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The sensory and nutritional quality of ‘sous vide’ foods
Food Control. Vol. 6, No. 1. pp. 4>52. 1945 Copyright 0 1995 Elscvier Science Ltd Printed in Great Britain. All rights reserved 0956~7135/95 $lO.lM + 0.00
‘SOUS-VIDE'REVIEW
The sensory and nutritional quality of ‘sous vide’ foods Philip G. Creed The sensory quality of ‘sous vide’ foods was the main factor which brought it to international attention before the microbiological risks became prominent. Few data are available which are consistent and provide quantitative scientific evidence for this method’s undoubted gastronomic appeal. Similarly, fewer data are available to support the supposed superiority in retention of vitamins. The wide range of products, processing conditions, processing equipment, methods for assessing sensory and nutritional quality calls for a more holistic approach to research. This paper reviews the objective work done so far and raises questions which need to be answered in order to help the method gain wider acceptance. Keywords: sous vide; sensory; nutrition; quality; vitamins; research needs
INTRODUCTION Since 1984 the ‘sous vide’ method has received a large amount of coverage in the technical press with applications ranging from commercial and institutional caterers, in-flight catering and schools, to retailers and food processors. Most coverage has concentrated on the risks of food poisoning and the steps needed to ensure the safety of the process but a significant proportion has shown an almost evangelical attitude towards the improved sensory and nutritional qualities of sous vide foods compared to those produced by conventional means. The comments from some of these sources (shown in Table I) provide ample anecdotal evidence that the sous vide method produces food of a high sensory quality with the added supposition that its nutritional qualities must be equally high. However, there is, as yet, little evidence for these opinions which can be supported by objective and scientific experiment. Table I can be summarized in the statement that the sous vide method produces a food with a better flavour, Department of Service Industries, Bournemouth University, Wallisdown, Poole, Dorset, BH12 5BB, UK. Presented at the First European ‘Sous Vide’ Cooking Symposium 25-26 March 1993, Leuven, Belgium. Received 1 August 1993; revised 3 March 1994; accepted 21 March 1994
colour, texture and nutrient retention that conventionally cooked foods. It should, however, be remembered that this evidence is put forward mostly on the basis of professional judgement by those who have already made a commitment to the sous vide method in time, resources and capita1 expenditure. This paper aims to outline methods available to provide the scientific evidence, to review the scientific evidence gathered so far and to suggest some of the questions which need to be addressed by future research projects.
TECHNIQUES FOR ASSESSING THE SENSORY QUALITY OF FOOD Sensory analysis has advanced greatly in recent years and is now regarded throughout manufacturing and research as an objective science. A number of factors have contributed to this advancement, including improved training programmes for sensory assessors, expansion of the range of sensory tests available, a greater range of availability of statistical techniques for data analysis and the introduction of sophisticated computer software packages, particularly in graphical representation, which has led to improved interpretation and presentation of the results of sensory analysis.
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Quality of ‘sous vide’ foods: P.G. Creed
Table 1 Examples of the subjective and anecdotal evidence for the sensory and nutritional qualities of sous vide foods Comment
Reference
‘ . no flavour is lost into the surrounding water or steam . . .’ ‘ . . . retains all of its natural flavour, along with more of its nutritional qualities . . .’ ‘ . . . the food retains all its flavours and fresh taste . . .’ ‘ . . . all the nutrients, flavor, texture and aroma of the food are locked in . . .’ ’ . . . raw or lightly cooked food product retains almost all its color, flavor and nutrients . . .’ ’ . . . enhances flavor and aroma . . .’ ‘ . . . the integrity and taste of the food is generally considered superb . .’ ‘ . . . the best lamb they’d ever tasted . . .’ ’ . . . tasted like real food . . .’ ’ . . . the process intensifies flavour . .’ ’ . . . superiority of flavour and natural nutrient retention . . .’ ‘ . . . food that tastes like it was freshly made . . .’ ‘ . . . the flavours can’t escape . . . more taste and smell . . . the texture of the food is constant . . .’ ‘ . . . seals in flavour, juices and nutrients . . .’ ‘. “sousvide” . . . products are rightly seen by the consumer as providing enhanced convenience and
Anon., 1987a Anon., 1987b Bacon. 1990 Baird,‘1990 Bertagnoli, 1987 Ivany, 1988 Kalinowski, 1988 Levine and Rossant, 1987 Levy, 1986 Manser, 1988b Nanleton. 1991 Pet&, 1998 Pring, 1986 Raffael, 1984
improved eating quality.’
‘* . ‘* . ’. . ’. . ‘. .
. does not harm the color, texture or flavour of food . . .’ . its flavour is highly praised in haute cuisine circles . . .’ . retain flavouring . . reduce the loss of vitamins and nutrients in the cooking process . , .’ . the quality of the final products are often far superior to foods prepared in the traditional manner.’ . flavour and tenderness are enhanced . . .’
Collectively, these advances allow the sensory analyst to construct models or images of products, to relate the results of laboratory tests to acceptability ratings from consumer studies and to predict the effect of changes in a product’s sensory profile on consumer acceptability. Many techniques are available which enable the investigation of the sensory properties of products ranging from simple difference tests to complex descriptive techniques. Several books have provided outlines of how and when the various tests should be used as well as updates in the field of sensory analysis (Amerine et al., 1965; Piggott, 1988; Thomson, 1988). The objectives of these tests can be summarized as follows: using difference tests to locate and identify differences between products or dishes; applying descriptive analysis to describe products quantitatively in terms of their sensory attributes and display graphically the results in two- and threedimensional models; using preference/acceptability tests to identify and quantify consumer acceptability for products; relating results from descriptive analysis and acceptability tests to predict consumer responses to changes in products by identifying those sensory attributes contributing to the direction and magnitude of the response; applying time-intensity measurements to investigate the appearance, duration, disappearance and linger of sensory attributes thereby assessing their relative contribution to the eating experience. Difference tests would be appropriate to determine simply if a dish cooked traditionally could be distinguished from the same dish cooked using the sous vide method. They would not, however, enable the investigator to know what those differences were or their magnitude on an absolute scale. Techniques for this would involve quantitative descriptive analysis (QDA)
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Food Control 1995 Volume 6 Number 1
Richmond, 1990 Sarpa, 1988 Sellers. 1990 Sessions, 1987 Somay, 1990 Stacey, 1985
which enables more information to be gathered (Stone et al., 1974). Powers (1988) gives an outline of descriptive methods including QDA, which he defined as: ‘developing a list of descriptive terms, screening wouldbe assessors for possible membership of a panel, training judges, using sufficient replication so that the performance of the assessors, the effectiveness of descriptive terms, product differences and possible interaction effects may be isolated and evaluated by statistical analysis, and expressing the results graphically as well as numerically.’ Stone and Side1 (1985) give procedures of the first stage of screening assessors, testing their ability to rank the basic tastes, identifying odours and discriminating differences as preliminaries to selection for using QDA. The second phase of training involves explaining the purpose of the method, the human mechanisms for sensing taste, odour, colour and texture, using descriptions from a standard vocabulary and standard scoring, the conditions used for testing and how the standard vocabulary can be devised in order to construct the forms used for scoring food products. Rutledge and Hudson (1990) and Rutledge (1992) have also provided guidance. The QDA technique has been used for: beer (Mielgaard et al., 1979; Mecredy et al., 1974), wine (Vedel et al., 1972), whisky (Shortreed et al., 1979), strawberries (Shamaila et al., 1992), apple juice (Dtirr, 1979), cider and perry (Williams, 1975), and apples (Williams and Carter, 1977).
SENSORY
QUALITY
OF CHILLED
FOODS
Several reviews have covered chilled foods but there is only one specifically on the sous vide method (Leadbetter, 1989) which concentrated on the microbiological hazards but also provided a brief history of sous vide and its preceding related systems [Nacka (Bjorkman
Quality of ‘sous vide’ foods: PG. Creed
and Delphin, 1966), AGS (McGuckian, 1969) and Capkold (Daniels, 1988)], packaging, equipment and the benefits of the system. Other reviews on the use of precooked chilled foods in catering have also covered food produced by the sous vide method: Robson & Collison (1989) reviewed sensory aspects; Glew (1990) covered sensory, nutritional and microbiological aspects, somewhat mistakenly outlining the method as being in use for the last 30 years and a variant of the now obsolete Nacka system, in preventing the loss of flavour volatiles and the over-cooking of delicate products such as fish; Mason et al. (1990) reviewed the sensory aspects of chilled foods, concluding that vacuum-packing may decrease juiciness and that the potential for enhancing the eating quality of particular products was real but exaggerated by the popular and catering press. They also highlighted the problems of comparing sensory studies due to the lack of standardization in product preparation, assessors, experimental design and test design: a comprehensive chapter on the sous vide method was also included in an extensive book on the technology and management of cook-chill catering (Light and Walker, 1990). General studies on chilled foods included work by Turner et al. (1984) who studied cabbage, peas, potatoes (four types) in hospital conventional, cook-chill, cook-freeze and cook-freeze-thaw systems producing data on colour, appearance, odour, taste and texture which allowed classification of samples into quality classes for easier comparison. This last idea of quality classes from Zacharias (1980) indicated the extent of the decline in sensory qualities during chilled storage.
SENSORY
QUALITY
OF SOUS VIDE FOODS
Two pieces of work (Light et al., 1988; Schafheitle and Light 1989a) studied the microbiological and sensory quality of some chicken, fish and vegetable dishes produced by the sous vide method and were summarized by Light and Walker (1990) and Schafheitle and Light (1989b). The work by Light et al. (1988) based on earlier work by Schafheitle et al. (1986) showed that chicken a la king was acceptable up to 14 days but courgettes provencale only up to 7 days and also suggested that the possible enhanced nutritional quality of sous vide food should be examined. A consumer panel was used to discriminate between fresh and samples stored for 7, 14 and 21 days. This implicitly assumed that there was no initial variation in the raw materials used for cooking conditions between the samples freshly prepared for each comparison at the different times. The trained panel assessed several characteristics - pungency, crispness, strength of flavour etc. as appropriate for the two dishes - to give mean quality scores at different storage times. However, no statistical evidence was provided to show if the differences in scores were significant. Later work by Schafheitle and Light (1989a, 1989b)
showed that chicken ballotine remained acceptable for up to 21 days stored at 13°C that the stored product was found to be different from the freshly produced dish using the Triangle Test but little difference was found using descriptive analysis - this outcome is somewhat surprising as it is generally agreed that a trained panel used for descriptive analysis would be far more discriminating than an untrained panel used for Triangle Tests (Pierson, personal communication, 1992). It was also concluded that combination ovens do not provide even heat transfer over all parts of the oven [c.f. work by Sheard and Church (1992) on variation in oven performance] and that more research is required on processing conditions, microbiological safety and migration of substances from the packaging to the food. Again it was assumed implicitly that there was no initial variation in the raw materials used for producing the fresh controls on the five separate occasions for the Triangle Tests. However, the times that the food was held at 80°C (the chosen pasteurization temperature) varied from 8 to 23 min, having already taken from 15 to 39 min to attain this temperature. This variation in heating conditions and the possible variation in raw material could have been the reason for the mean scores for appearance, odour, juiciness, flavour, chicken and vegetable texture showing no significant difference with an increase in chilled storage time from 0 to 21 days [except for the appearance at 14 days (p = 0.033) showing a poorer aspect]. The mean scores were, however, all at the positive end of the scale used. Church (1990) provided brief details of comparative trials where a significantly large proportion of assessors could distinguish sous vide from traditionally cooked chicken ballotine, vegetable rice and dauphinoise potatoes. This was confirmed by QDA as differences in ‘chicken juiciness, filling moistness, initial flavour and aroma depth’. It was not stated if these differences were statistically significant. Smith and Fullum-Bouchard (1990) compared chicken veloute prepared in cook-chill, cook-freeze and sous vide systems with a fresh sample at 1, 3 and 6 days storage at 4°C or -14°C and found no significant differences in aroma, appearance, flavour and tenderness. Gittleson ef al. (1992) assessed the sensory qualities of commercially produced sous vide salmon packed in two types of plastic packaging. Using QDA techniques, colour, flakiness, crumbliness, fish odour and overall acceptability were measured on nine-point scales over the 100 days of storage in ice at O-&C. Overall acceptability was found to be negatively correlated with fish odour and was satisfactory up to 12 weeks of storage (p
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Quality of ‘sous vide’ foods: P.G. Creed
included measurement of acceptability using eight factors for storage up to 4 weeks (Grant, personal communication, 1992). As part of a study on the use of irradiation on sous vide products, chicken breasts were assessed for offflavour and odours by Shamsuzzaman et al. (1992). It was found that the electron beam treatment had little effect on these two factors up to 55 days of storage at 2°C whereas untreated packs had spoiled between 30 and 42 days of storage. In recent unpublished work at Bournemouth University, multi-component foods (chicken/prawn, chicken/ bacon/pepper, haddock, minced lamb) encased in pastry with sauce were vacuum-packed in trays on an automatic machine, pasteurized and stored chilled (O3°C) or frozen (-18°C) for one week. No difference was found between chilled and frozen samples on scores for the acceptability of appearance, flavour, texture and aftertaste: all scores except for the lamb being at least 4.5 on a six-point scale - a level deemed satisfactory. Informal taste panels have been conducted by Levy (1986) and Manser (1988a) on sous vide products provided by Home Rouxl. Levy commented favourably on the texture of scallop and chicken liver mousses and the flavour of brill, fish mousse and poulet chasseur but unfavourably on vegetable dishes. Manser commented favourably on the intensity of flavour of carrots and lamb and all the sensory attributes of duck breast.
RELATED METHODS Two processes directly related to the sous vide method through the process of vacuum packing followed by cooking also provide relevant sensory data. The first described by Poulsen (1978) is the production of readycooked vacuum packed potatoes pasteurized at 9% 98°C for 40 min, cooled and stored at 0-5°C. The sensory data from Triangle Tests showed no preference for ready-cooked vacuum packed potatoes when compared with those normally prepared but a marked preference over canned potatoes. The second process, known as the cook-in-bag system, has been applied mainly to meats. One study (Jones et al., 1987) included comparisons between control pork roasts wrapped in PVC, stored at -20°C then cooked conventionally and roasts vacuum packed, cooked to 70°C in a 100°C water bath then stored for 14 and 28 days at 4” or -20°C then reheated. At 28 days, the frozen stored roasts were less juicy than the control and chilled stored roasts (pO.O5) over the storage period. Commercially produced turkey breast rolls prepared by vacuum packing and then cooked to an internal temperature of 71°C were studied over 87 days storage
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Food Control 1995 Volume 6 Number 1
at 4°C. Warner-Bratzler shear values from rolls produced at one of the two factories decreased significantly (pcO.05) (Smith and Alvarez, 1988). Buck et al. (1979) found that vacuum-packed beef roasts cooked to 60°C in a water bath at 60-61”C were significantly more tender (pO.O5) either immediately or after storage for 14 to 28 days at 4°C. The only significant change was in the intensity of beef flavour of the vacuum-packed roasts which decreased (pcO.05) (Stites et al., 1989).
TECHNIQUES FOR QUANTIFYING NUTRIENT RETENTION Quantifying the nutritional content of foods has long been of interest to many areas of human medicine and, more recently, due to requirements for nutritional labelling which may eventually cover cooked and chilled meals such as sous vide produced for sale to third parties. The vitamins of interest in this case will be those which are liable to decrease during the sous vide processing method, i.e. those sensitive to heat treatment: thiamin (vitamin B,), riboflavin (vitamin Bz) and ascorbic acid (vitamin C). Losses of minerals are not caused by heat but can occur by leaching into cooking water (British Nutrition Foundation, 1987). In the case of the sous vide method, the mineral content of the fresh food would probably be maintained. Calcium and sodium, for example, are both of interest to the consumer as regards maintaining an adequate supply of the former and limiting the intake of the latter.
VITAMIN DETERMINATION The strengths and weaknesses of various methods for determining the vitamin content of foods have been discussed by Lumley (1993), Eitenmiller (1990) and Gregory (1983). These methods include chemical methods such as: thiochrome for thiamin; microbiological methods where the dose response of microorganisms is measured; the radioimmume (enzyme-linked immunosorbent assay, ELISA) method of limited use; the time-consuming biological methods based on the growth response in rats and chicks and high performance liquid chromatography (HPLC) deemed to be the most developed and advancing method. Many HPLC methods have been devised to determine the amount of vitamins and other substances
Quality of ‘sous vide’ foods: P.G. Creed
Table 2
Percentage retention of vitamins in sous-vide processed meat and fish dishes (adapted from Watier, 1988)
Vitamin
Beef
Veal
Vitamin Vitamin B1 Bz
70 100
Vitamin B6 Pantothenic acid Vitamin Biotin B12
Vitamin A
Cod
ventional, cook-chill, cook-freeze and cook-freezethaw systems, producing data on vitamins B,, B2 and
C. Hunt (1984) reviewed nutrient losses in cook-chill and cook-freeze systems and concluded that vitamin C
Lamb
Pork
Salmon
91 52
77 100
90 100
90 100
85 63
100
100
100
100
85
100
conventional systems. Bognar (1990) provided the most
100
100
100
100
96
89
87 100 -
100 100 -
100 93 -
100 100
92 95 78
72 95 67
recent data in this area producing data on the vitamin loss during chilled storage of fruit and vegetables and the influence of chilling, storage and reheating condi-
losses in cook-chill
were not large when compared
with
tions on prepared foods. in food (Saxby, 1978). A very wide range of foods have been analysed using different extraction procedures to determine various combinations of vitamins. Examples of its use have been reviewed by Polesello and Rizzolo (1986) for water-soluble vitamins and more recently by Van Niekerk (1988). Most vitamin determinations using HPLC have analysed meal components with few analyses of prepared dishes such as those provided by the sous vide method. The only two examples found using HPLC for analysis are for white sauce (Nandhasri and Suksangpleng, 1986) and turkey bologna (Tuan et al., 1987). Other determinations of vitamins in foods such as meat loaf (Dahl and Matthews, 1980), beef patties, fried fish, (Ang et al., 1978), pot roast and gravy, beans and frankfurters (Ang ef al., 1973, beef stew, chicken g la king, shrimp Newburg, peas in cream sauce (Kahn and Livingston, 1970), cod au gratin (Jonsson and Danielsson, 1981), Italian spaghetti (Khan et al., 1982), beef stew (Nicholanco and Matthews, 1978) and pork casserole in retort pouches (Uribe-Saucedo and Ryley, 1982) generally made use of chemical methods recommended by the Association of Analytical Chemists (AOAC, 1984). present
RESEARCH ON THE NUTRITIONAL QUALITY OF CHILLED FOODS Little nutritional data is available on chilled foods prepared by the sous vide method but a large amount is available on changes occurring during processing (Bender, 1987) and foods prepared in cook-chill systems of which the sous vide method can be considered a variant. Bognar (1980) studied the vitamin A, B1 and B2 content of chilled meals prepared conventionally or by cook-chill, cook-freeze or sterilization methods as affected by chilling, storage and reheating conditions. Later worked on cook-chill systems found changes in vitamin content but not in the protein, fat, carbohydrates or mineral content of chilled and pasteurized chilled meals after 10 and 28 days storage at 2°C followed by reheating (Bognar et al., 1990). Data are available for many types of products at various times: sampled raw, just after cooking, just after cooling, during chilled storage, just after reheating and at the point of service. The work of Turner et al. (1984) included studies on cabbage, peas, potatoes (four types) in hospital con-
NUTRITIONAL QUALITY OF SOUS VIDE FOODS work aimed specifically at sous vide food (Buckley, 1987b) studied the stability of vitamin C in broccoli in a simulated sous vide system, finding that leaching and oxidation were avoided. The simulation consisted of cooking in a ‘pressure steamer for 4 min’. This would provide a temperature above 100°C and so, strictly speaking, would not be considered as a sous vide process. However, the results showed 86% retention of vitamin C, compared with the raw product after 5 days chilled storage. Broccoli boiled or pressuresteamed showed retention figures of 28 and 62%, respectively, after 1 day of chilled storage, falling to 9.4 and 7.3%, respectively, after 5 days storage. The method of determining vitamin C was described earlier (Buckley, 1987a). The most extensive work has been done in France (Watier, 1988; Watier and Belliot, 1990) and summarized by Watier and Belliot (1991). They studied the retention of the B vitamins in beef bourguignon, roast veal, roast lamb, roast fillet of pork, salmon and cod (Watier and Belliot, 1990) and of vitamin C, B1 and folates in potatoes, carrots, green beans and cauliflower (Watier, 1988). Their results are summarized in Tables 2 and 3. Comparisons were made with retention values from the literature and it was concluded from this that the sous vide method preserved ‘vitamins liable to oxidation better than traditional cooking, but this advantage was removed by storage and subsequent reheating’. This work has therefore not made a direct comparison between the same food cooked traditionally and that cooked by the sous vide method but does provide a guide to the influence of the sous vide method on vitamin retention in food. The earliest
Table 3 Percentage retention of vitamins in sous-vide processed vegetables after cooking, storage for 21 days and reheating (adapted
from Watier and Belliot, 1990) Vitamin Carotene Vitamin Vitamin Vitamin Vitamin Vitamin
E C B, BZ Bh
Potatoes
Carrots
Green beans
Cauliflower
63 86 100 80
89 100 86 67 100 100
100 100 40 86 71 79
69 56 86 63
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Quality of ‘sous vide’ foods: P.G. Creed
Smith and Fullum-Bouchard (1990) determined the vitamin C content using HPLC in the spinach component of chicken veloute prepared in cook-chill, sous vide (storage at 4°C) and cook-freeze systems (storage at -14°C) over 6 days, finding a decrease over the whole period for cook-chill samples and a decrease over the last half of storage for sous vide and cookfreeze samples. As part of a study on the use of irradiation on sous vide products, the thiamin content of chicken breast was determined using HPLC (Shamsuzzaman et al., 1992). It was found that sous vide cooking produced 98% retention based on the raw value (1.07 pg/g); increasing the electron beam treatment up to 2.9 kGy decreased this value to 86% ; after 27 days storage at 2”C, these figures fell to 87% and 80%, respectively. Other information on the retention of vitamins in sous vide foods has been quoted by Bognar et al. (1990). Although presented as figures relating to asparagus, broccoli in cream sauce, broccoli and chicken B la king produced by the sous vide method, the references cited (Kraxner, 1981; Erdmann and Klein, 1982; Ezell and Wilcox, 1959) provide no relevant data. For some data, it is likely that the authors intended to refer to Kossovitsas et al. (1973), who compared the quality of prepared meals (broccoli in cream sauce, chicken B la king and cod in cream sauce) held in chilled and frozen storage. This data, was based on the ‘Nacka’ method of preparation, so cannot provide a reliable estimate for the sous vide method. Bognar et al. (1990) also quote thiamin losses of 20-30% in sous vide processed roast beef joints (Cooksey et al., 1988) and 10% for vacuumpacked chicken and broccoli (Kraxner, 1981). The usefulness of the latter data are again doubtful. Work has also been done on ready-cooked vacuum packed potatoes (Poulsen, 1978). The method follows a similar procedure to sous vide, so that the nutritional results might be relevant. The vitamin C content fell from 26.8 to 24.9 mg/lOO g, a retention figure of 93% compared to a cited figure of 62% for boiled potatoes. Vitamin B1 fell from 77.6 to 73 pg/lOO g, a retention figure of 94%. No changes in the content of protein, fat, carbohydrates and minerals in foods produced by the sous vide method would be expected but some data is available on commercially produced turkey breast rolls prepared by the cook-in-bag process by vacuum packing and then cooking to an internal temperature of 71°C and stored for 87 days at 4°C. The only significant result was that amino nitrogen, non-protein nitrogen and pH did not change (p>O.O5) over the storage period (Smith and Alvarez, 1988).
CONCLUSIONS The main point arising from the previous sections indicates a lack of consistent information on the sensory and nutritional qualities of foods produced by the sous vide method which can be related to the proces-
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Food Control 1995 Volume 6 Number 1
sing conditions of temperature and time which will produce a safe product. The information available covers a wide range of products, processing conditions, methods for assessing sensory and nutritional quality and includes data with inherent variability. The latter could be present in the product itself, the processing conditions, the skills of the researchers (not all food scientists or technologists are chefs or all chefs scientists) and the difficulties of experimental design itself. This lack of information is one possible reason for the relatively slow market penetration and take-up of the sous vide method in the UK and elsewhere. The first European Sous Vide Cooking Symposium could be the first step towards a wider cooperation between researchers working in this area of food technology. The exchange of information and collaboration to avoid duplication of effort and to ensure consistent methods for product evaluation should be encouraged, although with commercial considerations this may not always be possible. Methods for quantifying nutrient retention and sensory qualities are now well developed and feasible for dishes prepared by the sous vide method. This means that it is possible to ask some fundamental questions which will draw together in a systematic and holistic way the strands of food safety, sensory quality, nutritional quality, consumer acceptance, process and equipment development and the skills of the professional chef. These questions might be: Is a dish prepared by the sous vide method significantly better in sensory qualities and nutrient retention than the same dish prepared conventionally under optimum conditions? If so, does the heating treatment this entails pasteurize the food adequately? How would any improvement in sensory qualities and nutrient retention over the same dish prepared conventionally be modified by chilled storage and reheating? Would any improvement in sensory qualities and nutrient retention over the same dish prepared conventionally be negated by frozen storage rather than chilled storage? How would any improvement in sensory qualities and nutrient retention over the same dish prepared conventionally be modified by the method of reheating? How do different food components and meals interact with each other and the food packaging to affect the outcomes of the previous questions? These questions need to be answered with regard to those dishes most available commercially. A study of the range of products offered by seven producers of sous vide foods in the UK, Ireland, France and the USA show that chicken-based dishes closely followed by fish-based dishes and vegetables are most readily available (Creed, unpublished data, 1992). These dishes also seem to be those which might be generally regarded as part of a ‘healthy’ eating regime and so the
Quality of ‘sous vide’ foods: P.G. Creed
study of their vitamin retention would be even more appropriate. This review has aimed to highlight the gaps in knowledge
relating
to
the
sensory
and
nutritional
aspects of foods prepared by the sous vide method. It also makes some basic proposals on research areas which,
when
completed,
will
enable
method to gain a solid foundation reliable knowledge.
the
sous
vide
of scientifically
Church, LJ. (1990) An Introduction to ‘Method Sous-Vide’ Department of Hospitality Management, Leeds Polytechnic, 46 pp. Cooksey, D.K., Klein, B.P. and McKeith, F.K. (1988) Effect of packaging process on thiamine retention and other characteristics of pre-cooked vacuum packaged beef roasts. Paper No. 653 presented at IFT Annual Meeting, New Orleans, June 1988 Dahl, C.A. and Matthews, M.E. (1980) Cook/chill foodservice system with a microwave oven: thiamin content in portions of beef loaf after microwave heating. J. Food Sci. 45, 608-612 Daniels, D. (1988) Sous vide and Capkold - two approaches to cooking ‘under vacuum’. The Consultant 21 (2) Spring, 26-28 D&r, P. (1979) Development of an odour profile to describe apple juice essences. Lebensmitt.-Wissensch. Technol. 12, 23-26
ACKNOWLEDGEMENTS
EitenmiBer, R.R. (1989) Strengths and weaknesses vitamin content of food J. Food Qual. 13, 7-20
The author gratefully acknowledges the financial assistante of British Gas, Unilever Research and Home Roux1 in this work.
Erdmann, J.W. and Klein, B.P. (1982) Harvesting, processing and cooking influences on vitamin C in foods. In: Ascorbic Acid: Chemistrv. Metabolism and Uses Advances in Chemistrv Series 200, (Ed;. P.A. Seib and B.M. Tolber) American Chemical Society, pp. 499-532
of assessing
Ezell, B.D. and Wilcox, M.S. (19.59) Loss in vitamin C in fresh vegetables as related.to wilting anh temperature. J. Agric. Food
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Bertagnoli, L. (1987) Pouches gain ground in food service. Restaurants Institutions 97 (lo), 29 April, pp. 178-180, 184 Bjorkman, A. and Delphm, K.A. (1966) Sweden’s Nacka hospital food system centralizes food preparation and distribution. Cornell Hotel Restaurant Admin.
Q. 7 (3), 84-87
Bognar, A. (1980) Nutritive value of chilled meals. In: Advances in Catering Technology (Ed. G. Glew) Applied Science Publishers, London, pp. 387-408 Bognar, A. (1990) Vitamin status of chilled food. In: Processing and Quality of Foods Vol. 3 Chilled Foods - the Revolution in Freshness (Eds. P. Zeuthen et al.) Elsevier Applied Science, London, New York, pp. 3.85-3.103 Bognar, A., Bohllng, H. and Fort, H. (1990) Nutrient retention in chilled foods. In: Chilled Foods - The State of the Art (Ed. T.R. Gormley) Elsevier Applied Science, London, New York, pp. 305-336 British Nutrition Foundation (1987) Food Processing - a Nutritional Perspective. Briefing Paper No. 11. 38 pp. Buck, E.M., Hickey, A.M. and Rosenau, J. (1979) Low-temperature air oven vs a water bath for the preparation of rare beef. J. Food Sci. 44, 1602-1605, 1611 Buckley, C. (1987a) The vitamin C content of cooked broccoli. Hotel Catering Res. Centre Lab. Rep. No. 238, Huddersfield Polytechnic, 6 pp. Buckley, C. (1987b) Storage stability of vitamin C in a simulated sous-vide process. Hotel Catering Res. Centre Lab. Rep. No. 240, Huddersfield Polytechnic, 2 pp.
Restaurant Management 2 (6), 43
Jonsson, L. and Danielsson, K. (1981) Vitamin retention in foods handled in foodservice systems. Leben.smitt.-Wissensch. Technol. 14, 94-96
Kahn, L.N. and Livingston, G.E. (1970) Effect of heating methods on thiamine retention in fresh or frozen prepared foods. J. Food Sci. 35, 349-351
Kalinowski, T. (1988) Ready, set, serve. Can. Hotel Restaurant 66 (6), 24-26
Khan, M.A., Klein, B.P. and Lee, F.V. (1982) Thiamin content of freshly prepared and leftover Italian spaghetti served in a university cafeteria food service. J. Food Sci. 47, 2093-2094 Kossovitsas, C., Navar, M., Chang, C.M. and Livingston, G.E. (1973) A comparison of chilled-holding versus frozen storage on quality and wholesomeness of some prepared foods. J. Food Sci. 38, 901-902
Kraxner, U. (1981) Qualitiit und Verhalten einiger Gemiisearten nach Lagerung in unterscheidlicher relativer Luftfeuchte. Doctoral Thesis, Technical University, Munich, 141 pp. Leadhetter, S. (1989) Sous-vide - a Technology Guide British Food Manufacturing Industries Research Association, Leatherhead, UK, 22 pp. Levine, J.B. and Rossant, J. (1987) ‘Sous-vide’ cooking: haute cuisine in a pouch? Business Week 13 July, pp. 104-105
Levy, P. (1986) A taste of technology. Observer Magazine 2 March, 58-60 Light, N. and Walker, A. (1990) Cook-Chill Catering: Technology and Management Elsevier Applied Science, London, pp. 157-178 Light, N., Hudson, P., Williams, R., Barrett, J. and Schatheitle, J.M. (1988) A pilot study on the use of sous-vide vacuum cooking as a production system for high quality foods in catering. Int. J. Hospitality Management 7 (1). 21-27
Food Control 1995 Volume 6 Number 1 51
Quality of ‘sous vide’ foods: P.G. Creed
Lumley, I.D. (1993) Vitamin analysis in foods. In: The Technology of Vitamins in Food (Ed. P. Berry Ottaway) Blackie Academic & Professional, Glasgow, pp. 172-232 McCuckian, A.T. (1969) The A.G.S. food system - chilled pasteurised food. Cornell Hotel Restaurant Admin. Q. May, pp. 87-92, 99 Manser, S. (1988a) Comparing notes. Taste November, p. 16 Manser, S. (1988b) The perfect vacuum. T&e November, pp. 35-36
Sessions, P. (1987) Sensible sous vide. Hospital Caterer May/June, p. 14 Shamaila, M., Powrl, W.D. and Skura, B.J. (1992) Sensory evaluation of strawberry fruit stored under modified atmosphere packaging (MAP) by quantitative descriptive analysis. J. Food Sci. 57 (5), 1168-1172, 1184 Sh-zraman, K., Chuaqui-Offermanns, N., Lucht, L., McDnugall, T. and Borsa. J. (1992) Microbioloeical and other characteristics of chicken b&as; meat following electron-beam and sous vide treatments. J. Food Prot. 55 (7), 528-533
Mason, LX, Church, I.J., Ledward, D.A. and Parsons, A.L. (1990) Review: The sensory quality of foods produced by conventional and enhanced cook-chill methods. Int. J. Food Sci. Technol. 25. 247-259
Sheard, M. and Church, I. (1992) Sous Vide Cook-Chill Leisure and Consumer Studies, Leeds Polytechnic, Leeds, UK, 47 pp.
Mecredy, J.M., Sonnemann, J.C. and Lehmann, S.J. (1974) Sensory profiling of beer by a modified QDA method. Food technol. 28 (ll), 36-37,40-41
Shortreed, G.W., Rickard, M.L., Swan, J.S. and Burtles, S.M. (1979) The flavour terminology of Scotch whiskey. Brewer’s Guardian 11,55-62
Mielgaard, M.C., Dalglelsh, C.E. and Clapperton, J.F. (1979) Beer flavour terminology. J. Inst. Brewing 85, 38-52
Smith, D.M. and Alvarez, V.B. (1988) Stability of vacuum cook-inbag turkey breast rolls during refrigerated storage. J. Food Sci. 53
Nandhasrl, P. and Suksangpleng, S. (1986) Application of high performance liquid chromatography to determination of seven water-soluble vitamins in white sauce. J. Sci. Sot. Thailand 12, 111-118 Napleton, L. (1991) Food packaging - frozen/chilled beats ambient. Frozen Chilled Foods August, p. 3 Nicholanco, S. and Matthews, M.E. (1978) Quality of beef stew in a hospital chill food-service system. J. Am. Dietetic Assoc. 72, 31-37 Petit, R. (1990) Clean - sous vide. Restaurant Hospital. 74, (ll), 150 Plggott, J.R. (Ed.). (1988) Sensory Analysis of Foods 2nd Edn, Elsevier Applied Science, London Polesello, A. and Rizzolo, A. (1986) Applications of HPLC to the determination of water-soluble vitamins in foods (a review 19811985). J. Micronutr. Anal. 2, 153-187 Paulsen, K.J. (1978) Vacuum-packaged Ready are Ready-to-serve
Foods?
cooked potatoes. In: How (Ed. K. Paulus) S. Karger,
Base]. pp. 138-146 Powers, J.J. (1988) Current practices and application of descriptive methods. In: Sensory Analysis of Foods (Ed. J.R. Piggott) Elsevier Applied Science, London, pp. 187-266 Ring, A. (1986) Sous-vide: boon or threat? Caterer Hotelkeeper 24 July, p. 19 Raffael, M. (1984) Revolution in the kitchen. Caterer Hotelkeeper 16 August, pp. 34-35 Richmond, M. (Ed.) (1990) The Microbiological Safety of Food Part I Report of the Committee on the Microbiological Safety of Food, HMSO, London, pp. 60,68,74-76, 87,93,95 Robson, C.P. and CoUl.um, R. (1989) Pre-cooked chilled foods. Cutering Health 1, 151-163 Rutledge, K.P. and Hudson, J.M. (1990) Sensory evaluation: method for establishing and training a descriptive flavor analysis panel. Food Technol. 44 (12), 78-84 Rutledge,
K.P. (1992) Accelerated training of sensory descriptive flavour analysis panelists. Food Technol. 46 (ll), 114-118
Saxby, M. J. (1978) Applications of high-performance liquid chromatography in food analysis. In: Developments in Food Analysis Techniques - I (Ed. R.D. King) Applied Science, London, pp. 125-153 Scarpa, J. (1988) Sous vide - fresh technology. Restaurant Business 87 (4), 1 March, pp. 136, 138, 140 Schafheitle, J.M. and Light, N.D. (1989a) Technical note: sous-vide preparation and chilled storage of chicken ballotine. Int. J. Food Sci. Technol. 24, l-205
,SchaIheiBe, J.M. and Light, N.D. (1989b) Sous-vide cooking and its application to cook-chill-what does the future hold? 1. Contemp. Hospitality Management 1, 5-10 Schatheitle, J.M., Wll, R., Barrett, J., Hudson, P. and Light, N. (1986) Sous-vide: the way ahead? In: The Future of Cook-chill Technology in Catering (Ed. N. Light), Workshop at Dorset Institute of Higher Education, (now Boumemouth University), Poole, Dorset, UK, 17-18 April Sellers, J. (1990) In the bag. Foodservice Equipment Suppliers Specialist 43 (3). 44, 46, 48
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(I), 46-48, 61
Smith, D.B. and Fullum-Bouchard, L. (1990) Comparative nutritional, sensory and microbiological quality of a cooked chicken menu item produced and stored by cook/chill, cook/freeze and sous vide cook/chill methods. Poster presented at Canadian Dietetic Association Annual Conference, 7 June, 6 pp. Sornay, M. (1990) The ‘sous vide’ system in catering. In: Chilled Foods - the Ongoing Debate (Eds. T.R. Gormley and P. Zeuthen) Elsevier Applied Science, London, pp. 99-100 Stacey, C. (1985) Hilton pioneers. Caterer Hotelkeeper 7 November, pp. 67, 70 Stites, C.R., McKeith, F.K., Bechtel, P.J. and Carr, T.R. (1989) Palatability and storage characteristics of precooked beef roasts. J. Food Sci. 54 (l), 3-6 Stone, IL, Side], J., Oliver, S., Waolsey, A. and Singleton, R.C. (1974) Sensory evaluation by quantitative descriptive analysis. Food Technol. 28 (ll), 24, 26, 28, 29, 32, 34 Stone, H. and Side], J.L. (1985) Sensory Evaluation Practices Academic Press, Orlando Thomson, D.M.H. (Ed.) (1988) Food Acceptability Elsevier Applied Science, London Tuan, S., Wyatt, J. and Anglemier, A.F. (1987) The effect of erythorbic acid on the determination of ascorbic acid levels in selected foods by HPLC and spectrophotometry. J. Micronutr. Anal. 3, 211-228
Turner, M., Mottlshaw, J., Zacharlas, R. and Bognar, A. (1984) Sensory quality and nutritive value of meals prepared from fresh and pre-processed components. In: Thermal Processing and Quality of Foods (Eds. P Zeuthen et al.) Elsevier, London, pp. 371402 U&e-Saucedo, S.M. and Ryley, J. (1982) A comparative study of ascorbic acid and thiamine retention in pouch sterilization and canning. Proc. Inst. Food Sci. Technol. 15 (3), 120-128 Van Niekerk, P.J. (1988) Determination of vitamins. In: HPLC in Food Analysb (Ed. E. Macrae) 2nd Edn, Academic Press, London, pp. 133-184 Vedel, A., Charle, G., Chamey, P. and Tourmeau, J. (1972) B-l. Le Vin - dt$nition B-2. Caracteres Organoleptiques Institut National des Appellations d’origine des Vins et Eaux-de-vie, Macon, France, cited by Powers, J.J. (1988) Watler, B. (1988) L’incidence des nouveaux proc6dCs sur les teneurs en vitamines des aliments. Inform. Di&&ique (3), 33-38 Watier, B. and Belliot, J.P. (1990) Aspect vitaminiques des produits de la IV’ gamme et de la V’ gamme. Inform. DiUtique (l), 42-48 Watier, B. and BeUiot, J.P. (1991) Vitamines et technologie industrielle rbcente. Cahiers Nutr. Di&&ique 26 (l), 23-26 Williams, A.A. (1975) The development of a vocabulary and profile assessment method for evaluating flavour contribution of cider and perry aroma constituents. J. Sci. Food Agric. 26,567-582 WlBiams, A.A. and Carter, C.S. (1977) A language and procedure for the sensory assessment of Cox’s orange pippin apples. J. Sci. Food Agric. 28, 1090-1104 Zacharlas, R. (1980) Chilled meals: sensory quality. In: Advances in Catering Technology (Ed. G. Glew) Applied Science, London, pp. 409-416
‘SOUS-VIDE'REVIEW
The sensory and nutritional quality of ‘sous vide’ foods Philip G. Creed The sensory quality of ‘sous vide’ foods was the main factor which brought it to international attention before the microbiological risks became prominent. Few data are available which are consistent and provide quantitative scientific evidence for this method’s undoubted gastronomic appeal. Similarly, fewer data are available to support the supposed superiority in retention of vitamins. The wide range of products, processing conditions, processing equipment, methods for assessing sensory and nutritional quality calls for a more holistic approach to research. This paper reviews the objective work done so far and raises questions which need to be answered in order to help the method gain wider acceptance. Keywords: sous vide; sensory; nutrition; quality; vitamins; research needs
INTRODUCTION Since 1984 the ‘sous vide’ method has received a large amount of coverage in the technical press with applications ranging from commercial and institutional caterers, in-flight catering and schools, to retailers and food processors. Most coverage has concentrated on the risks of food poisoning and the steps needed to ensure the safety of the process but a significant proportion has shown an almost evangelical attitude towards the improved sensory and nutritional qualities of sous vide foods compared to those produced by conventional means. The comments from some of these sources (shown in Table I) provide ample anecdotal evidence that the sous vide method produces food of a high sensory quality with the added supposition that its nutritional qualities must be equally high. However, there is, as yet, little evidence for these opinions which can be supported by objective and scientific experiment. Table I can be summarized in the statement that the sous vide method produces a food with a better flavour, Department of Service Industries, Bournemouth University, Wallisdown, Poole, Dorset, BH12 5BB, UK. Presented at the First European ‘Sous Vide’ Cooking Symposium 25-26 March 1993, Leuven, Belgium. Received 1 August 1993; revised 3 March 1994; accepted 21 March 1994
colour, texture and nutrient retention that conventionally cooked foods. It should, however, be remembered that this evidence is put forward mostly on the basis of professional judgement by those who have already made a commitment to the sous vide method in time, resources and capita1 expenditure. This paper aims to outline methods available to provide the scientific evidence, to review the scientific evidence gathered so far and to suggest some of the questions which need to be addressed by future research projects.
TECHNIQUES FOR ASSESSING THE SENSORY QUALITY OF FOOD Sensory analysis has advanced greatly in recent years and is now regarded throughout manufacturing and research as an objective science. A number of factors have contributed to this advancement, including improved training programmes for sensory assessors, expansion of the range of sensory tests available, a greater range of availability of statistical techniques for data analysis and the introduction of sophisticated computer software packages, particularly in graphical representation, which has led to improved interpretation and presentation of the results of sensory analysis.
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Quality of ‘sous vide’ foods: P.G. Creed
Table 1 Examples of the subjective and anecdotal evidence for the sensory and nutritional qualities of sous vide foods Comment
Reference
‘ . no flavour is lost into the surrounding water or steam . . .’ ‘ . . . retains all of its natural flavour, along with more of its nutritional qualities . . .’ ‘ . . . the food retains all its flavours and fresh taste . . .’ ‘ . . . all the nutrients, flavor, texture and aroma of the food are locked in . . .’ ’ . . . raw or lightly cooked food product retains almost all its color, flavor and nutrients . . .’ ’ . . . enhances flavor and aroma . . .’ ‘ . . . the integrity and taste of the food is generally considered superb . .’ ‘ . . . the best lamb they’d ever tasted . . .’ ’ . . . tasted like real food . . .’ ’ . . . the process intensifies flavour . .’ ’ . . . superiority of flavour and natural nutrient retention . . .’ ‘ . . . food that tastes like it was freshly made . . .’ ‘ . . . the flavours can’t escape . . . more taste and smell . . . the texture of the food is constant . . .’ ‘ . . . seals in flavour, juices and nutrients . . .’ ‘. “sousvide” . . . products are rightly seen by the consumer as providing enhanced convenience and
Anon., 1987a Anon., 1987b Bacon. 1990 Baird,‘1990 Bertagnoli, 1987 Ivany, 1988 Kalinowski, 1988 Levine and Rossant, 1987 Levy, 1986 Manser, 1988b Nanleton. 1991 Pet&, 1998 Pring, 1986 Raffael, 1984
improved eating quality.’
‘* . ‘* . ’. . ’. . ‘. .
. does not harm the color, texture or flavour of food . . .’ . its flavour is highly praised in haute cuisine circles . . .’ . retain flavouring . . reduce the loss of vitamins and nutrients in the cooking process . , .’ . the quality of the final products are often far superior to foods prepared in the traditional manner.’ . flavour and tenderness are enhanced . . .’
Collectively, these advances allow the sensory analyst to construct models or images of products, to relate the results of laboratory tests to acceptability ratings from consumer studies and to predict the effect of changes in a product’s sensory profile on consumer acceptability. Many techniques are available which enable the investigation of the sensory properties of products ranging from simple difference tests to complex descriptive techniques. Several books have provided outlines of how and when the various tests should be used as well as updates in the field of sensory analysis (Amerine et al., 1965; Piggott, 1988; Thomson, 1988). The objectives of these tests can be summarized as follows: using difference tests to locate and identify differences between products or dishes; applying descriptive analysis to describe products quantitatively in terms of their sensory attributes and display graphically the results in two- and threedimensional models; using preference/acceptability tests to identify and quantify consumer acceptability for products; relating results from descriptive analysis and acceptability tests to predict consumer responses to changes in products by identifying those sensory attributes contributing to the direction and magnitude of the response; applying time-intensity measurements to investigate the appearance, duration, disappearance and linger of sensory attributes thereby assessing their relative contribution to the eating experience. Difference tests would be appropriate to determine simply if a dish cooked traditionally could be distinguished from the same dish cooked using the sous vide method. They would not, however, enable the investigator to know what those differences were or their magnitude on an absolute scale. Techniques for this would involve quantitative descriptive analysis (QDA)
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Richmond, 1990 Sarpa, 1988 Sellers. 1990 Sessions, 1987 Somay, 1990 Stacey, 1985
which enables more information to be gathered (Stone et al., 1974). Powers (1988) gives an outline of descriptive methods including QDA, which he defined as: ‘developing a list of descriptive terms, screening wouldbe assessors for possible membership of a panel, training judges, using sufficient replication so that the performance of the assessors, the effectiveness of descriptive terms, product differences and possible interaction effects may be isolated and evaluated by statistical analysis, and expressing the results graphically as well as numerically.’ Stone and Side1 (1985) give procedures of the first stage of screening assessors, testing their ability to rank the basic tastes, identifying odours and discriminating differences as preliminaries to selection for using QDA. The second phase of training involves explaining the purpose of the method, the human mechanisms for sensing taste, odour, colour and texture, using descriptions from a standard vocabulary and standard scoring, the conditions used for testing and how the standard vocabulary can be devised in order to construct the forms used for scoring food products. Rutledge and Hudson (1990) and Rutledge (1992) have also provided guidance. The QDA technique has been used for: beer (Mielgaard et al., 1979; Mecredy et al., 1974), wine (Vedel et al., 1972), whisky (Shortreed et al., 1979), strawberries (Shamaila et al., 1992), apple juice (Dtirr, 1979), cider and perry (Williams, 1975), and apples (Williams and Carter, 1977).
SENSORY
QUALITY
OF CHILLED
FOODS
Several reviews have covered chilled foods but there is only one specifically on the sous vide method (Leadbetter, 1989) which concentrated on the microbiological hazards but also provided a brief history of sous vide and its preceding related systems [Nacka (Bjorkman
Quality of ‘sous vide’ foods: PG. Creed
and Delphin, 1966), AGS (McGuckian, 1969) and Capkold (Daniels, 1988)], packaging, equipment and the benefits of the system. Other reviews on the use of precooked chilled foods in catering have also covered food produced by the sous vide method: Robson & Collison (1989) reviewed sensory aspects; Glew (1990) covered sensory, nutritional and microbiological aspects, somewhat mistakenly outlining the method as being in use for the last 30 years and a variant of the now obsolete Nacka system, in preventing the loss of flavour volatiles and the over-cooking of delicate products such as fish; Mason et al. (1990) reviewed the sensory aspects of chilled foods, concluding that vacuum-packing may decrease juiciness and that the potential for enhancing the eating quality of particular products was real but exaggerated by the popular and catering press. They also highlighted the problems of comparing sensory studies due to the lack of standardization in product preparation, assessors, experimental design and test design: a comprehensive chapter on the sous vide method was also included in an extensive book on the technology and management of cook-chill catering (Light and Walker, 1990). General studies on chilled foods included work by Turner et al. (1984) who studied cabbage, peas, potatoes (four types) in hospital conventional, cook-chill, cook-freeze and cook-freeze-thaw systems producing data on colour, appearance, odour, taste and texture which allowed classification of samples into quality classes for easier comparison. This last idea of quality classes from Zacharias (1980) indicated the extent of the decline in sensory qualities during chilled storage.
SENSORY
QUALITY
OF SOUS VIDE FOODS
Two pieces of work (Light et al., 1988; Schafheitle and Light 1989a) studied the microbiological and sensory quality of some chicken, fish and vegetable dishes produced by the sous vide method and were summarized by Light and Walker (1990) and Schafheitle and Light (1989b). The work by Light et al. (1988) based on earlier work by Schafheitle et al. (1986) showed that chicken a la king was acceptable up to 14 days but courgettes provencale only up to 7 days and also suggested that the possible enhanced nutritional quality of sous vide food should be examined. A consumer panel was used to discriminate between fresh and samples stored for 7, 14 and 21 days. This implicitly assumed that there was no initial variation in the raw materials used for cooking conditions between the samples freshly prepared for each comparison at the different times. The trained panel assessed several characteristics - pungency, crispness, strength of flavour etc. as appropriate for the two dishes - to give mean quality scores at different storage times. However, no statistical evidence was provided to show if the differences in scores were significant. Later work by Schafheitle and Light (1989a, 1989b)
showed that chicken ballotine remained acceptable for up to 21 days stored at 13°C that the stored product was found to be different from the freshly produced dish using the Triangle Test but little difference was found using descriptive analysis - this outcome is somewhat surprising as it is generally agreed that a trained panel used for descriptive analysis would be far more discriminating than an untrained panel used for Triangle Tests (Pierson, personal communication, 1992). It was also concluded that combination ovens do not provide even heat transfer over all parts of the oven [c.f. work by Sheard and Church (1992) on variation in oven performance] and that more research is required on processing conditions, microbiological safety and migration of substances from the packaging to the food. Again it was assumed implicitly that there was no initial variation in the raw materials used for producing the fresh controls on the five separate occasions for the Triangle Tests. However, the times that the food was held at 80°C (the chosen pasteurization temperature) varied from 8 to 23 min, having already taken from 15 to 39 min to attain this temperature. This variation in heating conditions and the possible variation in raw material could have been the reason for the mean scores for appearance, odour, juiciness, flavour, chicken and vegetable texture showing no significant difference with an increase in chilled storage time from 0 to 21 days [except for the appearance at 14 days (p = 0.033) showing a poorer aspect]. The mean scores were, however, all at the positive end of the scale used. Church (1990) provided brief details of comparative trials where a significantly large proportion of assessors could distinguish sous vide from traditionally cooked chicken ballotine, vegetable rice and dauphinoise potatoes. This was confirmed by QDA as differences in ‘chicken juiciness, filling moistness, initial flavour and aroma depth’. It was not stated if these differences were statistically significant. Smith and Fullum-Bouchard (1990) compared chicken veloute prepared in cook-chill, cook-freeze and sous vide systems with a fresh sample at 1, 3 and 6 days storage at 4°C or -14°C and found no significant differences in aroma, appearance, flavour and tenderness. Gittleson ef al. (1992) assessed the sensory qualities of commercially produced sous vide salmon packed in two types of plastic packaging. Using QDA techniques, colour, flakiness, crumbliness, fish odour and overall acceptability were measured on nine-point scales over the 100 days of storage in ice at O-&C. Overall acceptability was found to be negatively correlated with fish odour and was satisfactory up to 12 weeks of storage (p
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Quality of ‘sous vide’ foods: P.G. Creed
included measurement of acceptability using eight factors for storage up to 4 weeks (Grant, personal communication, 1992). As part of a study on the use of irradiation on sous vide products, chicken breasts were assessed for offflavour and odours by Shamsuzzaman et al. (1992). It was found that the electron beam treatment had little effect on these two factors up to 55 days of storage at 2°C whereas untreated packs had spoiled between 30 and 42 days of storage. In recent unpublished work at Bournemouth University, multi-component foods (chicken/prawn, chicken/ bacon/pepper, haddock, minced lamb) encased in pastry with sauce were vacuum-packed in trays on an automatic machine, pasteurized and stored chilled (O3°C) or frozen (-18°C) for one week. No difference was found between chilled and frozen samples on scores for the acceptability of appearance, flavour, texture and aftertaste: all scores except for the lamb being at least 4.5 on a six-point scale - a level deemed satisfactory. Informal taste panels have been conducted by Levy (1986) and Manser (1988a) on sous vide products provided by Home Rouxl. Levy commented favourably on the texture of scallop and chicken liver mousses and the flavour of brill, fish mousse and poulet chasseur but unfavourably on vegetable dishes. Manser commented favourably on the intensity of flavour of carrots and lamb and all the sensory attributes of duck breast.
RELATED METHODS Two processes directly related to the sous vide method through the process of vacuum packing followed by cooking also provide relevant sensory data. The first described by Poulsen (1978) is the production of readycooked vacuum packed potatoes pasteurized at 9% 98°C for 40 min, cooled and stored at 0-5°C. The sensory data from Triangle Tests showed no preference for ready-cooked vacuum packed potatoes when compared with those normally prepared but a marked preference over canned potatoes. The second process, known as the cook-in-bag system, has been applied mainly to meats. One study (Jones et al., 1987) included comparisons between control pork roasts wrapped in PVC, stored at -20°C then cooked conventionally and roasts vacuum packed, cooked to 70°C in a 100°C water bath then stored for 14 and 28 days at 4” or -20°C then reheated. At 28 days, the frozen stored roasts were less juicy than the control and chilled stored roasts (p
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Food Control 1995 Volume 6 Number 1
at 4°C. Warner-Bratzler shear values from rolls produced at one of the two factories decreased significantly (pcO.05) (Smith and Alvarez, 1988). Buck et al. (1979) found that vacuum-packed beef roasts cooked to 60°C in a water bath at 60-61”C were significantly more tender (p
TECHNIQUES FOR QUANTIFYING NUTRIENT RETENTION Quantifying the nutritional content of foods has long been of interest to many areas of human medicine and, more recently, due to requirements for nutritional labelling which may eventually cover cooked and chilled meals such as sous vide produced for sale to third parties. The vitamins of interest in this case will be those which are liable to decrease during the sous vide processing method, i.e. those sensitive to heat treatment: thiamin (vitamin B,), riboflavin (vitamin Bz) and ascorbic acid (vitamin C). Losses of minerals are not caused by heat but can occur by leaching into cooking water (British Nutrition Foundation, 1987). In the case of the sous vide method, the mineral content of the fresh food would probably be maintained. Calcium and sodium, for example, are both of interest to the consumer as regards maintaining an adequate supply of the former and limiting the intake of the latter.
VITAMIN DETERMINATION The strengths and weaknesses of various methods for determining the vitamin content of foods have been discussed by Lumley (1993), Eitenmiller (1990) and Gregory (1983). These methods include chemical methods such as: thiochrome for thiamin; microbiological methods where the dose response of microorganisms is measured; the radioimmume (enzyme-linked immunosorbent assay, ELISA) method of limited use; the time-consuming biological methods based on the growth response in rats and chicks and high performance liquid chromatography (HPLC) deemed to be the most developed and advancing method. Many HPLC methods have been devised to determine the amount of vitamins and other substances
Quality of ‘sous vide’ foods: P.G. Creed
Table 2
Percentage retention of vitamins in sous-vide processed meat and fish dishes (adapted from Watier, 1988)
Vitamin
Beef
Veal
Vitamin Vitamin B1 Bz
70 100
Vitamin B6 Pantothenic acid Vitamin Biotin B12
Vitamin A
Cod
ventional, cook-chill, cook-freeze and cook-freezethaw systems, producing data on vitamins B,, B2 and
C. Hunt (1984) reviewed nutrient losses in cook-chill and cook-freeze systems and concluded that vitamin C
Lamb
Pork
Salmon
91 52
77 100
90 100
90 100
85 63
100
100
100
100
85
100
conventional systems. Bognar (1990) provided the most
100
100
100
100
96
89
87 100 -
100 100 -
100 93 -
100 100
92 95 78
72 95 67
recent data in this area producing data on the vitamin loss during chilled storage of fruit and vegetables and the influence of chilling, storage and reheating condi-
losses in cook-chill
were not large when compared
with
tions on prepared foods. in food (Saxby, 1978). A very wide range of foods have been analysed using different extraction procedures to determine various combinations of vitamins. Examples of its use have been reviewed by Polesello and Rizzolo (1986) for water-soluble vitamins and more recently by Van Niekerk (1988). Most vitamin determinations using HPLC have analysed meal components with few analyses of prepared dishes such as those provided by the sous vide method. The only two examples found using HPLC for analysis are for white sauce (Nandhasri and Suksangpleng, 1986) and turkey bologna (Tuan et al., 1987). Other determinations of vitamins in foods such as meat loaf (Dahl and Matthews, 1980), beef patties, fried fish, (Ang et al., 1978), pot roast and gravy, beans and frankfurters (Ang ef al., 1973, beef stew, chicken g la king, shrimp Newburg, peas in cream sauce (Kahn and Livingston, 1970), cod au gratin (Jonsson and Danielsson, 1981), Italian spaghetti (Khan et al., 1982), beef stew (Nicholanco and Matthews, 1978) and pork casserole in retort pouches (Uribe-Saucedo and Ryley, 1982) generally made use of chemical methods recommended by the Association of Analytical Chemists (AOAC, 1984). present
RESEARCH ON THE NUTRITIONAL QUALITY OF CHILLED FOODS Little nutritional data is available on chilled foods prepared by the sous vide method but a large amount is available on changes occurring during processing (Bender, 1987) and foods prepared in cook-chill systems of which the sous vide method can be considered a variant. Bognar (1980) studied the vitamin A, B1 and B2 content of chilled meals prepared conventionally or by cook-chill, cook-freeze or sterilization methods as affected by chilling, storage and reheating conditions. Later worked on cook-chill systems found changes in vitamin content but not in the protein, fat, carbohydrates or mineral content of chilled and pasteurized chilled meals after 10 and 28 days storage at 2°C followed by reheating (Bognar et al., 1990). Data are available for many types of products at various times: sampled raw, just after cooking, just after cooling, during chilled storage, just after reheating and at the point of service. The work of Turner et al. (1984) included studies on cabbage, peas, potatoes (four types) in hospital con-
NUTRITIONAL QUALITY OF SOUS VIDE FOODS work aimed specifically at sous vide food (Buckley, 1987b) studied the stability of vitamin C in broccoli in a simulated sous vide system, finding that leaching and oxidation were avoided. The simulation consisted of cooking in a ‘pressure steamer for 4 min’. This would provide a temperature above 100°C and so, strictly speaking, would not be considered as a sous vide process. However, the results showed 86% retention of vitamin C, compared with the raw product after 5 days chilled storage. Broccoli boiled or pressuresteamed showed retention figures of 28 and 62%, respectively, after 1 day of chilled storage, falling to 9.4 and 7.3%, respectively, after 5 days storage. The method of determining vitamin C was described earlier (Buckley, 1987a). The most extensive work has been done in France (Watier, 1988; Watier and Belliot, 1990) and summarized by Watier and Belliot (1991). They studied the retention of the B vitamins in beef bourguignon, roast veal, roast lamb, roast fillet of pork, salmon and cod (Watier and Belliot, 1990) and of vitamin C, B1 and folates in potatoes, carrots, green beans and cauliflower (Watier, 1988). Their results are summarized in Tables 2 and 3. Comparisons were made with retention values from the literature and it was concluded from this that the sous vide method preserved ‘vitamins liable to oxidation better than traditional cooking, but this advantage was removed by storage and subsequent reheating’. This work has therefore not made a direct comparison between the same food cooked traditionally and that cooked by the sous vide method but does provide a guide to the influence of the sous vide method on vitamin retention in food. The earliest
Table 3 Percentage retention of vitamins in sous-vide processed vegetables after cooking, storage for 21 days and reheating (adapted
from Watier and Belliot, 1990) Vitamin Carotene Vitamin Vitamin Vitamin Vitamin Vitamin
E C B, BZ Bh
Potatoes
Carrots
Green beans
Cauliflower
63 86 100 80
89 100 86 67 100 100
100 100 40 86 71 79
69 56 86 63
Food Control 1995 Volume 6 Number 1
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Quality of ‘sous vide’ foods: P.G. Creed
Smith and Fullum-Bouchard (1990) determined the vitamin C content using HPLC in the spinach component of chicken veloute prepared in cook-chill, sous vide (storage at 4°C) and cook-freeze systems (storage at -14°C) over 6 days, finding a decrease over the whole period for cook-chill samples and a decrease over the last half of storage for sous vide and cookfreeze samples. As part of a study on the use of irradiation on sous vide products, the thiamin content of chicken breast was determined using HPLC (Shamsuzzaman et al., 1992). It was found that sous vide cooking produced 98% retention based on the raw value (1.07 pg/g); increasing the electron beam treatment up to 2.9 kGy decreased this value to 86% ; after 27 days storage at 2”C, these figures fell to 87% and 80%, respectively. Other information on the retention of vitamins in sous vide foods has been quoted by Bognar et al. (1990). Although presented as figures relating to asparagus, broccoli in cream sauce, broccoli and chicken B la king produced by the sous vide method, the references cited (Kraxner, 1981; Erdmann and Klein, 1982; Ezell and Wilcox, 1959) provide no relevant data. For some data, it is likely that the authors intended to refer to Kossovitsas et al. (1973), who compared the quality of prepared meals (broccoli in cream sauce, chicken B la king and cod in cream sauce) held in chilled and frozen storage. This data, was based on the ‘Nacka’ method of preparation, so cannot provide a reliable estimate for the sous vide method. Bognar et al. (1990) also quote thiamin losses of 20-30% in sous vide processed roast beef joints (Cooksey et al., 1988) and 10% for vacuumpacked chicken and broccoli (Kraxner, 1981). The usefulness of the latter data are again doubtful. Work has also been done on ready-cooked vacuum packed potatoes (Poulsen, 1978). The method follows a similar procedure to sous vide, so that the nutritional results might be relevant. The vitamin C content fell from 26.8 to 24.9 mg/lOO g, a retention figure of 93% compared to a cited figure of 62% for boiled potatoes. Vitamin B1 fell from 77.6 to 73 pg/lOO g, a retention figure of 94%. No changes in the content of protein, fat, carbohydrates and minerals in foods produced by the sous vide method would be expected but some data is available on commercially produced turkey breast rolls prepared by the cook-in-bag process by vacuum packing and then cooking to an internal temperature of 71°C and stored for 87 days at 4°C. The only significant result was that amino nitrogen, non-protein nitrogen and pH did not change (p>O.O5) over the storage period (Smith and Alvarez, 1988).
CONCLUSIONS The main point arising from the previous sections indicates a lack of consistent information on the sensory and nutritional qualities of foods produced by the sous vide method which can be related to the proces-
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Food Control 1995 Volume 6 Number 1
sing conditions of temperature and time which will produce a safe product. The information available covers a wide range of products, processing conditions, methods for assessing sensory and nutritional quality and includes data with inherent variability. The latter could be present in the product itself, the processing conditions, the skills of the researchers (not all food scientists or technologists are chefs or all chefs scientists) and the difficulties of experimental design itself. This lack of information is one possible reason for the relatively slow market penetration and take-up of the sous vide method in the UK and elsewhere. The first European Sous Vide Cooking Symposium could be the first step towards a wider cooperation between researchers working in this area of food technology. The exchange of information and collaboration to avoid duplication of effort and to ensure consistent methods for product evaluation should be encouraged, although with commercial considerations this may not always be possible. Methods for quantifying nutrient retention and sensory qualities are now well developed and feasible for dishes prepared by the sous vide method. This means that it is possible to ask some fundamental questions which will draw together in a systematic and holistic way the strands of food safety, sensory quality, nutritional quality, consumer acceptance, process and equipment development and the skills of the professional chef. These questions might be: Is a dish prepared by the sous vide method significantly better in sensory qualities and nutrient retention than the same dish prepared conventionally under optimum conditions? If so, does the heating treatment this entails pasteurize the food adequately? How would any improvement in sensory qualities and nutrient retention over the same dish prepared conventionally be modified by chilled storage and reheating? Would any improvement in sensory qualities and nutrient retention over the same dish prepared conventionally be negated by frozen storage rather than chilled storage? How would any improvement in sensory qualities and nutrient retention over the same dish prepared conventionally be modified by the method of reheating? How do different food components and meals interact with each other and the food packaging to affect the outcomes of the previous questions? These questions need to be answered with regard to those dishes most available commercially. A study of the range of products offered by seven producers of sous vide foods in the UK, Ireland, France and the USA show that chicken-based dishes closely followed by fish-based dishes and vegetables are most readily available (Creed, unpublished data, 1992). These dishes also seem to be those which might be generally regarded as part of a ‘healthy’ eating regime and so the
Quality of ‘sous vide’ foods: P.G. Creed
study of their vitamin retention would be even more appropriate. This review has aimed to highlight the gaps in knowledge
relating
to
the
sensory
and
nutritional
aspects of foods prepared by the sous vide method. It also makes some basic proposals on research areas which,
when
completed,
will
enable
method to gain a solid foundation reliable knowledge.
the
sous
vide
of scientifically
Church, LJ. (1990) An Introduction to ‘Method Sous-Vide’ Department of Hospitality Management, Leeds Polytechnic, 46 pp. Cooksey, D.K., Klein, B.P. and McKeith, F.K. (1988) Effect of packaging process on thiamine retention and other characteristics of pre-cooked vacuum packaged beef roasts. Paper No. 653 presented at IFT Annual Meeting, New Orleans, June 1988 Dahl, C.A. and Matthews, M.E. (1980) Cook/chill foodservice system with a microwave oven: thiamin content in portions of beef loaf after microwave heating. J. Food Sci. 45, 608-612 Daniels, D. (1988) Sous vide and Capkold - two approaches to cooking ‘under vacuum’. The Consultant 21 (2) Spring, 26-28 D&r, P. (1979) Development of an odour profile to describe apple juice essences. Lebensmitt.-Wissensch. Technol. 12, 23-26
ACKNOWLEDGEMENTS
EitenmiBer, R.R. (1989) Strengths and weaknesses vitamin content of food J. Food Qual. 13, 7-20
The author gratefully acknowledges the financial assistante of British Gas, Unilever Research and Home Roux1 in this work.
Erdmann, J.W. and Klein, B.P. (1982) Harvesting, processing and cooking influences on vitamin C in foods. In: Ascorbic Acid: Chemistrv. Metabolism and Uses Advances in Chemistrv Series 200, (Ed;. P.A. Seib and B.M. Tolber) American Chemical Society, pp. 499-532
of assessing
Ezell, B.D. and Wilcox, M.S. (19.59) Loss in vitamin C in fresh vegetables as related.to wilting anh temperature. J. Agric. Food
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