While the exact answer depends on how you're storing the data, I think the methodology is the same:
- Create and store an empty list for every fruit name to store its reviews
- For each review, check each of the fruits to see if they appear. If a fruit appears in the comment at all, add the review to that fruit's list
Here's an example of what that would look like:
#The list of fruits
fruits = ['apple', 'banana', 'grape', 'orange']
#The collection of reviews (based on the way it was presented, I'm assuming it was in a dictionary)
reviews = {
    '1':['the banana was delicious','he called the firetruck','I had only half an orange'],
    '2':['I liked the banana','there was a worm in my apple','Cantaloupes are better then melons'],
    '3':['It could use some more cheese','the grape and orange was sour']
}
fruitDictionary = {}
#1. Create and store an empty list for every fruit name to store its reviews
for fruit in fruits:
    fruitDictionary[fruit] = []
for customerReviews in reviews.values():
    #2. For each review,...
    for review in customerReviews:
        #...check each of the fruits to see if they appear.
        for fruit in fruits: 
            # If a fruit appears in the comment at all,...
            if fruit.lower() in review: 
                #...add the review to that fruit's list
                fruitDictionary[fruit].append(review) 
This differs from previous answers in that sentences like "I enjoyed this grape. I thought the grape was very juicy" are only added to the grape section once.
If your data is stored as a list of lists, the process is very similar:
#The list of fruits
fruits = ['apple', 'banana', 'grape', 'orange']
#The collection of reviews
reviews = [
    ['the banana was delicious','he called the firetruck','I had only half an orange'],
    ['I liked the banana','there was a worm in my apple','Cantaloupes are better then melons'],
    ['It could use some more cheese','the grape and orange was sour']
]
fruitDictionary = {}
#1. Create and store an empty list for every fruit name to store its reviews
for fruit in fruits:
    fruitDictionary[fruit] = []
for customerReviews in reviews:
    #2. For each review,...
    for review in customerReviews:
        #...check each of the fruits to see if they appear.
        for fruit in fruits: 
            # If a fruit appears in the comment at all,...
            if fruit.lower() in review: 
                #...add the review to that fruit's list
                fruitDictionary[fruit].append(review)