDGEMENT W e t h a n k the A l b e r t a C a t t l e C o m m i s s i o n for the financial s u p p o r t o f this study. REFERENCES Allen, J. W. (1989). Nat. Provision., 201(6), 8. Bogh-Sorensen, L. & Olsson, P. (1990). In Chilled Foods: the S t a t e o f the Art, ed. T. R. Gormley. Elsevier Science Publishers, London, p. 245.
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