12 Consumer handling Since the mid-1980s there has been a considerable increase in legislation defining maximum temperatures during the production, distribution and retailing of chilled food. However, as soon as the food is purchased by the consumer, it is outside of any of these legislative requirements. Increasingly food poisoning incidents have been found to be due to mishandling of food in the home with insufficient refrigeration or cooling being the most fre- quent factor causing disease (WHO, 1992). Of the 1562 cases of food poi- soning reported during 1986–1988, 970 (62%) were caused in the home. Consumer handling of products may not be as intended or envisaged by the manufacturer. Many chilled products are purchased on the basis of the ‘fresh image’, but then frozen at home (Brown, 1992). After a chilled or frozen product is removed from a retail display cabinet it is outside a refrigerated environment whilst it is carried around the store and then transported home for further storage. In the home it may be left in ambient conditions or stored in the refrigerator/freezer until required. There are few published data on consumers’ attitudes to chilled food and their handling procedures in the home.The majority of the data quoted here have been obtained from a survey of 252 households which was funded by the Ministry of Agriculture Fisheries and Food (MAFF) in the UK (Evans et al., 1991). As part of the survey, participants were asked questions to assess their attitude to food poisoning, shopping habits and the length of time they stored chilled foods in the home. Monitoring was then carried out to determine the length of time and temperature foods were stored at in the home. These data were augmented with experimental data from labo- ratory studies on the performance of refrigerators and temperature changes during transportation to the home. 12.1 Consumer attitudes to food poisoning In the survey consumers were initially asked about their concern about food poisoning. The greatest number of participants (56.7%) were either only slightly concerned or not at all concerned about food poisoning. However, 31.7% of participants were concerned or very concerned about food poi- soning (answers were restricted to concern about food from shops and did not include concern about food poisoning due to restaurant or fast food- type meals or food). When asked to name foods that they considered might constitute a food poisoning risk most of the respondents (73%) considered poultry to be a problem. Raw poultry was considered to be a greater risk than cooked poultry. Meat was also considered likely to cause food poisoning with 66.7% of participants mentioning either raw or cooked meat as a potential problem (Fig. 12.1). 12.2 Shopping habits and transport from retail store to the home The frequency of shopping governs the length of time chilled food is stored in the home. Most consumers, 99.2% of the survey population, shopped on at least one day a week and few (16.3%) less than twice a week for chilled food. The greatest number (33.7%) shopped for food 3–4 days per week, closely followed by 26.2% who shopped 5–7 days per week and 23.8% who shopped on two days. Generally shopping was divided into trips for large quantities (defined as greater than one bag) and small amounts of food (less than one bag).The majority of households (84.5%) shopped for small quan- tities of chilled food on a variable basis, as required. 252 Meat refrigeration Bacon Sausages Pies Cook-chill foods Paté Meat – raw Poultry – cooked Meat – cooked Poultry – raw 0 1020304050607080901010 Number of occurrences Fig. 12.1 Meat products considered a food poisoning risk (source: Evans et al., 1991). Most participants in the survey carried out their main shopping between 1 and 5 miles from their homes and few householders travelled more than 5 miles to shop. Most people (85.3%) used a car to transport their main shopping home. Small quantities of food were generally bought close to the home, reflecting the availability of shops in the towns surveyed. Most house- holders (87.6%) who bought small quantities of food transported it home either on foot or by car. Unprotected chilled food will warm up during transportation. Survey results showed that consumers took on average 43 min to bring meat, fish or dairy items home from the shops and place them in a refrigerator. The greatest number of items were transported home and placed in a refriger- ator within 13 min. Although most people bought food home well within 60 min there were a number of items which took far longer to be bought home (up to 2 days) and placed in a refrigerator. Although insulated bags and boxes are widely sold, only a small per- centage of consumers (12.7%) used them to transport some of their food home. The vast majority (87.3%) of people did not use any means of pro- tecting food from temperature gain during transportation. Increases in product temperatures during transportation can be consid- erable. In investigations, the temperatures of 19 different types of chilled product (including a variety of meat products) were monitored during a simulated journey from the supermarket to home (James and Evans, 1992a). One sample of each product was placed in a precooled insulated box containing eutectic ice packs and the second left loose in the boot of the car. The car was then driven home and the product removed and placed in a domestic refrigerator after a total journey time of 1 h. Additional investigations looked at 9 types of frozen product, including frozen chicken, meat pie, lasagne and pizza (Evans, 1994). Products were purchased and transported to the Research Centre where the products were tempered and temperature sensors inserted into the geometric centre of each food and where possible a second sensor inserted just below the surface of the sample. The products were then refrozen to a temperature of ca. -25 °C and then transferred to a car where the above procedure was repeated.After a journey time of 1 h the products were placed in the freezer section of a domestic refrigerator. The ambient temperature during both journeys ranged from 23 to 27 °C. Initial product temperatures of the chilled meats measured when the food reached the car ranged from 4 °C to over 20 °C (Table 12.1). Some of the meat product temperatures in samples placed in the boot rose to around 30 °C during the 1 h car journey whilst most of the samples placed in the insulated box cooled during the car journey except for a few at the top of the box which remained at their initial temperature. Product temperatures in the frozen foods were close to -25°C when placed in the car. Temperatures of products placed in both the cold box and at ambient temperature rose during the 1 h journey. Temperatures of Consumer handling 253 chickens and meat pies placed at ambient temperature reached tempera- tures approaching 10°C. Frozen meat products in the cold box kept below -10 °C for the period of the journey. Thin sliced chilled products showed the highest temperature changes during transport, whereas temperature gains in thicker products such as chicken and paté (Fig. 12.2) were smaller. A similar trend was seen with the frozen products. After being placed in the domestic refrigerator, ‘warm’ chilled products required ca. 5 h before the temperature at the surface was reduced below 7 °C. ‘Warm’ frozen products placed in a domestic freezer required at least 5 h to reduce centre product temperatures to below -15 °C. Predictions made using a mathematical model that calculated bacterial growth from temperature/time relationships indicated that increases of up to 1.8 generations in bacterial numbers (Table 12.2) could occur in the chilled foods during this transport and domestic cooling phase. The model assumed that bacteria required a time to acclimatise to the change in tem- perature (the lag phase) and that no acclimatisation had occurred during 254 Meat refrigeration Table 12.1 Maximum temperatures (°C) measured in meat products after being transported for 1 h in the boot of a car without protection or within a cooled insulated container Product Unprotected Cool box Minced beef 18 9 Sausage (raw) 28 15 Smoked ham 30 14 Beef pie 24 7 Sausage roll 28 12 Lasagne 21 6 Source: Evans et al., 1991. 0 60 120 180 240 300 360 Time (min) 0 5 10 15 20 25 30 T emper ature ( ° C) Ambient pate centre Ambient pate surface Cool box pate centre Cool box pate surface ? ? ? ? ′ ′ ′ ′ Fig. 12.2 Temperature changes in paté during domestic transportation (source: Evans et al., 1991). display. If this rather optimistic assumption was not made then up to 4.2 doublings of pseudomonas and growth of both salmonella and listeria were predicted. Only very small increases in bacterial numbers (<0.4 genera- tions) were predicted in products transported in the insulated box, owing to the maintenance of lower product temperatures. Although it was unable to prevent bacterial growth, the cold box did ensure that bacterial growth was minimal and was substantially less than if transported in ambient conditions. 12.3 Refrigerated storage in the home The length of time consumers store chilled foods after purchase will affect their safety. In the survey consumers thought that the majority of meat and meat products (raw meat, cooked meat, raw poultry, cooked poultry, pre- pared meals, pizza/quiche, cold pies and sausages) would store well for 2 days. However, a number of people considered that these foods could be stored for more than 7 days and sometimes as long as 30 days. Most par- ticipants thought that products such as bacon and paté could be stored for up to a week, although a few people considered that storage of up to 30 days was acceptable. Fresh fish was generally considered to store less well, with most participants stating that they would only store fish for 1 day or less. The range in anticipated storage life for different food types varied con- siderably. Opinions on the storage lives of individual foods ranged from 0.5 to 7 days (range 6.5 days) for cold pies and sausages to between 0.5 to 30 days (range 29.5 days) for paté and bacon. The minimum storage life for all meats and meat products was either a quarter or half a day.A small number of householders thought that they could store chilled foods for periods of up to 30 days. Bacon and paté were both thought to be acceptable after this Consumer handling 255 Table 12.2 Maximum temperature measured and increase in bacterial numbers in chilled foods during 1 h in car followed by 5 h in a domestic refrigerator Product Conditions Maximum Pseudomonas Clostridium temperature (generations) (generations) (°C) Paté Ambient, car 25 1.5 0.4 Cool box, car 13 <0.4 0.0 Chicken – raw Ambient, car 24 1.6 0.2 Cool box, car 4 0.0 0.0 Chicken – Ambient, car 28 1.8 0.7 cooked Cool box, car 12 0.0 0.0 Prawns Ambient, car 37 1.3 1.6 Cool box, car 14 0.0 0.0 Source: Evans et al., 1991. period by a small proportion of participants. Cooked meat and poultry were also thought to store for up to 21 days by a few householders (Fig. 12.3). It was interesting to note that although poultry and meat were con- sidered a likely cause of food poisoning, participants did not necessarily consider that these foods had short storage lives. It is therefore possible that people do not associate storage time as being related to any food poison- ing problem. Consumers do not always ‘practice what they preach’. When the food stored in consumers’ refrigerators was examined, actual storage times were generally greater than storage times stated in the questionnaire. Almost 67% of the food was kept for longer periods. Actual storage times were greater than the stated storage time for all meat, fish and dairy items except pies which were thought to have an acceptable storage life of 3.3 days and were stored for 3.2 days (Table 12.3). 12.4 Temperatures in domestic food storage The refrigerator is a common household device and very few households in the UK do not own a refrigerator or fridge-freezer for storage of chilled foods. Fridge-freezers have become increasingly popular in the last 20 years in the UK and now provide almost 50% of the market (Anon, 1990). These 256 Meat refrigeration 02468101214161820224262830 Prepared meal Pizza/quiche Pie Pate Sausage Bacon Burger Poultry – cooked Poultry – raw Meat – cooked Meat – raw Days Maximum Mode Minimum ?′ Fig. 12.3 Minimum, modal and maximum storage lives of chilled meat and meat products stated by householders (source: Evans et al., 1991). figures were almost replicated in the survey where only 3 of the households did not own a working refrigerator and 49.4% owned a fridge-freezer. Almost 32% owned an ice box type refrigerator and the rest larder refrigerators. The temperature at which a refrigerator operates is critical for the safe storage of chilled food. Recommendations concerning the microbiological safety of foods advise that maximum temperatures in domestic refrigera- tors should not exceed 5 °C (Richmond, 1991). Consumers in the survey were therefore asked what temperature they tried to operate their refrigerator. Nearly all participants were unable to name actual temperatures and gave answers based on the method they used to set the temperature dial (Fig. 12.4). A large number of people (32.8%) set their refrigerators according to the weather, setting the refrigerator to a lower temperature (higher setting) in the summer. It was interesting to note that although 38 participants had a thermometer in their refrigerator only 30 actually used the information to set their refrigerator temperature. To evaluate temperatures within each refrigerator, a miniature data logger with 3 air and 2 product sensors was placed into the refrigerator to monitor temperatures every 8 s and to record mean temperatures every 5 min for a period in excess of 7 days. Air temperature sensors were posi- tioned in the top, middle and bottom sections of the refrigerator and a sim- ulated food product (87 mm diameter by 28 mm high disc of ‘Tylose’, a food substitute, in a petri dish) placed on the middle shelf. Sensors were placed in the geometric centre and centrally on the surface of the Tylose disc (Fig. 12.5). Results showed that the mean temperature over 7 days (evaluated from top, middle and bottom sensors) ranged from -1 to 11°C. The overall mean air temperature for all the refrigerators in the survey was 6 °C, with 70% of refrigerators operating at average temperatures above 5°C (Fig. 12.6). An investigation carried out in Northern Ireland found similar results with 71% of refrigerators having a mean internal temperature above 5°C (Flynn Consumer handling 257 Table 12.3 Mean actual and perceived storage life Food Actual mean Perceived mean storage life (days) storage life (days) Meat – raw 3.9 2.4 Meat – cooked 5.4 4.5 Poultry – raw 3.3 2.5 Poultry – cooked 3.9 2.4 Bacon 8.2 6.6 Sausages 5.6 4.1 Paté 10.3 4.1 Pies 3.2 3.3 Source: Evans et al., 1991. et al., 1992). A US study of food discard patterns and reasons found 21% of refrigerators were at or above 10 °C (Van Garde and Woodburne, 1987). An analysis of percentage time spent between certain temperatures, calculated for all refrigerators, showed that the greatest proportion of time 258 Meat refrigeration 17.2% Refrigerator ‘feels’ cold 12% Recommendation on thermometer 2% Other 28% Same setting all the time 32.8% According to weather 0.25% No setting on refrigerator 7.6% Manufacturers recommended setting Fig. 12.4 Methods used to set refrigerator temperature (source: Evans et al., 1991). Simulated food product with thermistor probes at the surface and centre Data logger 1,2 and 3 – thermistors placed in air at top, middle and bottom of refrigerator Crisper 3 2 1 Door Freezer Fig. 12.5 Position of miniature data logger and sensors within refrigerator (source: Evans et al., 1991). (80.3%) was spent between 3 and 8.9 °C. Only small amounts of time were spent above 9 °C (Fig. 12.7). However, only 4 refrigerators (1.6%) in the whole survey operated below 5 °C during all the monitoring period and 33.3% of refrigerators spent all their time above 5 °C. A further analysis showed that in 69.9% of refrigerators the warmest place was in the top and in 45.1% the coolest place was in the middle (Table 12.4). However, the top of the refrigerator was not always the warmest and the bottom was not always the coldest place (Table 12.5, Fig. 12.8). The mean temperature range within a refrigerator was found to vary between refrigerator types. Ice box refrigerators had the smallest range Consumer handling 259 14 – 14.9°C 13 – 13.9°C 12 – 12.9°C 11 – 11.9°C 10 – 10.9°C 9 – 9.9°C 8 – 8.9°C 7 – 7.9°C 6 – 6.9°C 5 – 5.9°C 4 – 4.9°C 3 – 3.9°C 2 – 2.9°C 1 – 1.9°C 0 – 0.9°C – 1 to – 0.1°C – 2 to – 1.1°C – 3 to – 2.1°C – 4 to – 3.1°C – 5 to – 4.1°C 0 5 10 15 20 25 Percentage Fig. 12.6 Overall mean temperatures for all refrigerators in survey (source: Evans et al., 1991). Table 12.4 Position of highest temperature within refrigerators investigated Position % of refrigerators Highest mean Lowest mean temperature temperature (°C) (°C) Top 69.9 20.3 Middle 8.1 45.1 Bottom 22.0 34.6 Source: Evans et al., 1991. (average 1.8 °C), whereas the range in temperature in fridge-freezers and larder refrigerators was nearly twice as great (average of 3.4 °C in fridge- freezers and 3.7 °C in larder refrigerators) (Table 12.6, Fig. 12.9). A survey carried out in China found higher ranges in temperature within domestic refrigerators with only 2.3% of the refrigerators surveyed operating with a temperature range of less than 6°C: 34.1% had differences of 8–12 °C, 34.1% in the range of 12–14 °C and 29.5% differences greater than 14 °C (Shixiong and Jing, 1990). The time for storage of frozen foods in the UK is based on the star-rating system (Table 12.7). This was introduced in the early 1960s relating equip- ment capability with frozen food keeping quality (Ware, 1974; Sanderson- Walker, 1979). 260 Meat refrigeration 0 5 10 15 20 25 30 35 40 15 – 16.9°C 13 – 14.9°C 11 – 12.9°C 9 – 10.9°C 7 – 8.9°C 5 – 6.9°C 3 – 4.9°C – 1 – 0.9°C 1 – 2.9°C – 3 to – 0.9°C – 5 to – 2.9°C Percentage Fig. 12.7 Frequency distribution of temperatures in all refrigerators (source: Evans et al., 1991). Table 12.5 Positions of lowest and highest mean temperatures in refrigerators investigated Refrigerator % of lowest mean % of highest mean type temperatures in: temperatures in: Top Middle Bottom Top Middle Bottom Ice box 48.1 41.6 10.4 28.6 11.7 59.7 Fridge-freezer 10.6 45.5 43.9 84.6 8.9 6.5 Larder 0.0 50.0 50.0 100.0 0.0 0.0 Source: Evans et al., 1991. The advantages of the star-rating system (Sanderson-Walker, 1979) are that (1) it is a simple and easily understood method of giving consumers some indication of recommended storage time at home; (2) it is supported by the equipment manufacturers (who indicate the performance of their Consumer handling 261 Overall Top Middle Bottom Product surface Product centre Ice box Larder Fridge-freezer 0.00 2.00 4.00 6.00 8.00 Temperature ( ° C) Fig. 12.8 Mean temperatures within refrigerator types investigated (source: Evans et al., 1991). Larder Fridge-freezer Ice box 0 5 10 15 20 25 30 35 40 45 Percentage of occurrences <1 1–1.9 2–2.9 3–3.9 4–4.9 5–5.9 6–6.9 7–7.9 8–8.9 9–9.9 10–10.9 11–11.9 12–12.9 Range of temper ature ( ° C) Fig. 12.9 Range of mean temperature distribution within refrigerator types inves- tigated (source: Evans et al., 1991). appliances using the star-rating) and frozen food manufacturers (who indi- cate storage times using the star-rating on their packets). However, few data have been published on temperatures and actual con- sumer use of domestic freezers. 12.5 Performance testing of domestic refrigerators After purchase, chilled food can spend a period of between a few hours and many weeks in a domestic refrigerator. However, few data have been pub- lished on the temperature performance of domestic refrigerators either under controlled conditions or in use. Data can be found on energy con- sumption (Dlugoszewski and Minczewski, 1984), evaporator coil design (Karpinski, 1984), and the shelf-life advantages to be gained with product stored in a special refrigerator containing a 0°C chamber with fan air circu- lation (Olsson, 1988). Current standards for domestic refrigerators contain some temperature tests that are carried out under controlled conditions on empty, closed refrigerators. In domestic use, refrigerator doors are opened regularly, are not usually empty but range from near empty to crammed full, and often food at ambient temperature, or above, is placed in them. Some data have been published from experiments carried out on exam- ples of 3 types of refrigerator (James et al., 1989; James and Evans, 1992b). These were a 6 cubic foot dual compressor fridge-freezer (no.1), a 6 cubic foot single compressor fridge-freezer (no.2) and a 4 cubic foot free stand- ing domestic refrigerator with an ice box compartment (no.3). 262 Meat refrigeration Table 12.6 Temperature range in refrigerator types investigated Range in temperature (°C) Ice box Fridge-freezer Larder Minimum temp range 0.2 0.1 0.5 Maximum temp range 7.0 12.04 9.0 Mean temp range 1.8 3.4 3.7 Source: Evans et al., 1991. Table 12.7 Star-rating system Storage temperature Storage time Capability of equipment a35 Not warmer than -6 °C Up to 1 week Storage only a35a35 Not warmer than -12 °C Up to 1 month Storage only a35a35a35 Not warmer than -18 °C Up to 3 months Storage only a35a35a35a35 Not warmer than -18 °C Up to 3 months Can freeze down fresh food from room temperature to -18 °C as well as store Source: Evans et al., 1991. 12.5.1 Performance of empty appliances When set to the manufacturers recommended setting, temperatures in the ice box refrigerator (no.3) were uniform and low with a minimum of -1.4 °C on the bottom shelf and a maximum of 5.9 °C in the door. Average temperatures were between ca. 0.5 and 1.5°C on the shelves and just above 3 °C in the door with a cycle of less than 0.5 °C. There was a much larger temperature range in the two fridge-freezers, 1.7–14.3 °C in no.1 and -6.7 to 10.7 °C in no.2.Average temperatures were far less uniform in the chilled food compartment of the fridge-freezers. In fridge-freezer no.1 the average temperature of the top shelf was up to 5 °C higher than that measured on the middle shelf which was the coolest area in the appliance. Highest average temperatures of ca. 7.5 and 10 °C were measured on the top shelves of the fridge-freezers. In fridge-freezer no.2 the average temperature on the bottom shelf reached -2 °C at the minimum point in the temperature cycle. 12.5.2 Performance of loaded appliances Loading 12 packs (dimensions 100 ￥ 150 ￥ 25 mm) of Tylose (a simulated food) that had been precooled to 5 °C into the ice box refrigerator reduced the mean temperatures by between 1.2 and 2.0 °C (Table 12.8). The tem- perature change caused by loading was similar in magnitude in fridge- freezer no.2 where the mean temperature of the top shelf rose by 0.7 °C and the mean at other positions dropped by between 0.5 and 1.1°C. It was also noted that the length of the refrigeration cycle increased from ca. 0.75 to 1 h. In fridge-freezer no.1 the magnitude of the temperature cycle was substantially reduced. The magnitude and position of the maximum Consumer handling 263 Table 12.8 Maximum, minimum and mean temperatures on shelves and in door of refrigerators Ice box Fridge-freezer Fridge-freezer no. 1 no. 2 Position Empty Loaded Empty Loaded Empty Loaded Top shelf Maximum 2.1 1.2 14.3 6.0 10.7 11.1 Minimum 0.7 -1.2 6.6 2.4 4.7 5.2 Mean 1.5 0.3 10.2 3.8 7.3 8.0 Middle Maximum 2.2 0.4 8.0 6.9 5.4 4.9 shelf Minimum -1.0 -2.6 1.7 4.3 0.9 -0.1 Mean 1.4 -0.6 6.3 5.5 3.6 2.9 Bottom Maximum 1.6 4.0 8.0 9.8 5.0 3.7 shelf Minimum -1.4 -3.0 2.4 5.7 -6.7 -5.8 Mean 0.7 -0.6 6.7 8.1 2.1 1.0 Door Maximum 5.9 3.3 8.0 8.4 6.5 6.7 Minimum 0.9 -0.4 5.3 0.8 2.2 0.5 Mean 3.2 2.0 6.9 3.8 4.2 3.7 Source: Evans et al., 1991. temperature was also influenced by loading, from a value of 14.3 °C and located on the top shelf, to a reduced value of 9.8 °C and a location on the bottom shelf. 12.5.3 Effect of loading with warm (20 °C) food products Food is often loaded ‘warm’ into refrigerators after purchase from retail stores. Loading a small amount of warm (20°C) food, (2 joints (ca. 17.5 ￥ 7.6 ￥ 3.6cm, 195 ± 10 g) and 2 drumsticks (ca. 12 ￥ 6 ￥ 3cm, 120 ± 10g)) of simulated chicken (Tylose) showed up the poor cooling performance of domestic refrigerators. Over 2 h was required in the ice box refrigerator to reduce the surface temperature of the drumsticks and portions to 7°C compared with over 5 h in the fridge freezer (Table 12.9). Drumsticks in the domestic refriger- ator always cooled faster than the larger portions. However, in the fridge- freezer, portions on the middle shelf cooled faster than drumsticks positioned on the top shelf. 12.5.4 Effect of door openings In normal operation refrigerator doors are opened and left open for dif- ferent periods while food is loaded and unloaded. In the ice-box refrigera- tor single door opening of either 3 or 6 min was compared with 3 or 6 1 min openings over a 1 h period. Immediately after a 3 min door opening the average air temperatures ranged between 5.5 and 16 °C compared with 0–2°C before the opening (Fig. 12.10). Within ca. 1h of the door being closed the average tempera- tures had been reduced to within 1 °C of their normal value. With 3 1 min door openings the average temperatures tended to increase progressively with each subsequent opening and the degree of temperature recovery reduced (Fig. 12.11). After a single 6 min door opening the average air temperature rose to between 7.5 and 16.5 °C (Fig. 12.12). The air temperature slowly recovered over the next 2 h. After each of 6 successive 1 min door openings the air temperature was warmer and the temperature after 9 min of recovery higher (Fig. 12.13). 264 Meat refrigeration Table 12.9 Time taken (h) to cool products from 20 to 7°C Ice box Fridge-freezer Surface drumstick 2.2 5.3–8.6 Deep drumstick 2.5 5.4–8.6 Surface portion 2.2 5.1–7.5 Deep portion 3.4 5.9–8.0 Source: Evans et al., 1991. 12.6 Performance testing of domestic freezers Despite their widespread ownership and use, few data have been published on the temperature performance of domestic freezers. Much of what has been published has primarily been for the benefit of the consumer, giving practical instructions for ‘home freezing’, with little basis in scientific fact. Consumer handling 265 Top average temp Mid average temp Bottom average temp Door average temp 18 16 14 12 10 8 6 4 2 –2 0 0.0 0.5 1.0 1.5 Time (h) T e mperature ( ° C) Fig. 12.10 Three min door opening (source: Evans et al., 1991). Top average temp Mid average temp Bottom average temp Door average temp 18 16 14 12 10 8 6 4 2 –2 0 0.0 0.5 1.0 1.5 2.0 2.5 Time (h) T emperature ( ° C) Fig. 12.11 Three 1 min openings at 20 min intervals (source: Evans et al., 1991). Bailey’s work remains one of the few studies on the practical performance of domestic freezers (Bailey, 1974). Data can also be found on modelling the cooling curve of a food in a direct cooling-type domestic fridge-freezer (Murasaki et al., 1981). Freezer standards and manufacturers’ claims place the emphasis on the ability to freeze fresh food within a 24h period. The basis of this need to 266 Meat refrigeration Top average temp Mid average temp Bottom average temp Door average temp 18 16 14 12 10 8 6 4 2 –2 0 0.0 0.5 1.0 1.5 2.0 2.5 Time (h) T e mperature ( ° C) Fig. 12.12 Single 6 min door opening (source: Evans et al., 1991). Top average temp Mid average temp Bottom average temp Door average temp 2.52.01.51.00.50.0 18 16 14 12 10 8 6 4 2 –2 0 Time (h) T emperature ( ° C) Fig. 12.13 Six 1 min openings at 10 min intervals (source: Evans et al., 1991). ‘fast freeze’ is that the organoleptic properties of foods are affected by freezing rate owing to their influence on ice crystal formation. It is true that freezing causes irreversible damage to the microstructure of meat, and it has long been shown by photomicrographs that the faster meat is frozen, the smaller the ice crystals produced. Above a certain rate these are pre- dominantly intracellular, when by implication the damage to cell walls should be minimal. However, numerous other factors affect the ultimate size of ice crystals. They can grow in cold storage, at a rate that is faster the higher the temperature and the more it fluctuates. In meat they become progressively larger the longer the period between slaughter and freezing; in rapidly frozen, well-conditioned meat they are as large as in the case of very slowly frozen fresh meat. Furthermore, extracellular ice crystal for- mation is not necessarily disruptive because muscle cell wall membranes are elastic, unlike those of vegetable tissue. Evidence in the literature indicates that organoleptically there would appear to be no differences between meat, after thawing and cooking, frozen in 24 h, 48 h or even longer, and there are no adverse reports on bac- teriological deterioration of meat slowly frozen from the chilled state in this way (Bailey, 1974; 1976). In any case, even the best rates of freezing attain- able in domestic food freezers are very slow in real terms. No experiments have established whether there is any merit in requiring the freezing of meat to be completed within 24 h. The recommendation is to set the freezer at its lowest temperature up to one day before adding a new batch of food for freezing. The setting should be returned to the norm (about -18 °C) the next day and no more than one tenth of the total capacity of the freezer should be used for freez- ing fresh food in any 24 h period. 12.7 Conclusions It is clear from the data presented that the temperature of chilled and frozen meats can rise to unacceptably high values if transported without insulation in a car boot.These data were obtained in June 1989, a very sunny period, but higher ambient temperatures are not uncommon in mid- summer. The predictions carried out show that substantial increases in bacterial numbers can occur during transportation and subsequent re- cooling. It is not difficult to think of even worse situations where chilled products reside in the open backs of estate cars for many hours on hot summer days. However, a combination of increased consumer education and the use of insulated/precooled containers should solve this particular problem. The basic design of domestic refrigerators has not changed in the last 50 years although their use and lately the type, complexity and microbio- logical sensitivity of the foods stored in them has markedly changed. Consumer handling 267 Designers have responded to market demands for more compact appli- ances and more features, for example chilled drink and ice dispensers, but temperature control is only advertised as a sales point on more expen- sive multicompartment refrigerators. Consumers now purchase and store a wide range of ready meals and other chilled products and they have demanded, and obtained, substantial reductions and in some cases the total elimination of preservatives and additives in these products. New chilled products are therefore inherently more bacterially sensitive and require closer temperature control than their predecessors. If current predictions that eating habits will change from the current pattern of set meals, to all day grazing, then the consequence is likely to be a demand for and purchase of more preprepared chilled foods and more visits to domestic refrigerators. These results indicate that current refrigerators are unlikely to be able to maintain foods at the temperatures desirable for chilled products when subjected to more frequent door openings and the addition of ‘warm’ food. The appliances differ substantially in their ability to respond to door open- ings and loading with warm (20°C) food and both these operations are crucial to domestic operation. Since current and proposed standards only test empty closed appliances, these differences are not apparent to the con- sumer who only knows that they are tested to the relevant standard. These limited experiments indicate the problems in providing consumers with general recommendations and simple temperature test procedures. Rec- ommendations such as ‘by keeping the top of a fridge at 5 °C, the bottom should be at 0°C’ (Anon, 1990) may be applicable to the majority of appli- ances but not pertinent to specific appliances. The consumer study of 252 households indicated that higher tempera- tures than desirable are to be found in current domestic refrigerators and that there is a need to educate consumers about the need for lower tem- peratures. This need should subsequently create a demand for domestic refrigerators that will maintain low temperatures under normal operating conditions. The data presented on the temperature performance of refrig- erators in the laboratory indicates the need for different international standards that relate to food safety, quality and consumer usage of domes- tic refrigeration. Experimental investigations into the performance of a typical household freezer have shown that only relatively small quantities of meat joints of weight 1.5 kg, packaged and loaded as in a domestic situation, can be frozen from +5 to -10 °C in 24 h. However, evidence in the literature indicates that organoleptically there would appear to be no differences between meat frozen in 24 h, 48 h or even longer, and there are no adverse reports of bac- teriological deterioration of meat slowly frozen from the chilled state in this way. No experiments have established whether there is any merit in requir- ing freezing to be completed within 24 h. 268 Meat refrigeration 12.8 References anon (1990), Fridge-freezers,The Association for Consumer Research, Which? May 1990, 286. bailey c (1974),The problems of domestic deep freeze cabinets, in Cutting C L Meat Freezing: Why and How? 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