Effects of prefreezing treatments on the structure of strawberries and jams

Contents
ABSTRACT.......................................... .................................................. .............. 5
PREFACE........................................... .................................................. ................ 7
LIST OF ORIGINAL PUBLICATIONS...................................... ........................ 8
ABBREVIATIONS .................................................. .......................................... 12
1. INTRODUCTION...................................... .................................................. 13
1.1 Strawberry .................................................. ......................................... 14
1.2 Model of the primary cell wall .................................................. .......... 15
1.2.1 The chemistry of textural compounds..................................... 17
1.2.1.1 Cellulose, hemicellulose and lignin .......................... 18
1.2.1.2 Pectins........................................... ............................ 19
1.2.1.3 Proteins .................................................. ................... 22
1.3 Texture studies of cell walls .................................................. .............. 23
1.3.1 Bright-field, epifluorescence and CLS microscopy ................ 23
1.3.2 FT-IR microspectrometry................................. ....................... 25
1.4 Pretreatments .................................................. ..................................... 28
1.4.1 CaCl2 .................................................. ..................................... 28
1.4.2 PME .................................................. ...................................... 30
1.4.3 Sucrose .................................................. .................................. 32
1.4.4 Vacuum infusion .................................................. ................... 32
1.5 Freezing and thawing .................................................. ........................ 33
1.6 Jam making............................................ .............................................. 35
1.6.1 Ingredients....................................... ........................................ 36
1.6.1.1 Strawberries .................................................. ............ 36
1.6.1.2 Pectin .................................................. ...................... 37
1.6.1.3 Sucrose........................................... ........................... 39
1.6.1.4 Acids and preservatives ............................................ 40
1.6.2 Traditional jam making .................................................. ......... 41
1.6.2.1 Open pans .................................................. ............... 44
1.6.2.2 Vacuum pans .................................................. .......... 44
1.6.2.3 Continuous cooking .................................................. 45
1.6.2.4 Filling and cooling........................................... ......... 46
1.7 Aims of the present study .................................................. .................. 48
2. MATERIALS AND METHODS........................................... ....................... 49
2.1 Strawberries...................................... .................................................. . 49
2.2 CaCl2, sugar and enzyme............................................ ......................... 49
2.3 Pretreatments .................................................. ..................................... 49
2.4 Jam making............................................ .............................................. 50
2.5 Microscopical studies .................................................. ........................ 50
2.5.1 Bright-field, epifluorescence and CLS microscopy ................ 50
2.5.2 FT-IR studies........................................... ................................ 51
2.5.2.1 DRIFT measurements ............................................... 51
2.5.2.2 FT-IR microspectrometric studies ............................ 52
2.6 Physical analyses .................................................. ............................... 52
2.6.1 Drip loss and soluble solids............................................ ......... 52
2.6.2 Firmness .................................................. ................................ 52
2.6.3 Jam colour .................................................. ............................. 53
2.7 Chemical analyses .................................................. ............................. 53
2.7.1 pH................................................ ............................................ 53
2.7.2 Calcium........................................... ........................................ 53
2.7.3 Sugar, titrable acidity and vitamin C....................................... 54
2.8 Sensory evaluation........................................ ....................................... 54
2.9 Experimental design .................................................. .......................... 55
2.10 Statistical analysis .................................................. ............................. 56
3. RESULTS AND DISCUSSION........................................ ........................... 57
3.1 Microstructure .................................................. ................................... 57
3.1.1 Bright-field, epifluorescence and CLS microscopy
(Publications I–IV)............................................. ..................... 57
3.1.1.1 Fresh berries (I–II) .................................................. .. 57
3.1.1.2 Pretreated frozen berries (II–IV)............................... 62
3.1.2 FT-IR studies........................................... ................................ 67
3.1.2.1 DRIFT measurements (I) .......................................... 67
3.1.2.2 Fresh berries (I)............................................... .......... 69
3.1.2.3 Pretreated frozen berries (II–III)............................... 70
3.2 Physical quality .................................................. ................................. 73
3.2.1 Drip loss and soluble solids (Publications II–V)..................... 73
3.2.2 Firmness (Publications II–IV)............................................ ..... 73
3.2.3 Jam colour (Publication IV) .................................................. .. 74
3.3 Chemical quality........................................... ....................................... 75
3.3.1 pH (Publications III–V)............................................ ............... 75
3.3.2 Calcium (Publications III–IV)........................................... ...... 75
3.3.3 Sugar, titrable acidity and vitamin C (Publication IV)............ 75
3.4 Sensory quality (Publications III–IV)........................................... ....... 76
3.5 Modeling (Publication V)................................................ .................... 78
4. FEASIBILITY OF THE STUDY............................................. .................... 81
5. CONCLUSIONS .................................................. ........................................ 84
REFERENCES .................................................. ................................................. 86
APPENDICES
Publications I–V

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