Heat Treatments PDF

1  DEPARTMENT FOOD TECHNOLOGY, SAFETY AND HEALTH FOOD STRUCTURE & FUNCTION RESEARCH GROUP Heat treatments Prof. dr. ir. Koen Dewettinck  Basic knowledge Technical thermodynamics Heat transfer...

1  DEPARTMENT FOOD TECHNOLOGY, SAFETY AND HEALTH FOOD STRUCTURE & FUNCTION RESEARCH GROUP Heat treatments Prof. dr. ir. Koen Dewettinck  Basic knowledge Technical thermodynamics Heat transfer Psychrometrics Heat addition Heat removal Emerging technology FOOD PRESERVATION EATING QUALITY Destruction of microorganisms Low temperature preservation High pressure processing Heat treatments Freezing Irradiation Drying Microwave heating Baking, roasting and frying Pulsed electric field Separation techniques Ultrasound 3 Heat treatments Preservation processes eliminate the potential for foodborne illness Principle: heating → holding → cooling ̶ heating and cooling rate ̶ temperature ̶ holding time ̶ Blanching DIFFERENT ̶ Pasteurisation ̶ Sterilisation Fellows – Food processing technology – Chapter 11, 12 and 13 4 Blanching 5 Aim of blanching Destroy enzymatic activity in vegetables and some fruits ̶ Lipoxygenase Negative effects on product quality: colour, ̶ Polyphenoloxidase texture, off-odours, off-flavours, nutrient ̶ Polygalacturonase breakdown ̶ Chlorophyllase ̶ Catalase Marker enzymes; peroxidase is the most ̶ Peroxidase heat resistant 6 Aim of blanching Under-blanching → more damage enzymes are not inactivated tissue damage → enzymes come into contact with substrates 7 Aim of blanching Blanching = pre-treatment ̶ after preparation of raw material ̶ sometimes combined with peeling/cleaning Before freezing and drying ̶ enzymes active during storage ̶ micro-organisms grow during thawing or rehydration Before canning ̶ avoid enzyme activity during heating of the can 8 Determination of the time-temperature combination Optimal time-temperature combination — size and shape of the food — thermal conductivity of the food — blanching temperature/method — convective heat-transfer coefficient (water or steam) Objective — specified temperature at thermal center — specified degree of peroxidase inactivation — specified retention of vit. C Typically 1-15 min at 70-100°C 9 Unsteady state heat transfer Convective heating by steam/hot water Conduction of heat within the food 10 Equipment Heating ̶ steam blanchers: tunnel, rotarory drum, fluidised bed ̶ hot-water blancher ̶ microwave blanching Cooling ̶ cold air ̶ cold water (sprays) 11 Steam tunnel blanching Mesh conveyer carries food through steam atmosphere Source: http://www.key.net/products/steam- blancher 12 Steam tunnel blancher: IQB Individual quick blanching (IQB) → overcome the problem of uniform heating of multilayered foods Heating in one layer Cooling with Holding in fog spray deep bed for uniform T 13 Steam tunnel blancher: IQB IQB with pre-conditioning = preliminary drying (air at 65°C) → surface water evaporates → surface absorbes condensing steam during IQB 14 Rotary drum steam blancher + compact design + reduced energy consumption + reduced water consumption + lower costs 15 Batch fluidised bed blancher A mixture of air and steam fluidises and heats the product + fast and uniform heating + shorter processing times + smaller losses of water-soluble components + reduction in effluent 16 Hot water blanching Different designs: Reel blancher → a slowly rotating cylindrical mesh drum, partly submerged in hot water Pipe blancher → hot water is recirculated through a pipe, food is metered in Blancher-cooler → a single conveyor through pre-heating, blanching and cooling 17  Steam vs hot water: Trade-off between costs and product quality Steam + Little loss of water-soluble components - Limited cleaning → washers + Less effluent - Uneven heating in piles + Better energy efficiency - Some loss of mass + Better product quality - Complex equipment - Difficult to clean Hot water + Less complex equipment - More effluent + More even heating - Risk: contamination with thermophilic bacteria + Less floorspace - Physical damage of food 18 Cooling after blanching Cold air - evaporation → weight loss Cold water - increased leaching losses + absorbing water increases overall yield Cold water sprays + higher nutrient retention 19 Microwave blanching + faster and more uniform heating + reduced energy costs + lower nutrient losses Ohmic blanching Infrared blanching - higher investment cost Example: mushroom blanching 20 Effect on foods Membrane damage → loss of cell turgor → nutrient/flavour losses Disruption of subcellular organelles KZ Katsaboxakis. In: P Zeuthen, JC Cheftel, C Eriksson, M Jul, H Leniger, P Linko, G Varela, Eds., Thermal Processing and Quality of Foods, London: Elsevier Applied Science, 1984, pp. 559–565 21 Effect on foods: texture Softening ̶ Large pieces and certain types of food ̶ Calcium chloride to reduce softening 22 Effect on foods: colour Removal of intercellular gases from tissues and surface dust → alters the wavelength of reflected light → brightens the colour Food pigments ~ D-value → sodium carbonate or calcium oxide to protect chlorophyll 23  Effect on foods: chlorophyll H H From: Food science and culinary arts by Mark Gibson (2018) Chlorophyll a Chlorophyll b Heat 1. Loss of carbon/hydrogen tail → water-soluble → leaching out Enzymes (chlorophyllase) 2. Magnesium is replaced by hydrogen → colour change 24  Effect on foods: chlorophyll Solution: alkaline solution (sodium carbonate) - high concentrations reduce firmness H H Chlorophyll a Chlorophyll b Heat 1. Loss of carbon/hydrogen tail → water-soluble → leaching out Enzymes (chlorophyllase) 2. Magnesium is replaced by hydrogen → colour change 25 Effect on foods: vitamins and minerals Leaching → loss of water-soluble components Ascorbic acid = water soluble, thermally labile and subject to enzymatic breakdown → indicator! Blanching and cooling Surface area-to-volume ratio of vegetable method 26 Effect on foods: vitamins and minerals Vitamin loss depends on — Variety and maturity — Food preparation methods — Surface area-to-volume ratio — Blanching and cooling method — Time-temperature combination — Ratio water to food 27 Effect on micro-organisms ̶ Reduce surface contamination ̶ Important for heat sterilisation → less spoiled containers 28 Pasteurisation 29 Definition Any process, treatment, or combination thereof, that is applied to food to reduce the most resistant micro-organism(s) of public health significance to a level that is not likely to present a public health risk under normal conditions of distribution and storage. Definition evolved towards ‘any process’ → irradiation, high-pressure processing, pulsed electric fields, membrane technology 30 Aim of pasteurisation 31 Aim of pasteurisation 32 Pasteurisation of milk pH ≈ 6.7 → destruction of pathogens Some spoilage mo are more resistant → refrigeration Alkaline phosphatase = indicator enzyme → D-value similar to pathogens High-temperature-short-time (HTST) → better retention of nutritional values and sensory quality 33 Pasteurisation of milk Source: Dairy processing handbook, Tetra 34 Pak Processing Systems AB, 2003 Equipment Packaged foods ̶ Hot water Similar to blanching ̶ Water sprays equipment ̶ Steam Unpackaged foods ̶ Plate heat exchanger Singh & Heldman – Introduction to food engineering – Chapter 4 p. ̶ Tubular heat exchanger 266-273 ̶ Scraped surface heat exchanger 35 Hot water pasteurisation ̶ glass, metal or plastic containers ̶ little risk of thermal shock ̶ batch or continuous Source: http://www.zacmi.com/pdf/reserved/Pasteurizer.pdf 36 Pasteurisation with water sprays Tunnel: pre-heating, heating, cooling Water recirculation → save energy and water Source: http://www.khs.com/nc/en/press/press-articles/specialist- articles/press-release/pressrelease/extensive-package-of- advantages.html?type=98&print=1 37  From: www.foodnavigator.com Steam pasteurisation + Faster heating + Shorter residence time + Smaller space requirements Almond pasteurization: — Propylene oxide gas (non-organic) Examples: surface steam — Steam pasteurisation (organic) pasteurisation of fish and meat, nuts pasteurisation 38 Equipment Packaged foods ̶ Hot water Similar to blanching ̶ Water sprays equipment ̶ Steam Unpackaged foods ̶ Plate heat exchanger Singh & Heldman – Introduction to food engineering – Chapter 4 p. ̶ Tubular heat exchanger 266-273 ̶ Scraped surface heat exchanger 39 Plate heat exchanger Parallel stainless-steel plates → rubber gaskets prevent intermixing Source: http://www.alfalaval.com/products/heat-transfer/plate-heat- exchangers/gasketed-plate-and-frame-heat-exchangers/ 40 Plate heat exchanger 41 Plate heat exchanger ̶ Counter or parallel flow ̶ Patterns → increased turbulence → better heat transfer 42 Plate heat exchanger Food properties low-viscosity liquid foods particulates: < 0.3 cm Dairy and beverage industry Deposition of milk proteins → fouling decrease heat transfer pressure drop Source: https://www.tempco.it/blog/54/fouling-factor-negli-scambiatori-a-piastre/ 43 Plate heat exchanger + maintenance is easy + capacity increase → more plates + low capital investment + energy conservation → regeneration 44  45 Heating Regeneration City water COOLING Chilled water Glycol Regeneration Regeneration 46 Tubular heat exchanger Double-pipe heat exchanger: counterflow and parallel flow 47 Tubular heat exchanger Double-pipe heat exchanger: counterflow and parallel flow Source: http://www.platetypeheatexchangers.com/double_pipe_heat_e 48 xchanger.html Tubular heat exchanger Triple-tube heat exchanger Obstructions to create turbulence 49 Tubular heat exchanger Shell-and-tube heat exchanger 1 tube pass 2 tube passes 50 Tubular heat exchanger Shell-and-tube heat exchanger Fluid flows over the tubes (not parallel) Application: heating in evaporation systems 51 Tubular heat exchanger Shell-and-tube heat exchanger Source: http://www.souheat.com/ 52 Tubular heat exchanger Shell-and-tube heat exchanger Source: http://www.souheat.com/ 53 Tubular heat exchanger Coiled heat exchanger — high capacity of high viscous foods — particle integrity up to 25 mm Product flows through coil-shaped tube Media flows around the tube Source: https://www.tetrapak.com/processing/heat-exchangers/tetra-pak- coiled-heat-exchanger 54 Scraped-surface heat exchanger Minimize heat resistance due to film buildup and fouling Scraping → rapid heat transfer to small product volume Rotation speed → depends on product Steam, hot water, brine or refrigerant 55 Scraped-surface heat exchanger 56 Scraped-surface heat exchanger -35°C – 190°C Applications: heating, whipping, gelling, emulsifying, plasticizing, crystallizing Foods: soups, peanut butter, baked beans, tomato paste, ice cream 57 Effect of pasteurisation on foods Mild heat treatment → minor changes to sensory and nutritional characteristics → shelf-life extended by few days or weeks Fruit juice → colour deterioration due to polyphenoloxidase → deaeration → BUT loss of volatile components 58 Sterilisation 59 Sterilisation Destroy vegetative microbial cells, spores and enzymes → shelf-life up to 6 months at room temperature In-container sterilisation (retorting) — canned vegetables and meat, baby foods, milk,… — pre-cooking → minimum heating before consumption — disadvantage: substantial change in quality Ultra-high-temperature (UHT) — mainly liquid foods — aseptic packaging after heat treatment — advantage: increased quality 60 In-container sterilisation C-value = cook value Achieve the required change in sensory characteristics Higher than time needed for sterilisation — z-value nutrients and chemicals related to cooking ~ 25-45°C — z-value micro-organisms ~ 7-12°C 61 In-container sterilisation: processing time Time for adequate sterilisation — Heat resistance of micro-organisms and/or enzymes — pH — Heating conditions — Size and shape of container Heat penetration — Physical state of the food Processing time = f(heat resistance and heat penetration) 62 In-container sterilisation: processing time Heat resistance → part: destruction of micro-organisms — Depends on the pH of foods → pH-dependent strain/type — Low-acid foods: Clostridium botulinum as reference (most pathogenic micro-organism) — Acid foods: Enzyme inactivation (less severe heat treatment) — Commercial sterility concept (12D process) 63 In-container sterilisation: processing time Rate of heat penetration Unsteady state heat transfer process 1. convection 2. conduction 1 2 3 3. convection and/or conduction Steam or Food Wall pressurized Major problem: low rate of heat water penetration for solid or viscous foods Solution: agitation 64 In-container sterilisation: processing time Rate of heat penetration: measurement Geometric centre 1/5th of container height 65 In-container sterilisation: processing time Rate of heat penetration depends on ̶ Product ̶ Process ̶ Package 66 In-container sterilisation: processing time Rate of heat penetration depends on PRODUCT Consistency ~ type of heat transfer — convection: liquid and particulate foods — conduction: solid foods Convection Convection Convection Conduction Conduction Conduction Water-based Non-water-based >> > 67 In-container sterilisation: processing time Rate of heat penetration depends on PRODUCT Consistency ~ type of heat transfer — convection: liquid and particulate foods — conduction: solid foods Composition ~ thermal conductivity 68 In-container sterilisation: processing time Rate of heat penetration depends on PROCESS Retort temperature: ∆T = driving force Heat transfer medium: saturated steam = most effective — Steam pressure balances the pressure developed inside the container — Velocity ~ rate of heat transfer Agitation: end-over-end agitation and axial agitation — ↑ effectiveness of convection currents, ↑ heat transfer in viscous or semi-solid foods Type of retort: batch or continuous 69 In-container sterilisation: processing time Rate of heat penetration depends on PACKAGING Size of container — Volume: small is faster than large — Height: tall promote convection current — Higher surface area:volume ratio of tray, pouches or flat cans → faster heat penetration Container material ~ thermal conductivity: metal > glass/plastic Headspace volume — Static retort: headspace insulates food surface → reduce heat penetration — Agitated retorts: movement of headspace gas bubble mixes convective/conductive heating foods 70 In-container sterilisation: method Exhausting = removal of air ̶ Reduces strain on seals ̶ Prevents corrosion ̶ Prevents oxidative changes in the food Methods ̶ Hot filling ̶ Cold filling + preheating with lid partially sealed ̶ Mechanical removal (pump) ̶ Steam flow closing 71 In-container sterilisation: method Heating with saturated steam — condensing steam → latent heat — air in retort → insulating layer around can → venting Heating with flames — flame temperature ~ 1770°C — high rates of heat transfer — high internal pressure → small cans 72 In-container sterilisation: method Heating with hot water hot water with over-pressure of air (300kPa, 121°C) glass containers — lower thermal conductivity → risk of thermal shock — slower heat penetration, longer processing times flexible pouches — better heat transfer, minimal overheating — energy saving, shorter processing time — difficulty: influence of heat on the polymer and seals 73 In-container sterilisation: method Cooling — sprays of potable water — compressed air to equalise pressure 74 In-container sterilisation: equipment Batch retorts 75 In-container sterilisation: equipment Batch retorts Source: http://www.dft- technology.de/en/products/steam-water-spray.html 76 In-container sterilisation: equipment 77 In-container sterilisation: equipment 78 In-container sterilisation: equipment Continuous retorts + more gradual change in pressure + greater thermal efficiency + reduced process time - higher capital cost 79 In-container sterilisation: equipment Continuous: rotary steriliser Source: http://www.jbtfoodtech.com/en/Solutions/Equipment/Sterilizers/Rotary- Pressure-Sterilizer 80 In-container sterilisation: equipment Continuous: hydrostatic (tower) steriliser Source: Dairy processing handbook, Tetra Pak Processing Systems AB, 2003 81 In-container sterilisation: equipment Continuous: hydrostatic (tower) steriliser 82 In-container sterilisation: effect on foods Nutrients Lipids and carbohydrates hydrolysis → remain available Protein coagulation → some loss in biological activity Water-soluble vitamins → brine or syrup Lipid-soluble vitamins → oxidation 83 In-container sterilisation: effect on foods Meat Pigments — Brown metmyoglobin — Maillard reaction and caramelisation Flavour and aroma — Pyrolysis, deamination and decarboxylation of amino acids — Maillard reaction and caramelisation — Lipid oxidation Texture — Coagulation and hydrolysis of collagen → loss of waterholding capacity — Gelatin solubilisation 84 In-container sterilisation: effect on foods Fruit and vegetables Pigments: chlorophyll and carotenoids conversion Flavour and aroma: Degradation, recombination and volatilisation of aldehydes, ketones, sugars, lactones, amino acids and organic acids Texture — Hydrolysis of pectic materials — Starch gelatinisation — Loss of cell turgor Watch the video about canned tomato soup available on Ufora 85 In-container sterilisation: effect on foods Milk Pigments: Maillard reaction and caramelisation Flavour and aroma: cooked flavour due to whey protein denaturation 86 Ultra high temperature (UHT) Heating foods in thin layers + aseptic packaging → improved product quality → shelf-life: at least 6 months without refrigeration Examples: Liquid foods: milk, fruit juice and concentrates, cream, yoghurt drink, salad dressing and ice cream mixes Foods with small particles: cottage cheese, baby foods, tomato products, soups and rice desserts 87 UHT: foods with larger solids Enzyme inactivation at the centre: overcooking of the surface → limiting particle size Agitation to improve rate of heat transfer and temperature distribution → product damage Settling of solids in holding tube → overlong holding times → variable proportions of solids in the filled product 88 UHT: processing temperature and time t,T-combination for required F0-value — fastest moving particle — longest time needed for heat transfer from liquid to particle center Typically 130-150°C for a few seconds 89 UHT: method Air removal Constant volume → avoid under-processing Saving energy Reducing oxidative changes 90 UHT: method Air removal Direct methods Heating — steam infusion — steam injection Holding Indirect methods Cooling — plate heat exchangers — tubular heat exchangers — scraped-surface heat exchangers 91 UHT: method Air removal Heating Holding Cooling Prevents boiling during holding and cooling Back-pressure 92 UHT: method Air removal Heating Holding Cooling Back-pressure Surge tank Aseptic packaging 93 UHT: method 94 Dairy processing handbook, Tetra Pak Processing Systems AB, 2003 UHT: heating equipment Direct methods — steam infusion — steam injection Indirect methods — plate heat exchangers Heat exchangers — tubular heat exchangers pasteurisation — scraped-surface heat exchangers 95 UHT: steam-infusion Spreaders → product flows in thin sheets Direct contact between steam and product → rapid T increase Steam condensed steam Latent heat Food heated food Mixture 96 UHT: steam-infusion Water addition is desirable → water is flashed off: vacuum cooling system Application: soups, milk, ice cream mixes,… 97 UHT: steam-injection Extra information Steam is divided in small bubbles in the food product Source: Dairy processing handbook, Tetra Pak Processing Systems AB, 2003 98 UHT: direct heating methods Direct indirect Steam infusion steam injection + fastest heating + fastest heating of both methods + fastest cooling + greater control of processing conditions - low-viscosity products + no hot surfaces → less risk for localised overheating - potable steam = expensive + higher-viscosity products - low energy regeneration - spray nozzle bloackage - low process flexibility - separation of components in some foods 99 UHT: indirect heat exchangers Plate heat exchangers + Relatively expensive + Less floor space and water consumption + Energy efficient (>90% energy regeneration) + Flexible in changing the operation + Easily inspected - Pressures limited by the plate gaskets to approximately 700 kPa - Low liquid velocities - Low flow rates, cause uneven heating and solids deposits - Gaskets susceptible to high temperatures - Limited to low viscosity liquids - Careful initial sterilisation of the large mass, necessary for uniform expansion - Liable to fouling 100 UHT: indirect heat exchangers Tube-and-shell heat exchanger + Few seals and easier maintenance of aseptic conditions + Operation at higher P (7000-10000 kPa) + Higher liquid flow rates (6m/s) + Turbulent flow at tube walls + More uniform heat transfer - Difficulty in inspecting the surfaces for food deposits - Limited to relatively low-viscosity food - Lower flexibility to change in production - Larger-diameter tubes cannot be used - Any increase in production rate requires duplication of the equipment 101 UHT: aseptic packaging No high temperature and pressure during processing! Cartons are pre-sterilised — UV or ionising radiation — hydrogen peroxide or heat Maintaining sterile condition — UV — Positive air pressure of filtered air (prevent entry of contaminants) 102 UHT: effect on foods Milk ̶ little caramelisation and Maillard browning ̶ age gelation = sudden sharp increase in viscosity due to aggregation of casein micelles ̶ negligible vitamin losses → improved sensory and nutritional quality as compared to in- container sterilisation 103  Sterilisation: in-container sterilisation UHT Criteria In-container sterilisation UHT Product sterilisation Unsteady state Precise, isothermal Process calculations Fluids Routine, convection Routine Particulates Routine, conduction or broken heating Complex Low acid particulate processing Routine Becoming more common Other sterilisation required None Complex (process equipment, containers, lids, aseptic tunnel) Energy efficiency Low >30% saving Sensory quality Not suited to heat-sensitive foods Suitable for homogeneous heat-sensitive foods Nutrient losses High Minimal Value added Lower Higher 104  Sterilisation: in-container sterilisation UHT Criteria In-container sterilisation UHT Stability Shelf stable at ambient temperatures Shelf stable at ambient temperatures Suitability for microwave heating Only glass and semi-rigid container Most semi-rigid and rigid containers (not aluminium foil) Production rate 600-1000 min-1 ≈ 500 min-1 Labour/handling costs Higher Low Downtime Minimal (mostly caused by seaming and Re-sterilisation needed if loss of sterility in labelling) filler or steriliser Flexibility for different container sizes Need different container delivery equipment Single filler for different container sizes and/or retorts Survival of heat resistant enzymes Rare Common in some foods (e.g. milk) Post-process additions Not possible Possible (e.g. probiotics added before filling) 105  DEPARTMENT FOOD TECHNOLOGY, SAFETY AND HEALTH FOOD STRUCTURE & FUNCTION RESEARCH GROUP Heat treatments Prof. dr. ir. Koen Dewettinck