J Sci Food Agric 1990,53, 101-10$

Kinetics of the Thermal Destruction of Thiamine in the White Flesh of Rainbow Trout (Sulmo guirdneri)

Suparno,* A J Rosenthall and S W Hansong

Department of Food and Fisheries, Humberside Polytechnic, Nuns Corner, Grimsby DN34 SBQ, UK

(Received 13 October 1988; revised version received 16 August 1989; accepted 9 March 1990)

ABSTRACT

Thin slices of white flesh j iom rainbow trout (Salmo gairdneri Richardson) were given diflerent time-temperature treatments and the D and z values for total thiamine were determined. The z value (26°C) was in good agreement with previously reported data but the D values were considerably lower than published results. Thiamine appears to be less thermally stable in the fish muscle than was previously thought. This has serious repercussions when optimising thermal processing with respect to nutrient retention.

Key words: Thiamine, D-value, z-value, salted boiled fish, in-vivo nutrient retention.

INTRODUCI'ION

Of all the nutrients in food, thiamine has been most widely studied in relation to heat treatment. Consequently, the pathways of thermal degradation of thiamine are largely known (Dwivedi and Arnold 1973). Farrer (1955) reviewed the thermal degradation of thiamine in various foods. Numerous factors have been shown to influence the rate of destruction, including temperature, pH, buffer system and electrolyte concentration. Below pH 4.0 thiamine is very stable even at high temperatures but, if the pH is slightly alkaline, then considerable losses may occur even on moderate heating.

* Present address: Research Institute of Fish Technology, Jakarta, Indonesia. $ Present address, to which correspondence concerning this paper should be sent: Oxford Polytechnic, Gipsy Lane, Oxford OX3 OBP, UK. 8 Present address: Superior Software Ltd, Skinner Lane, Leeds LS7 IAX, UK.

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J Sci Food Agric 0022-5142/90/S03.50 0 1990 SCI. Printed in Great Britain

102 S u p a m o , A J Rosenthul, S W Hanson

Thiamine is relatively abundant in flesh foods such as fish. A popular product in South East Asia is salted boiled fish. An important consideration in its manufacture is maximum retention of its nutritional value. Thiamine is not evenly distributed throughout the musculature of fish; it is more abundant in the more physiological active dark flesh.

Suparno (1988) carried out extensive investigations into the nature of the thiamine present in rainbow trout and changes in it during thermal processing. He showed that both free thiamine and thiamine pyrophosphate are present in the flesh. In its uncooked state thiamine pyrophosphate predominates but on heating the concentration falls rapidly, with an initial increase in free thiamine content. Extended heating induces further losses in the pyrophosphate ester and losses in the free thiamine.

Historically, microbial safety has been the primary concern in the thermal preservation of foods. More recently, loss of nutrients and changes in the sensory properties have become increasingly used in determining process conditions. The general method proposed to determine microbial destruction during thermal processing (Ball and Olson 1957) has been widely adopted for both nutrients and organoleptic changes (Lund 1975). A comprehensive account of this general method is given by Stumbo (1973). Fundamental to this general method is a knowledge of the rate of isothermal destruction of the microorganism being considered (ie the decimal reduction time or D value) and the relationship between the rate of isothermal destruction and temperature ( z value). Armed with this information and a knowledge of the likely original number of microorganisms (No) , one can accurately predict the number ( N ) after a specified process.

For an isothermal process:

N = N o x (1)

where t is the heating time (minutes). The relationship between D values at different temperatures (T, and T,) is:

(2)

For convenience it is common practice to compare the rates of destruction with a standard reference temperature (T,) whose D value is known. For microbial spores the reference temperature of 121°C is commonly adopted. Equation (2) thus becomes :

D,/D, = lO(r~-rz)/z

(3) L = 1@Ti-Tr)/z

where L (the lethality) is the proportion of the decimal reduction time at the reference temperature, equivalent to each minute at temperature TI.

For any particular process the lethalities can be determined at each temperature and integrated with time to give the equivalent destruction at the reference temperature and hence, using eqn (l), the resulting number of microorganisms.

As far as thiamine degradation reactions are concerned, it is generally assumed that first order kinetics apply (Mauri et a1 1989) ie

C / C , = e-k' (4)

Thermal destruction of thiamine in rainbow trout 103

where C and C , are the concentration of the nutrient at time t and time zero respectively, and k is the reaction rate constant.

Clearly there is a similarity between eqns (1) and (4), the term k being analogous to the inverse of the D value.

The influence of temperature on the rate of a chemical reaction is given by the Arrhenius equation:

(5)

where ko is a frequency constant and E is the activation energy. Clearly a plot of log, k versus reciprocal temperature will yield a straight line

whose slope (- E / R ) could be considered analogous to the z-value. Despite their different origins, the terms D and z value have been extended from

microorganism destruction to rates of destruction of chemical species. It is obvious that, unless the D and z values for the component being investigated are accurately known, Ball and Olson’s general method is useless. Since thermal destruction is usually endothermic, the D and z values are dependent on the rate at which heat can flow through the material in question and hence must be determined individually for materials of different thermal properties. Furthermore, like microorganisms and nutrients, food materials are often denatured by heating and consequently their thermal properties may change with their temperature/time history.

k = ko . e( -E IRT)

MATERIALS AND METHODS

Rainbow trout (Salmo gairdneri Richardson) of similar size and weight (300-350 g and approximately 3 cm wide at the centre of the body) were obtained from a local trout farm. In the experiments to determine the D and z values of thiamine in the fish muscle, the fish were cut along the base of the abdominal cavity to remove the gut and internal organs. As is the practice during the manufacture of salted, boiled fish in South East Asia, the whole fish were immersed in 250 g litre- NaCl for 2 h, drained, covered with a plastic sheet and left overnight in a cold store at 3-5°C. Using a sharp knife and taking care to avoid the dark meat, fillets 3 mm x 40 mm x 60 mm were excised and placed in a 70 mm x 100 mm retortable pouch. The air was evacuated and the pouch was carefully sealed. These sealed pouches were placed in a single layer in a wire basket and heated in a laboratory retort for various times at various temperatures. After the allotted process times the steam was turned off. The retort was then gradually depressurised to atmospheric pressure over a period of 5 min whereupon the pouches were immediately removed from the retort and allowed to cool at room temperature. To compensate for thiamine destruction during the come-up and shutdown periods, a sample was brought to the processing temperature and immediately cooled. This sample was designated zero time process and its thiamine content was used as the initial thiamine concentration.

Experiments were carried out in triplicate. Total thiamine was determined as follows. The contents of the pouch were homogenised and 2-3 g was accurately weighed and transferred to a test tube. Hydrochloric acid (25 ml 0.1 M) was added and the sample was digested in a boiling water bath for 30 min. After cooling, the

104 Suparno, A J Rosenthal, S W Hanson

pH waschecked and, iffound to be greater than 4.0, the solution was discarded. The solution was made up to 100 ml with distilled water, 75 ml was taken and its pH was adjusted to 4.5-5.0 by the addition of 15 g litre-' sodium acetate solution. To the solution was added 5ml of a lOOg litre-' fungal amylase solution (Miles Laboratory Inc, Leverkusen, FRG) and it was incubated in a water bath between 37 and 40°C overnight. To six 15-ml centrifuge tubes, each containing about 0.5 g of KCl, 2 ml of digest was added, followed by 1 ml of 3.75 M sodium hydroxide solution. The tubes were shaken briefly to mix the contents. Into the first threetubes two drops of 10 g litre-' potassium ferricyanide solution were added. These tubes were gently rotated to mix the contents. Isobutanol(5 ml) was added to each of the tubes which were then shaken well to dissolve the thiamine in the solvent. The tubes were centrifuged at lOOxg for 5min and the isobutanol was removed for fluorimetric analysis in a Shimadzu SP540 spectrofluorimeter, with an excitation wavelength of 370 nm and an emission wavelength of 426 nm. The blank readings (tubes not treated with ferricyanide) were subtracted from the test readings and the concentration of thiamine was determined from a standard curve. All reagents used were prepared as described by the AOAC (1980).

RESULTS AND DISCUSSION

To compensate for natural variation in the thiamine concentrations between fish the results have been expressed as a percentage remaining after heating. The zero time process was carried out to compensate for thiamine destruction during the initial heating and post-process cooling. Thus the zero time process involved heating the samples to the process temperature and then immediately cooling them. The thiamine content of all zero time samples, regardless of process temperature, was treated as 100%.

The mean percentage thiamine retained for the various temperature-time histories and the best line fit for each isothermal process at various times are shown in Fig 1. The slopes of these lines may be converted to D values by extrapolating to 10% retained (ie 90% lost) and taking the corresponding time.

Figure 2 is a plot of the logarithm of the D value versus the corresponding temperature. The slope of this line, 26°C per log cycle, is the z value for the thermal destruction of thiamine in rainbow trout white fish muscle. This is in good agreement with published data for other foods (eg Lund 1975). However, the D values are much lower than those previously reported. A D value at 121-1°C of 76 min is obtained, contrasting with literature values ranging from 1 15 to 254 min at the same temperature.

While it is recognised that both free thiamine and thiamine pyrophosphate are present in the white muscle of rainbow trout, it is known that the pyrophosphate ester is less heat stable, being converted to free thiamine (Farrer 1955; Suparno 1988). Furthermore, since both free thiamine and thiamine pyrophosphate are biologically active, for practical purposes it is the total thiamine that is of interest nutritionally.

The presence of organic matter has previously been shown to exhibit a protective

Thermal destruction of thiamine in rainbow trout 105

- 104°C ---o--- 10Q'C

115'c 12 1°C - 127°C

.... * ....

0 50 100 150 200 250

5 1.8 -;:;I , , , , ,y 1.6 1.5

100 105 110 115 120 125 130 Time (minutes) Temperature ('C)

Fig 1. Thiamine degradation at various Fig 2. Thermal destruction time for thiamine. temperatures.

effect on some components during heating (eg bacterial endospores). On the other hand, natural materials may contain interfering agents which contribute to the rate of component loss (eg thiaminase enzymes naturally present in the flesh of the fish). At first sight, model systems appear to be an attractive solution to these problems for determining the kinetics of nutrient breakdown. They exhibit consistent thermal properties, contain fixed initial concentrations of a known thiamine species and are devoid of interfering agents. However, the results they produce do not necessarily resemble what happens in real systems. It is clear from this work that a lower D value is obtained in fish muscle than in other systems reported, ie thiamine is destroyed more rapidly than would be expected from previous work.

Most of the published work on the thermal degradation of thiamine has been carried out in model systems such as buffer solutions. The concentrations of thiamine used have not always reflected realistic concentrations present in foods. Farrer (1955) reported slower rates of thiamine degradation for solutions in excess of 10 pg ml-' than in the less concentrated solutions which are found in most food systems. Moreover, the rate of thiamine destruction at different temperatures depends on the form of thiamine present. For example, thiamine hydrochloride, which has been the subject of several investigations, is considerably more stable than thiamine mononitrate or thiamine pyrophosphate (Farrer 1955; Labuza and Kamman 1982).

Since the chief purpose of knowing the D and z values of nutrients is to optimise processing to achieve their maximum retention, the results obtained in this study are significant. It is necessary to know the actual D and z values for thiamine in real foodstuffs in order to gain maximum nutrition from processed flesh foods, primary sources of thiamine in the diets of many developing nations.

CONCLUSION

An accurate knowledge of D and z values for nutrients in each food being processed is essential to achieve the maximum nutrient retention. D and z values obtained for one nutrient in one particular food are not necessarily the same as for that same

106 Supamo, A J Rosenthal, S W Hanson

nutrient in anotherrThis situation is partly attributable to the existence of different chemical entities all possessing the same potential physiological activity but differing in the rates at which they decompose during processing. The D and z values of total thiamine in the white flesh of rainbow trout have been determined during steam cooking: they are 76 min (at 121°C) and 26°C respectively.

ACKNOWLEDGEMENTS

The authors would like to thank Winrock International for funding and Mr D Whiteley for preparing the artwork. Much of the work was carried out at Humberside Polytechnic.

REFERENCES

AOAC 1980 Official Methods of Analysis (13th edn), ed Horowitz W. Association of Official

Ball C 0, Olson F C W 1957 Sterilization in Food Technology. McGraw Hill, New York. Dwivedi B K, Arnold R G 1973 Chemistry of thiamine degradation in food products and

model systems: a review. J Agric Food Chem 21 54-60. Farrer K T H 1955 The thermal destruction of vitamin B, in foods. In: Advances in Food

Research, 6 , eds Mrak E K & Stewart G F. Academic Press, New York, pp 257-311. Labuza T P, Kamman J F 1982 Comparison of the stability of thiamine salts at high

temperature and water activity. J Food Sci 47 664-665. Lund D B 1975 Heat processing. In: Principles of Food Science Part I I Physical Properties of

Food Preservation, ed Fennema 0 R, Karel M & Lund D B. Marcel Dekker, New York,

Mauri L M, Alzamora S M, Chirife J and Tomio M J 1989 Review: kinetic parameters for thiamine degradation in foods and model solutions of high water activity. In t J Food Sci Techno1 24 (1) 1-10.

Stumbo C R 1973 Therrnobacteriology in Food Processing (2nd edn). Academic Press, New York.

Suparno 1988 Studies on improved methods for processing salted-boiled fish and changes in nutrients and quality during thermal processing. PhD dissertation. Council for National Academic Awards, UK.

Analytical Chemists, Washington, DC.

pp 31-92.